Exposing For Snow

Baseball in Snow, March 2008
1/320sec @f/8, ISO200 | 24-70mm f/2.8L on Canon 30D

I was going to hold off a bit on the "snow exposure" commentary since things in Cincinnati had warmed up into the 60's within about a week of the 2008 Winter Storm. However, since Chicago got slammed with snow this week and because the winter temperatures have returned to the 'nati I may as well talk snow!

Bump Exposure Compensation Up
The typical advice when it comes to shooting snowscapes is to operate your camera as usual, but to set the Exposure Compensation to a value of +1EV or so. This works and you'll get nice white snow, but why is it necessary?

Middle Gray Metering
The meter inside your camera is a reflective meter, meaning that it is reading the light bouncing off of the surface you are focused on and determines how to expose the scene. The camera is setup to give an exposure which will render the metered subject as middle gray (18% gray).

Of course, there are a lot of complex algorithms involved but fundamentally this is what happens.

So, if your scene is primarily made up of bright white snow, the camera's meter will attempt to expose all of that nice snow as a dingy middle gray - hence an underexposure of the scene. By setting the Exposure Compensation to +1EV, you are telling the camera to "Expose the scene based on the meter reading plus one full stop".

By making the gray snow twice as bright (i.e., plus one stop) you now have white snow & a properly exposed image!

Especially Important for Digital Photography
While getting a proper exposure in-camera is always important, it is especially so with digital cameras. Because of the way a digital sensor works, the most information is stored in the brightest two stops of the exposure. More info is in my digital basics talk and on The Luminous Landscape, but the key is to understand that a digital sensor counts the number of photons that hit its surface.

Because it takes twice as many photons to double the luminance and because those photons are counted one by one, the digital sensor uses twice as many bits to describe the levels in the brightest stop as it does the next-brightest stop and so on. It is a linear system.

The reason this matters is because while you can lighten an underexposed scene in post processing you'll soon reach a point where the computer is trying to reveal detail where there is little information available. This results in posterization (blotchy) artifacts in the shadows that can ruin an otherwise great photo.

The key is to expose the scene properly so that you're capturing the detail without blowing out the highlights - often easier said than done, but that's why we use meters, histograms, technique, and practice to help us out!

Incident Metering as Another Option
When I was out shooting the 2008 storm I didn't employ the +1EV technique discussed above. Because the sky was heavily overcast and the light was very consistent I pulled out my light meter and took an incident reading. The difference between incident and reflective readings is that while the reflective system must assume that the light is bouncing off of a surface of a given brightness (well, reflectivity), the incident meter is evaluating the amount of light emitted from the light source.

This allows you to obtain a correct exposure for any subject illuminated by that light source. Since I wasn't concerned that the light would change (the clouds were thick and even), this was my best option. The meter read 1/400sec @ f/8.0 (ISO 200) so by locking that in I was able to photograph any scene lit by that light confident that the exposure would be correct.

The key benefit to this approach vs. the in-camera reflective meter is that the reflective meter is influenced by the contents of the scene. A scene with a lot of snow would register differently than one that has a little snow and many darker buildings - combine the latter scene with a +1EV and you can end up overexposing the snow!

When I encountered a situation where the light changed a bit, such as under the bridges or when the snow slowed, I would adjust the exposure time slightly in order to compensate. This technique certainly requires a bit more attention, but it delivers highly consistent results.

If you don't have access to an incident meter you could achieve similar results by using your camera's meter to derive the correct exposure (use your histogram) and then lock those settings in via manual mode.

Post Processing Snow Images
Although most all of the images had the proper exposure there was still some improvements to be had through the magic of post processing. Working in Lightroom, I typically had to increase the Blacks slider several notches in order to achieve a true black point in the photographs.

Why? Well, in these images there was a whole lot of blowing snow which meant that there were more gray tones than true shadows. As the snow blows across the shadow areas it is recorded on the sensor and shows up gray.

While this may be what was actually recorded, it looks very flat in print and doesn't resemble what the scene looked like in real life. We fix it with the Blacks slider (or curves) and pull back the shadow areas so that we get close to a true black point. The images now have a nice contrasty pop & will look great on print.

Remember: By properly exposing for the snow we've captured the scene with the most possible information - we now have a lot of headroom to pull back shadows & tweak highlights without worrying about artifacts.

Works for (gasp!) Film Too...
I was with my buddy Andy last night watching the XU Musketeers beat WVU and he brought along his prints from the storm shoot. Andy shoots 35mm film but other than that there was no difference in our approach to exposure - I took the reading and helped him to dial in the settings on his Pentax body (after compensating for his 400ASA film; meaning that his exposure was 1/800sec @ f/8).

The prints looked great - though there was "no" post-processing, in reality the lab did the same things I did with the black point before printing. ...Just because you're not changing things in Photoshop doesn't mean that this isn't occurring on your behalf at the lab! Hence the term "digital darkroom" being used to describe the post process of the digital workflow.

Andy commented on how clear & crisp everything looked and - referring to other dark, gray snow scenes that he's seen before - how much of an improvement proper exposure makes. He certainly had some nice shots in there - too bad I can't share them with you since they're currently stuck in that little print envelope! :)

Learning More about Proper Exposure
One of the best books I've read about photography is Understanding Exposure by Bryan Peterson. Whether you're shooting film or digital, if you're interested in getting serious about your photography and taking it to the next level then this is the first book you should read. A little knowledge in the fundamentals goes a long way.