Review: Samyang/Rokinon 14mm f/2.8 ED AS IF UMC

Looking at the 14mm f/2.8 wide angle lens

Some our more striking commercial shots have come with the use of extreme wide angle views, and Keith regularly uses the Canon 14mm lens on a full frame 1Ds Mk3.
Unfortunately for those wanting to experiment with such views, the Canon lens comes in at around £1850
However, if you're on a budget, the Samyang 14/2.8 lens is available for a bit over £300. What's the differences and how well does the Samyang work?
Keith has been lent one for a few weeks to test out, and looks to see how well it works.

• Note - page has many pics - may load slowly!

The lens appears elsewhere around the world under the Samyang, Bower, Rokinon and other names, and is available from numerous suppliers, and in several different mounts (Nikon, Sony, 4/3, Pentax - Canon EF tested here)

Finding the 14mm (USA) - Amazon | B&H | Adorama

Wide lenses

Back in the days I used 35mm film, I'd have considered 24mm as a fairly wide lens. With my 2004 Canon 1Ds (11MP full frame) I moved to the EF16-35 2.8L, and these days its the EF14mm and TS-E17mm (tilt/shift) that offer my widest rectilinear views.

I mention rectilinear ('straight lines') since I also have the Canon 8-15 zoom fisheye lens [review] which most definitely does not keep straight lines straight (unless they pass through the centre of the field).
As an architectural photographer, straight lines are important, and buildings, with their regular patterns, are great at showing lens distortions in this area.
I'd heard that the Samyang 14mm was a good lens but suffered from such serious distortions that it might not be good enough for 'serious' work. Was this so, could it be fixed, and how difficult was this 'everything manual' lens going to be to use?
I'm testing the lens on a 'full frame' 35mm/FX sensor. On a Canon APS-C body it gives an 'equivalent' of ~22mm and 21mm on a Nikon DX body (it is available for other mounts too)
This is not a 'lens review' full of charts and measurements, but more a feel for what the lens was like to use, and the kinds of results you can expect.

The 14mm f/2.8 ED AS IF UMC Lens

You may find this lens appearing under a number of different guises (Rokinon/Bower) around the world.
It's made in Korea, and ours was kindly loaned by the UK (trade) distributor Intro 2020

The lens is supplied with a lens cap, and rear cap. The lens hood is fixed, and an integral part of the lens body.
There is a soft bag supplied for storing the lens.

Some specifications

I should note that I don't do detailed measurement based reviews of lenses - I know there are people who love that sort of stuff, but MTF charts will have to be found elsewhere...

Parameter	Value
Focal length:	14 mm
Maximum aperture:	2.8
Angle of view:	115.7°
Minimum focusing distance:	0.28 m
Minimum aperture:	22
Number of aperture blades:	6
Focus adjustment:	manual
Optical construction:	14 lens elements, 10 groups, 1 ASP, 1 Hybrid ASP, 2 ED, 3 HR
Available mounts:	Canon EF, Four Thirds, Nikon F, Pentax, Samsung NX, Sony A
Dimensions (for Canon EF):	96.1 x 87 mm
Weight (for Canon EF):	552g

The Canon mount version has no electronic communication with the camera.
That means everything is manual. You need to manually focus and manually set the aperture.
There is no confirmation from the camera AF system, and you need to manually set your shutter speed and ISO - welcome to 'real photography' as some might like to call it ;-)

I have heard it said that you can glue an AF confirm chip to the mount, but this wasn't my lens to modify...
It also means that there is no exif data relating to the lens, so if such numbers matter to you, write down the settings.
The red alignment dot would be easier to use if it were also on the main lens body.
As you can see in the shot below, there is a distance scale on the lens, and combined with quite a long focus throw (how far the focus ring rotates) manual focus is smooth and easy to set.

Given the importance of that distance scale I'll come back to issues of manual focus in more detail later.
Two aspects I'm aware that not everyone will agree with ;-)

There is no hyperfocal distance scale - good!
From my experience they are usually pretty meaningless decoration, which fool the unwary into believing they offer some deep insight into how lenses focus ;-)
Filters - pah, who needs them (no, not even the option for 'gel' filters at the back to reduce image quality)

The aperture blades are not round, so do give a slight shape to OOF highlights at anything other than f/2.8
The curved aperture here is the off-axis aperture viewed with light coming through the light table below.
Move your mouse over the image to see the six sided aperture at f/5.6

If you were wondering why the first view wasn't a full circle, it's because we're looking in at a very oblique angle, and there is internal vignetting - this is perfectly normal for any wide angle lens. It's known as optical vignetting, and as you can see from the f/5.6 image, usually goes if you stop down a bit.
There is an example later, showing the considerable difference that going from f/2.8 to f/8 makes in an actual shot.

