Reflected vs Incident Light

January 27, 2005

I will assume that the majority of you reading this are not trained photographers. This is not a judgement. I state it only so the next few minutes that I spend typing won't seem pointless.

Cameras are equipped with light meters so that consumers don't end up with horribly over, or under, exposed images. They are very convenient, and if the camera batteries are checked with some regularity, will deliver fairly reliable results so long as you are initiated into the mystery cult of reflected light.

That's right, there's something that your in-camera light meter isn't telling you. A dark secret, if you will, that will sabotage all your brilliant photographs of snow, white ceilings, and polar bears under bridal veils in blizzards. It is the secret of 18% gray.

You see, the light meter on your camera doesn't measure the actual amount of light falling on a subject, it measures the amount of light reflected off that subject. This reflected light enters your camera's light sensor(s) and based upon how much reflected light is registered the meter makes a guess about how much actual (or incident) light is hitting the subject. That's right, it guesses. 

Now some people might find that endearing. It's a nice bit of personification when you think about it— complex sensor(s) and processors do their best to make an educated guess on behalf of the human. . . But it is absolutely maddening if you want a photograph of a white subject to demonstrate a broad tonal range but still read as a white subject.* I know, because it would be some time before anyone helped me understand why all of my pictures of white subjects looked like I'd put a 20% gray filter between the lens and the subject.**

This issue of reflected light and the camera's best guess works great for many subjects because most images contain a fairly broad range of items that are all reflecting light to various degrees. When you average out all these different quantities of reflection you get an overall cast of 18% gray in the image. By assuming that, the camera, more often than not, takes a decently exposed image. However, if you are shooting anything that is predominantly dark (low amount of reflected light) or exceedingly bright (high amount of reflected light) the camera won't compensate and will continue to read as if the image had a balance of tones that averaged out to about 18% gray. Then it becomes the human's problem to compensate for the discrepancy by either allowing more light in to the lens than the meter says to, or allowing less light in (also against the meter's suggestion). In other words, the user has to make a judgement that contradicts what the technology states to do. 

While that seems easy, don't underestimate the power our products have over us now. Our modern culture has embraced the idea of the product being right for decades and I'd say that, generally, we are a long way from the days of inquisitive tinkering and experimentation.*** Even I tend to defer to the light meter when I know it's wrong. I'll take the image at the exposure it wants as insurance against my own arrogance in assuming it to be incorrect.

That's why I love the Photo Phazer. It has no controls of any kind other than ON, OFF, and SHOOT. Like most other cameras it has a set of sensors that find the "perfect" exposure. And like most other cameras, when left to its own devices, it turns the line between a white wall and the white ceiling into a brooding abstraction in neutral gray.

*Excessively dark, or predominantly black, subjects are equally difficult to capture. A good rule of thumb is to not try and photograph your black labrador standing against a black car at night.

**In all these examples I'm assuming that I'm photographing with black and white film, not color. Hence, when I refer to 20% gray it is because I think of black and white film as being a single spectrum, with white being at 0%, black being at 100%, and there being 99 steps of gray between the two. Nevertheless, all of the same issues regarding light meters apply to color film as well, but there are many more spectrums to consider; red, blue, etc.

***Even though it was just those tendencies that led to such technical marvels in the first place. I suppose that nowadays the cost of the equipment is the most prohibitive barrier to amateur inquiry.