Some advice on exposure and the 18% gray card

As late as yesterday I was talking about how to get good shots in the snow . The main problem is that the exposure meter is deceived integrated into the machine from the reflectivity of snow and underexpose the picture by about 1 stop.
One of the more practical advice on how to overcome this problem is aa overexposing during shooting, but now you need an explanation more "technical" to introduce the topic of today already dealt with previously , but now expanded.
Exposure meters, whether they are mounted on a compact digital camera is a SLR or a rangefinder camera, work with the principle of reflected light. Are calibrated to measure the reflectivity of medium gray is equal to about 18% of the light that hits it, and from this measurement provide the aperture-coupled time.
Consequently, when you point the meter reflection (integrated in the camera) on the white will be an underexposure because the reflectivity of the white is 36% (twice the medium gray), but when you bet on black or a person very dark on a black background will be overexposed because the reflectivity of the black of course with a lower reflectance value of about 9%.
Hence the need to forcibly overexpose when shooting in the mountains, or in general when you take a very bright light subjects to compensate for the errors. That exposure compensation can be a way to take pictures in difficult situations that send your meter in crisis.
Another way is to use a gray card 18%, purchased for twenty euros provided in the most photo shops. This product, very popular with the first analog photography, has fallen a bit 'into disuse with the advent of digital but personally I find salvation in many situations more complex to manage.
Operation is relatively simple, the card is placed on or near the subject to be recorded and this exposure is calculated using a reading of reflected light. The shot must be totally covered the surface of the card or you have to make a spot metering.
It seems almost superfluous to point out that the light hitting the subject must be the same as hitting the otherwise gray card exposure will be totally distorted. In the event that taken with wide angle lenses are very minor problems. Just for the wider angle of view is more likely to preclude or at least evenly lit scenes feature a wide range of colors.
A very handy trick (read Blog Marco Crupi ) if we did not have the card with us is to frame your subject and zoom all the way to act on the focus ring so strongly blur the subject. The colors will blend into a dough that will simulate the 18% gray and then simply expose and focus to achieve significant results.