First impressions

The lens is of plastic construction, but it's a very solid plastic, and matched to a metal lens mount. If you drop it, don't expect to find a convenient repair service.
Wide open at f/2.8 the lens focuses moderately easily using the viewfinder of my Canon 1Ds Mk3, but that's a very good viewfinder. I've also quite a bit of experience using manual focus lenses (tilt/shift for example) so it's not an issue.
If this is your first manual focus lens, then expect to put in quite a bit of practice. A combination of prefocusing and working at f/8 means that many of the example shots I'll show, needed little real thought about focus when I wanted to press the shutter.
You get a lot of depth of field at f/8 on a 14mm lens - learn to use it.
I'm testing the lens with manual setting of exposure - you can use aperture priority, but do experiment to see how your particular camera works with a lens like this. I use manual exposure settings a lot, so this aspect didn't trouble me.

The distance settings...

For a quick check of the accuracy of the distance settings, I set up the camera in our kitchen, photographing a pack of thin batteries on a door (they were at hand, had no depth and fine writing on them).
The distance I set, was to the film plane (that little ø mark just below the hot shoe).

At only half a metre, using 10x liveview at f/2.8, there is plenty of detail and the focus is easy to adjust with live view.

The view from the camera shows a rather bowed view of my kitchen door.

At one metre you get a feel for just how wide the coverage is for a 14mm lens. However, the distortion is now quite noticeable, as is the fact that it's not a nice simple barrel distortion, which is easy to 'fix'.

At 2 metres, the fine detail isn't showing clearly, even at 10x magnification on the rear view. It's still fairly easy to focus at f/2.8 though. At f/8 it does take quite a bit of rocking the focus back and forth to decide the sharpest point.

The view at two metres - all the walls and doors really are straight and true, even if the house dates from the 1880's.

I'd originally thought of providing a basic table, showing the difference between indicated focus (on the lens) and the actual distance.
However, it appears that the effective focus point also varies slightly with aperture.
So, I'd suggest you try this one out for yourself, if you are going to rely on the distance scale. In particular, use something like the moon to find out where the infinity focus point is for your lens at several different apertures. At f/8 on my setup, infinity focus was just slightly in (shorter) from where marked.
Now, if this sounds bad, it's probably because those of you in the past who used distance scales, never actually checked their accuracy ;-) Accuracy in this area comes at a price (think cine lenses with their extended focus throw)
It so happens that my preferred way of using such wide angles lenses often relies on setting the lens to infinity, rather than the rather vague 'hyperfocal' technique (I'll show an example in a bit).

Using the lens

As well as some test shots, I decided to just go for a walk round near where I live, along the canal in Leicester, and try and give a feel for the sort of view that 14mm gives (shots at f/8, infinity focus, unless noted).
I like using the EF14mm for 'street' photography every so often. The powerful lines and perspective, along with being able to get in really close are fun (and not part of my day to day photo work).
Now you've seen the really obvious distortion that the lens shows in my kitchen, look at the subsequent examples and see how often it shows up in more general shots.
The churchyard at St. Mary de Castro late one winter afternoon (more pics) (colour not boosted by nearly as much as you might think in this one).

The large horse chestnut tree in the churchyard.

Blocks of flats along the canal.

Afternoon colours on a day like this are intense. A slight chill in the air adds to the calmness.

There are always plenty of swans about, who assume that if you are holding anything, it's possibly some bread...
This shot manually set to 2 metres focus on the lens, with the camera just held out and pointed in the direction of the swan.

At f/8 you've a lot of depth of field, so even if the distance scale isn't perfect, a rough guess will work.

Lens Corrections

Let's get back to that serious warping of straight lines...
The shot below, of an absolutely superb local hardware store, shows the wavy 'moustache' distortion of lines across the frame.
Fortunately, software exists that, with appropriate profiles' can fix this to some extent. I wish that DxO optics Pro supported this lens with my 1Ds mk3, since its corrections are really good. It doesn't, so what about ACR in Photoshop/Lightroom?
It turns out that you can make profiles to fix distortions (incl. vignetting and chromatic aberration).
It's much easier though, to find an existing profile and modify it - I took one for the Canon 5D Mk2 and edited it to work with my 1Ds3.
Move your mouse over the image below to see the effect.

Note the slight loss of coverage at the edges of the image.
Try this one - under a canal bridge (mouse over it to see).

Note that the corrected version shows how I was not quite square on to the bridge structure (all the outdoor shots are hand held). Some care is needed here, since it's the distorted version of the image that you see in the viewfinder.
Another bridge shot... is this corrected?

Another view of the red girders. Is this corrected?

And finally, a look along the canal. How about this one? Any geometry correction?

Actually, none of the last three had any corrections applied.
The distortions are quite strong, but just not very noticeable in many images, but when they are, it's painfully obvious ;-)
The correction settings file I used with Photoshop CS5 came from Thomas Berndt at www.photo-worX.de
Here it is in the lens correction filter, in Photoshop, at the bottom of the list.

