I do not usually cover technical issues in photography, but today I am going to. The issue addresses the problem that occurs when the camera sensor gets confused because there is either a lot of white or a lot of black in a scene. I am thinking about this problem because I am seeing a fair number of images with snow in them. This is a textbook scenario for sensor confusion. Let me set up the story.

In 2007, my husband, and more important, the love of my life died. My mother died less than a year later in 2008. Four months after that, at the end of 2008, I sold the Subway restaurant franchises that our family had run for 17 and 1/2 years. To say that I started 2009 unmoored hardly describes the situation. Against all odds, I found a path to a new life when I made the decision to take a basic digital photography class. Truly the photography gods smiled when they sent the (now late) Bahman Farzad to be my teacher.

We used his book, The Confused Photographer’s Guide to Photographic Exposure and the Simplified Zone System. It was perfect for me. After years of not understanding the Exposure Triangle, I finally got it. I also came to understand the old school Zone System and its particular application to a problem that digital camera sensors have.

Mr. Farzad was given to grand teaching gestures. I like it when teachers are not subtle. One of our in-class assignments was to take a picture of a large white card and then a large black card. Nothing else could be in the picture. Mind you, I was a beginner, so I was shooting on Automatic. The main challenge was getting the camera to take the picture. Of course, the automatic focus could not focus on a solid color. I did not see that coming. I do not remember how I solved the problem, but eventually I did. The resulting two images were shocking. The picture of the white card was gray. So was the picture of the black card.

Camera sensors have to have some rule about how to determine the proper exposure. Here it is, albeit, overly simplified: Look at the scene, measure the blacks and whites and grays, average them out and then choose an exposure to set the average to middle gray. This produces a very nice exposure almost all of the time. However, it does not work well when the scene is predominantly white or black. There is total failure when the scene is all white or black.

Let’s look at the white card like the camera sensor did. There were no grays or blacks in the frame, only white. So the sensor did what sensors do, it averaged the white to white and then underexposed it so that the resulting image came out middle gray. It then looked at the black card, averaged the black and dutifully followed the rule. It overexposed the image so it came out, yes indeed, middle gray.

Photo by Kate Johnson Wehr. Used with permission. This is a good example of a properly exposed color snow scene.

How does that apply to taking pictures of snowy scenes? If there is truly a lot of white in the scene, you may have to intervene to get the proper exposure. If left to its own devices, the camera sensor will average the scene to very light and expose, actually underexpose, to get the whole scene to a nice middle gray. What you have then is gray snow. The solution is to override the camera sensor by overexposing the photo by either one or two f-stops. Simple right? If you have an unusually large amount of white in a scene, you may have to overexpose. If you have an unusually large amount of black in scene you may have to underexpose. Photography is so counterintuitive.

Photo by Denis Buchel. Used with permission. You can see Denis’ Facebook Gallery at Denis Buchel, his Instagram Gallery at @denisbuchel and his website at Denis Buchel. This is an example of a properly exposed b&w snow scene.

When taking street photography, I typically focus and spot meter on a face. Most of the time it is a Caucasian face, which, as luck would have it, is typically middle gray. I tend to have a consistently good exposure doing that. When I do not have a good exposure, I can always change the exposure using a dial on my camera. In the case below, I spot metered on a face, but not on a human face. The meter sensored primarily black, so it overexposed to move the image toward middle gray. I had to apply exposure compensation.

Dedicated to Street

Getting the Right Exposure in a Snowy Scene