Metering in the snow

Today was one of those rare days in southern England when it snowed!  We can wait years, but so far its happened twice this winter!  Go out and you will likely meet a number of keen photographers at well-known local spots.

For those experienced in using a camera in all sorts of conditions the fact that metering will be somewhat confused by snow is no surprise, but it may well catch out those newer to the hobby.

Its a good idea to have some idea what your particular camera meter will behave like when shooting snowy landscapes. While exposure errors can be fixed relatively easily in RAW files, its always more satisfying to get it right in-camera and pushing a file too far can increase noise textures and damage image quality.

Camera metering systems are designed to give a 0EV (exposure value) for a typical image that contains a variety and balance of tones between white and black.  This is what we would consider to be a normal exposure, not too dark or too light.  It is commonly referred to as 18% grey.

However, if a scene contains considerably more dark or light tones than "normal" the metering will try to compensate to achieve a 0EV exposure.  If the tones are dark the metering will tend to overexpose the image to bring the tonal values up and if the tones are light it will tend to underexpose the image to bring the tones down.  In snow we have a similar issue to the bride's white dress...there is an imbalance in the tones owing to the large amount of white, which causes the metering to underexpose the image.

The exact amount of underexposure in snowy conditions will depend on the quality of light and how much of the composition contains the snow or other tones, like sky, buildings or people.

Today I found that the quality of the light had a huge effect on metered exposure.

Images taken in shade required 1.3 stops of positive exposure compensation to get a histogram pushing nicely up to the right and a correct exposure.  This was one of those.

Images taken in direct sunlight only required about 0.3 stops of compensation, like the following one.  I am imagining that much of the difference is because when there is direct sun, the blue sky is a more neutral tonal value than the white or light grey cloud.

In most exposure modes, such as shutter or aperture priority, the photographer would dial in the appropriate amount of exposure compensation using the dedicated button and command dial or the camera menu.

Even if the photographer shoots in manual exposure mode the camera is still using the same metering algorithms as in any other mode.  It will therefore continue to show a neutral 0EV exposure on the viewfinder meter when the final image will actually be underexposed just as much as in any other exposure mode.  Rather than using exposure compensation (usually unavailable in manual mode) the photographer should dial in the required amount of "overexposure" above the 0EV value....in the case of the above examples, the same 1.3 or 0.3 stops.

With a basic understanding of what the meter is likely to do we can set the camera up as soon as we start shooting and make any minor adjustments much faster.  The advantage of digital is that we can take a test exposure, check the histogram and by the second shot we should have nailed correct exposure.