My own modified version is here if anyone wants to experiment [Samsung 14mm Lens correction profile for Canon 1Ds Mk3 - zip file].
There are some versions available for newer versions of Photoshop/Lightroom from Adobe - unfortunately, when I last checked, not for many cameras.

Closer focus

The shot below at f/2.8 was focused on the text by moving backwards/forwards after setting the lens to minimum focus - it's hand held. At f/2.8 the round aperture gives a nice out of focus blur.

The cropped shot below was focused on one of the window highlights on the salad.

The swan is very close - manually set at 1 metre and f/8 - camera held at arms length, no viewfinder used. Did I mention not putting your hand too near hungry swans?

Flare?

In the shot below, I've pushed up the colour, to show the flare, shooting directly into the sun.

Still at f/8, the six blade aperture shows in the lights and the flare from the just out of shot street lamp.

It's worth noting that at 14mm it only takes a slight pointing upwards to get very noticeable convergence of verticals (see my Why I use shift lenses article for more).
One more example with the sun in the corner of the frame shows the effect, but also shows the minimal chromatic aberration you get with this lens.

Detail at 100%.

Vignetting

A quick example from outside my house shows the appreciable difference it makes with this lens in going from f/2.8 to f/8.
Move your mouse over the image to see the difference.

It's reduced further at f/11, but you get significant vignetting with any wide lens.

Detail

The shot below, is at f/8, with the lens set at infinity (actually slightly less on this lens at f/8). You can see a bit of vignetting, but much less noticeable, even compared to f/5.6

The bottom of the frame is obviously not sharply focused. However, at f/8, I can work out the resolution (when focused at infinity) as 14/8mm or 1.75mm - more than enough to show blades of grass. This simple calculation lies at the heart of my preferred choice of infinity focus for most of my landscape work - See my Why I don't bother with hyperfocal distance charts article for more).
The top left hand corner is shown below at 100%
There have been no corrections of any sort applied. Chromatic aberration is virtually absent.

The centre of the frame (100%) shows good detail, on what was a pretty low contrast day.
Do go back to the full frame shot above and note just how small a part of the image this is.

Compared to my much more expensive Canon lens, this is good. Sharpness and contrast is slightly down with the Samyang at f/8, but in the corners there is distinct (albeit minor) chromatic aberration with the EF14 2.8L II.
This isn't going to be a side by side shoot out, but it's worth briefly looking at some other comparisons.

A brief comparison with the EF14 2.8L II

I've set up the camera on a tripod, looking down the street where I live.
I actually took rather a lot of pictures, but reduced size shots on a web page show very little IMHO, and my boredom threshold in such testing is crossed very rapidly ;-)

The two lenses side by side - mouse over the image to see the lens caps.

The Samyang lens is lighter and plastic bodied, whilst the Canon just feels very solid and hefty. With just a quick look into the front of the lenses, there are more reflections visible in the Samyang, but not a lot, and experience with using it doesn't show too much flare or reduced contrast.
Move your mouse over the image of the street below (Canon) too see the Samyang version of the shot.
The most striking difference is the distortion.

Note too the warmer look of the Samyang lens and its slightly wider coverage (both shots processed at the same colour temperture).
Image contrast is better and residual slight barrel correction is very easily corrected with the Canon. At f/2.8 or f/4 the differences are pronounced, but by f/11 it's close (if you're not shooting architecture ;-)

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The Samyang beats the EF14 2.8L II for chromatic aberration at all settings
So, the EF14 is much better in many ways (autofocus for example) - but so I should hope, for £300 vs £1800.
My suspicion is that for many users the Samyang represents exceedingly good value, particularly if auto focus isn't vital and you can perfect your software lens correction workflow.
For use on a crop sensor body, the distortions are even easier to fix, but IMHO, a crop body defeats the whole idea of getting a 14mm lens...

Conclusions

If you're interested in the Samyang 14mm (or whatever it's called where you are) then consider what it is you want to use it for?
For some people the lack of AF and having to manually stop down the lens to the working aperture will be a show stopper.
However I found it perfectly easy to use out and about, by making use of good depth of field at f/8 and an ability to roughly estimate distances.
Exposure was set manually in all the tests here, and relies on the fact that if the light changes enough to need to alter the shutter speed, ISO or aperture, then you should easily be able to notice it. It's hardly difficult with a bit of practice, and you'll develop a much better intuitive feel for scene lighting, which is no bad thing. You could use aperture priority if you must, but I'd suggest taking the time to learn a bit more about light, since as well as manual focus, it isn't going to harm your photography ;-)
Close up working needs care, or a tripod and liveview. In many respects, difficulties I've heard about using a lens like this reflect lack of experience and skills - the cost of digital images (i.e. effectively nothing) mean that if you want to learn the skills, it's easy to just go out and experiment (it's where the majority of my skills come from...)
Getting the best from ultra wide lenses like this takes a lot of thought - expect quite a steep learning curve when it come to appreciating just what works and what doesn't. The strong perspective and emphasis of what's close, need some serious thought about composition, particularly if your previous idea of 'wide' was 24mm ;-)
As an architectural photographer I really noticed the complex geometric distortions of this lens, enough to render it completely useless for some of my work. But hold on, add in software correction, and I could find much more use for it.
There is also the fact that in many photos I took, even though I knew how bad the distortion is, I couldn't see it...
Take a bit of care in reading any reviews with tables of figures about lens performance, such wide lenses are exceedingly difficult to test accurately and repeatably without a very expensive test bench setup.
Taking several shots hand-held, it was easily able to stitch them into much wider views, such as this one of St. Mary de Castro.

Stitched with Autopano Giga, which handles the lens distortions with ease.

Build quality?

Despite being a plastic bodied lens, it's robust and solidly built, but to my mind, could be improved in a few areas. The red dot for aligning the lens with my camera mount is on the metal part of the lens hidden by the lens mount - I could do with it at the side of the lens.
The lens cap fits firmly, and showed no inclination to fall off.
The precise accuracy of the focus scale has been widely questioned - test it for yourself if you get the lens. Of particular importance would be the infinity point (which will also vary with larger changes of temperture)

Other mounts

The range of lens mounts that this lens is available for makes it an interesting option to explore, but as I've said, I feel it needs a bigger sensor to get the real benefit of its 'width'.

Overall

An absolute bargain, if its features (fully manual action, no AF confirm) and foibles (geometrical distortion) are not a problem for you.

Summary

Buying the Samyang/Rokinon/Bower 14mm f/2.8 lens
We make a specific point of not selling hardware, but if you found the review of help please consider buying the lens, or any other items at all, via our links. It won't cost more, but we do get a small contribution towards running the site (thanks!).
In the USA: Amazon | B&H | Adorama
In the UK
Amazon UK (Canon) | Amazon UK (Nikon)
Park Cameras | Calumet |Wex

The Samyang (Rokinon) 14mm f/2.8 ED AS IF UMC Lens provides an extremely wide angle view. With good optical quality, but noticeable geometric distortions that may or may not be an issue.
The fully manual lens is available from numerous suppliers, and in several different mounts
(Nikon, Sony, 4/3, Pentax - Canon EF tested here)

Article first published December 2013 - Comment below, or discuss with Keith on Google+, where there are also many much higher resolution images taken with this lens.
If you're new to the site, it has many hundreds of such reviews and articles - Articles index page.

Sigma 24-105mm f/4.0 DG OS HSM Art Lens Review

Your general purpose lens selection short list just got one lens longer. The Sigma 24-105mm f/4.0 DG OS HSM Art Lens, featuring great image quality, excellent build quality and optical stabilization, definitely deserves your consideration. This lens is a pleasure to use, it looks great and the results from it look even better.
Sigma makes a wide variety of lenses, but until now, has been missing a stabilized full-frame format general purpose lens. As such, the 24-105mm f/4 OS lens addition to the Sigma lineup is completely welcomed by me.

Focal Length
By (my) definition, a general purpose lens is one that will cover a very wide range of typical photography subjects. This lens type is usually the first I tell people to get with their first DSLR camera. The focal length range is the biggest factor in my considering a lens to fall in the general purpose lens category. For those of you using a full frame format sensor, the 24-105mm focal length range found in this lens will supersede my recommended general purpose lens focal length range by a very comfortable margin on the long end.
Here is a pair of examples showing what this focal length range looks like:

 24mm  |  35mm  |  50mm  |  70mm  |  80mm  |  105mm      24mm  |  30mm  |  35mm  |  50mm  |  70mm  |  105mm 

The first set of examples utilizes the printed-on-the-lens focal lengths. For the second set, I attempted to hit 28mm (I estimated slightly long) and omitted 80mm as it is relatively close to 70mm. The 24-70mm focal length range is a very common one. The comparison above shows the specific advantage of having up to 105mm available.

Those on the APS-C (1.6x FOVCF) format will see an angle of view equivalent to 38.4-168mm on a full frame DSLR. This range is somewhat lacking from a wide angle perspective, but it has a very attractive long end. If your focal length needs gravitate toward the long end, a 24-105mm lens will be very useful to you.
As I said, by definition, a general purpose lens has a very wide range of uses. One of my favorite such uses is for landscape photography. It is not hard to compose excellent 24mm landscape images that evoke a sense of presence and at 105mm, it is even easier to find attractive landscape compositions.
People are of course hugely-popular general purpose lens subjects, and with the focal lengths up to 105mm included, this lens allows great perspective for even relatively tightly framed portraits. Zoom out to 24mm and capture the entire group and/or the environment your subject is in.

The number of other uses for this lens is as vast as your imagination. Products, cars/trucks/boats/bikes, art, cities, beaches, mountains, sports (best-suited for outdoor sports), parties, pets and much more.

Aperture
In the Sigma press release, "Amir-Hamzeh added that this new lens combines the largest possible fixed aperture to zoom ratio that will maintain optimal integrity ..." Interesting is that, until now, the Sigma line has been completely void of fixed f/4 max aperture zoom lenses. An f/4 aperture is large, but f/4 allows 1/2 as much light to reach the sensor (one stop darker) as the f/2.8 zooms allow. But you will not find any 24-to-something zoom lenses reaching longer than 70mm with an f/2.8 aperture available.
If shooting action in low light, I'm going to want one of the f/2.8 variants that will allow me to use a shutter speed that is twice as fast. If shooting the night sky, I prefer f/2.8 (the earth is in action). You of course need to decide if f/4 is wide enough for your needs.
I can tell you that the image quality from this lens is indeed, as Amir-Hamzeh mentioned, "optimal" (much more to come). And great is that you have the max f/4 aperture available at all focal lengths. Unlike with a variable max aperture lens (an f/4-5.6 lens for example), you can set a wide open aperture exposure that remains constant over the full zoom range.

Optical Stabilization
Adding greatly to the usefulness of this lens is optical stabilization. OS makes a huge difference when handholding this lens – delivering both sharper results at long shutter speeds and better-framed results thanks to a stabilized viewfinder.
Great for video recording is that this OS implementation is practically silent. Unique to this OS implementation is that it is always active if the camera is powered on and the OS switch is in the on position. And, I can hear the very faint OS noise even when OS is switched off. The difference in long exposure handheld image quality is vastly different with OS switched off, but the OS system is always running. I've never worried about battery drain when using stabilization (including with this lens), but I expect that always-on stabilization uses some additional power.
At 24mm, my keeper rate was nearly 100% with a .3 second exposure which is about 3 stops of assistance for me. About 60% of my images were sharp at .4 seconds, but my keeper rate dropped quickly beyond this exposure time.
At 105mm, I am getting mostly sharp results at 1/10 second with very gradual drop-off until just fewer than 50% of my images were sharp at 1/5 second. The keeper rate then plummets as the exposure duration increases. About 75% of images remain sharp at the approximately 4-stop of assistance 1/6 sec shutter speed.
These tests were done under ideal conditions. If you run up a mountain and take a shot under high wind conditions, I can assure you that your .3 second exposure is not going to be sharp. But IS is still going to be beneficial in this situation. It is just that the benefit is shifted to shorter exposures.
Got ideal conditions? Shoot a lot of frames and you will likely capture a sharp image (you probably only need 1) at even longer exposures than I'm talking about here. Below is a modestly-sharpened example of a .3 second 105mm handheld capture.

 OS On  |  OS Off 

I'm sure you agree that the difference between these two images is quite noticeable. OS works. Unless the subject is moving. A shorter exposure is needed to stop subject motion.

Image Quality
Since a general purpose lens is frequently the most-used lens in the kit, and since the lens can be as important (or more important) than the camera, it makes complete sense to get a high performance general purpose lens model. And in the performance category, image quality is my most-cared-about feature. Great news is that the Sigma 24-105mm f/4.0 DG OS HSM Art Lens performs extremely well and delivers especially-sharp images.
At 24mm through 35mm, this lens is very sharp across the frame with some minor full frame corner improvement showing at f/5.6. Using a 1 or two stop narrower-than-max aperture usually delivers better image sharpness, but the only reason to stop this lens down to f/8 in the 24-35mm range is for increased depth of field or a longer exposure.
Though still quite sharp at 50mm, I can see a very slight decline in wide open aperture image sharpness across the frame. Stopping down to f/5.6 results in a very slightly sharper center of the frame and a more-noticeable improvement in corner sharpness. Sharpness at 70mm is very similar to sharpness at 50mm, though the corners show more clean-up continuing at f/8. Still, image quality in the 50-70mm range is very good – only bested slightly by the 24-35mm results.
At 105mm, wide open aperture image sharpness takes a more noticeable decline. The across-the-frame improvement at f/5.6 is strong with corners showing slight improvement at f/8. While 105mm f/4 is usable, f/5.6 will make you smile big.
Especially affecting 105mm image quality is relatively strong CA (Chromatic Aberration). Most zoom lenses exhibit some CA toward the border of their image circle at both extents of the focal length range and this is the case with the 24-105 OS. The overall amount of CA is not unusual, but CA at 105mm is slightly more noticeable than at 24mm. Here is a 105mm worst-case example:

CA is one of the most easily software-correctable lens flaws.

Using a lens with a full frame image circle on an APS-C body typically results in no noticeable light falloff in the frame corners even at wide open apertures and this is the case with the Sigma 24-105. Full frame camera owners will, as usual, see light falloff when using this lens at wide apertures and still slightly at narrower apertures at the widest angle focal lengths. To be more specific, at f/4, expect about 3 stops of vignetting in the corners at 24mm and about 2.5 stops at 105mm. The least amount of light falloff occurs at approximately 35mm where a just-visible 1 stop of shading is seen in f/4 corners. At f/5.6, corner shading drops under 1 stop over most of the focal length range except at 24mm where 1.2 stops of corner shading remains visible even at f/11.
Two days prior to my initial lens loan return deadline, the skies finally cleared for a flare test. And I'm glad that I got this test opportunity because, while not flare-free, this lens performs very well with a bright light source in the frame. The sun in the case of this test.
You can count on a 24-something mm full frame zoom lens to deliver barrel distortion on the wide end and pincushion distortion at the long end. And the Sigma holds to this tradition. At 24mm, you are going to see moderately-strong barrel distortion – the center of the frame bulges outward. Zooming to a longer focal length changes the distortion to the pincushion type – with the sides pinching in toward the center. At 28mm, distortion is negligible, but moderately strong pincushion sets in soon after.
Following is a comparison of the 24-105's distortion at 24mm vs. 28mm. While distortion will not be recognizable in many scenes, a straight line near the edge of the frame will make it clear. Distortion also makes it difficult to judge when the camera is perfectly level – something I am challenged to do without optical complications.

24mm | 28mm

Zooming out slightly to 28mm makes a large difference in the squareness of the results. Distortion can of course be corrected in software, but this correction is destructive at the pixel level. Again, this lens performs normally in regards to distortion.
The Sigma 24-105 OS has a 9-rounded-blade aperture. As a generalization, the more aperture blades the better, but blade count is not a sole differentiator in defocused image quality. More aperture blades tend to deliver more-rounded out-of-focus highlights with a stopped-down aperture (as do rounded blades). Significant to some will be that a lens' aperture has an odd or even blade count. An even blade count in a significantly stopped-down aperture will turn point light sources into stars with the same number of points as aperture blades. Odd blade count apertures will deliver twice as many star points as blades – 18 points in this case. Here is an example:

I'm not sure if I like odd or even blade counts better, but more important is the quality of the foreground and background blur. And in this regard, the Sigma performs nicely. Specular highlights will show some bright borders and a couple of concentric circles inside, but they are nicely rounded even when stopped down and the circles centers are rather smooth.

Blur 1 | Blur 2 | Blur 3

Below is a crop from near the top left corner of a Canon EOS 5D Mark III-captured image. Those are stars and stars are supposed to be round. Most lenses do not deliver round stars in the frame corners due to coma and the Sigma is again normal in this regard.

Overall, I didn't find any below average Sigma 24-105mm f/4.0 DG OS HSM Art Lens image quality attributes, and the much-better-than-average image sharpness this lens delivers is going to result a lot of these lenses sold.

Focusing
The Sigma 24-105mm f/4.0 DG OS HSM Art Lens features Sigma's HSM (Hypersonic Motor) focusing system. This particular HSM implementation is silent during use. Focus speed is reasonable, but not quite as fast as the Canon equivalent I am used to.
Especially important is that AF accuracy has been good. In one-shot mode, most of my images have been properly focused. I thought that the focus speed would be an issue with action photography, but I was pleased with the good AI Servo AF performance I encountered. This lens does not have the best-available AF system for action sports, but unless the subject was very close, my results were mostly in-focus. Overall AF performance is quite acceptable.
This lens is compatible with the Sigma USB Dock. The USB dock allows the lens' firmware to be updated (bug fixes, compatibility updates, etc.) and allows precise focus calibration.
FTM (Full Time Manual) focusing is enabled and the lens does not change size with focusing (internal focusing). Some subject size change is noticed with focusing.
This lens is not parfocal. If you change the focal length, refocus.
The manual focusing ring is very smooth with an ideal amount of rotational resistance. The 90° rotation is adequate for fine-focusing needs.
The Sigma's 17.7" (450mm) MFD (Minimum Focusing Distance) delivers a 0.22x MM (Maximum Magnification). A .22x MM is a reasonable value and is in line with the other lenses in and near this class.

Model	MFD		MM
Canon EF 24-70mm f/4 L IS USM Lens	7.9"	(200mm)	0.70x
Canon EF 24-105mm f/4 L IS USM Lens	17.7"	(450mm)	0.23x
Sigma 24-105mm f/4.0 DG OS HSM Art Lens	17.7"	(450mm)	0.22x
Nikon 24-120mm f/4G AF-S VR Lens	17.7"	(450mm)	0.24x
Nikon 24-70mm f/2.8G AF-S Lens	14.4"	(366mm)	0.27x
Canon EF 24-70mm f/2.8 L II USM Lens	15.0"	(380mm)	0.21x
Sigma 24-70mm f/2.8 EX DG HSM Lens	15.0"	(380mm)	0.19x
Tamron 24-70mm f/2.8 Di VC USD Lens	15.0"	(380mm)	0.20x

No one is going to confuse 0.22x for a macro lens, but modestly-small subjects are fine targets for this magnification.

Build Quality & Features
I've had all of Sigma's Global Vision lenses available as of this review date and can tell you that I love the look and feel of all of these lenses. From the classy finish to the sharply ribbed rubber rings that feel great to the smooth overall dimensions (except the raised switch area in this lens), these lenses have great design quality.

Select View:  MFD | ∞    MFD | ∞    w/ Hood:  MFD | ∞    MFD | ∞    Rotated        Compare >

Like all of the other lenses in its class, the Sigma 24-105 OS extends when zooming to a longer focal length. The length of this extension is 1.86" (47.3mm). This lens features a double-extending lens barrel, but the extension remains solid even when fully extended. The zoom and focus rings are also solid – and very smooth. The switches are firm and snap into position – you are not likely to inadvertently change one of these.

I know. You saw that the focus ring is behind the zoom ring and thought to yourself, "Oh no, Bryan is going to complain about that." Well, I don't want to let you down. I still like rear-positioned zoom rings and front-positioned focus rings best. The camera and lens balance point is at the rear of this lens. That means that my thumb and first finger goes into the nicely rounded rear section of the lens and from that hold position, I reach forward to the zoom ring. And that means that the focus ring is in the middle, available for accidental rotation.
I want there to be no chance that I unintentionally rotate the focus ring after focus is acquired. However, this position is not such a big deal for this lens. Partly because this focus ring is rather narrow, partly because the zoom ring is raised and partly because the zoom ring rotation resistance is firm.
When I reach forward, the large, raised zoom ring is very easy to find. This ring is solid and very smooth with just the right amount of resistance. No zoom extension lock switch is provided. Though I encountered some reverse gravity zooming when shooting with the lens extended and directed straight upward, I have not encountered any typical gravity zooming. Even when shaking the lens. I can't predict what will happen after a long period of use, but ... the lens is great as-is.
Note that the zoom and focus rings rotate in the Canon direction – opposite that of Nikon lenses.
The same reasons that keep the focus ring out of the way also make it harder to use. Being small with ribbing covering only a portion of it, the focus ring is not nearly as easy to find and use as the zoom ring. Especially in the dark.
The weight of this lens is noticeable, but not excessive. It is a bit heavier than the other f/4 lenses in its class and still slightly heavier than most 24-70 f/2.8 lenses. The size of this lens is very similar to the other lenses included in the chart below.

Model	Weight		Dimensions w/o Hood		Filter	Year
Canon EF 24-70mm f/4 L IS USM Lens	21.2 oz	(600g)	3.3 x 3.7"	(83.4 x 93mm)	77mm	2012
Canon EF 24-105mm f/4 L IS USM Lens	23.7 oz	(670g)	3.3 x 4.2"	(83.5 x 107mm)	77mm	2005
Sigma 24-105mm f/4.0 DG OS HSM Art Lens	31.2 oz	(885g)	3.5 x 4.3"	(88.6 x 109.4mm )	82mm	2013
Nikon 24-120mm f/4G AF-S VR Lens	23.7 oz	(670g)	3.3 x 4.1"	(84 x 103mm)	77mm	2010
Canon EF 24-70mm f/2.8 L II USM Lens	28.4 oz	(805g)	3.5 x 4.4"	(88.5 x 113mm)	82mm	2012
Nikon 24-70mm f/2.8G AF-S Lens	31.8 oz	(900g)	3.3 x 5.2"	(83.82 x 132.08mm)	77mm	2007
Sigma 24-70mm f/2.8 EX DG HSM Lens	27.9 oz	(790g)	3.4 x 3.7"	(86.6 x 94.7mm)	82mm	2011
Tamron 24-70mm f/2.8 Di VC USD Lens	29.1 oz	(825g)	3.5 x 4.3"	(88.2 x 108.5mm)	82mm	2012

For many more comparisons, review the complete Sigma 24-105mm f/4.0 DG OS HSM Art Lens Specifications using the site's Lens Spec tool.

When you pick up this lens, it has enough size and mass to feel substantial, but not that much to become burdensome. I've carried this lens for long periods of time with no remorse for having done so. Especially after seeing my images later.
Note that this is not a weather sealed lens.

As with all Sigma lenses, the lens hood is included in the box. The rear of this modestly-sized hood has a rubberized surface that, in addition to the molded-in ribbing, makes it easy to grasp for bayonet installation/removal. I seldom take this hood off or even reverse it. Having the hood in ready-to-use position makes using the lens faster to use – which is especially helpful for a lens that I leave on the camera ready for quick use around the house or from the bag. My bag for this lens has been the Lowepro Toploader 70 AW. It provides just the right amount of room for a DSLR mounted to the 24-105 OS with the hood in ready-to-use position. I often forgo lens cap use when casing in this way, making the kit even faster to use.

Sigma includes their padded, zippered nylon case with this lens. The case is quite protective and a nice included accessory. A 1.75" (440mm) loop is provided for belt attachment. I personally prefer the Sigma cases to the minimally protective Canon lens pouches.
Sigma recently improved their lens cap design, making them easier to grasp for installation or removal even inside the installed hood.
The 82mm filter threads this lens has received are becoming increasingly common. Take note that using a standard thickness circular polarizer filter will increase light falloff in frame corners. I highly recommend using a slim model such as the B+W XS-Pro.

All Global Vision lenses receive a designation of "A" for Art, "C" for Contemporary or "S" for Sports. As I said before, I love what Sigma is doing with their lenses right now, but I'm not yet a fan of potentially limiting the usefulness of a lens to one of these categories. And, I was not sure why this lens received an "A" (vs. a "C" perhaps?). I posed this question to Sigma with the response being "The 24-105 was placed in the Art line because it fits the criteria best- large fixed aperture designed for a more artistic vision." I'm more of the opinion that an f/4 aperture is medium wide, but it is indeed fixed. The excellent image quality this lens delivers will indeed enhance your artistic expression.
Here is a look at the Sigma 24-105 OS positioned between three other f/4 zoom lenses.

The above lenses are, from left to right, in their fully retracted positions, the following:

Canon EF 24-70mm f/4 L IS USM Lens
Canon EF 24-105mm f/4 L IS USM Lens
Sigma 24-105mm f/4.0 DG OS HSM Art Lens
Nikon 24-120mm f/4G AF-S VR Lens
The same lenses are shown below in their fully extended states with their lens hoods in place.

While all of these lenses (and the f/2.8 zooms as well) can be compared both optically and physically in the site's comparison tools, the Canon 24-105 L IS has been my most-used Canon lens to date. And this is the lens I really wanted to compare to the Sigma.

Compared to the Canon EF 24-105mm f/4 L IS USM Lens
The Sigma enters life with a big challenge on the Canon side of the fence. Canon's focal length range and max aperture equivalent lens has been a long-time favorite that delivers great overall performance at a reasonable price. The bottom line is that I could be very happy with either of these lenses as my sole general purpose zoom and that they are more similar than different.
From a wide open aperture sharpness perspective, the Sigma performs better in the mid and peripheral areas of the image circle at 24mm through 28mm. The two lenses are nearly equivalent from 35mm through 70mm. The Canon is sharper at 105mm. When using a zoom lens, many of us tend to use the two focal length extremes the most. Sigma wins the 24mm contest and Canon wins at 105mm.
But that's at f/4. Stop down to f/5.6, and you'll be hard pressed to find a difference in sharpness between these lenses.
The Sigma has slightly more light falloff at long end but shows less flare. The Canon has less pincushion distortion in the mid focal lengths.
The Sigma focuses more quietly than the Canon, but the Canon focuses a bit faster than the Sigma. The Canon has a larger and better-positioned focus ring with more rotation (122° vs. 90°). The Canon uses smaller filters (77mm vs. 82mm), but the advantage should go to the size that is already in your kit. The Canon lens is lighter and slightly smaller – and is weather sealed.
So, the Sigma presents a strong challenge to this Canon lens. Price is probably going to make the difference to many.
Price
Price, for most of us, matters. And the good news is that the Sigma 24-105mm f/4.0 DG OS HSM Art Lens has a modest price tag – especially for what you get. If you compare this lens' price to the price of the Canon 24-105 L, it looks like a very good deal. If you compare the price of this lens to the frequently-been-available white box version of the 24-105 L (a lens pulled from a camera and lens kit), the Sigma's price is not such an advantage.

The reviewed Sigma 24-105mm f/4.0 DG OS HSM Art Lens was loaned to me by Sigma. I usually buy my review lenses online/retail, but Sigma made a production version of this lens available to me before this lens hit retailer warehouses. So I accepted their offer.
The Sigma 24-105mm f/4.0 DG OS HSM Art Lens is available in Canon (reviewed), Nikon and Sony/Minolta. My standard disclaimer: There are potential issues with third party lenses. Since Sigma reverse engineers (vs. licenses) manufacturer AF algorithms, there is always the possibility that a DSLR body might not support a (likely older) third party lens. Sometimes a lens can be made compatible by the manufacturer, sometimes not. There is also the risk of a problem that results in the lens and body manufacturers directing blame at each other. Compatibility with the Sigma USB Dock is risk reducing as Sigma can release firmware updates for the lens. Sigma USA's 3-year warranty is superior to Canon's standard 1 year warranty (Sigma's international warranty is also 1 year).

I've tested a lot of lenses that made me anxious to move on. This is not one of them. I would have no problem making this my primary general purpose lens.
The Sigma 24-105mm f/4.0 DG OS HSM Art Lens' long focal length range will take a broad swath out of your angle of view needs. This great-looking lens is a joy to use and the results from it are even exciting to view. For a reasonable price, the Sigma 24-105mm f/4.0 DG OS HSM Art Lens reliably delivers the excellent image quality you are looking for from your DSLR camera.

http://www.the-digital-picture.com/Reviews/Sigma-24-105mm-f-4.0-DG-OS-HSM-Art-Lens.aspx