What is exposure anyway? A colleague of mine says it is the amount of light required to record an image. She’s absolutely right of course. Her answer made me thing about the quality of my question. I believe that the question to ask is “What is a correct exposure?” That answer is much more useful. A correct exposure reveals the true tone of the subject. Makes sense. Getting there is a different story. How did I find the exposure for the photo above? Read on…
How much light is correct?
There can be only one exposure that reveals the true tone of the subject. Finding that exposure is tricky until you understand how the light meter in your camera works. That light meter measures the light bouncing off the subject. This meter is called a reflected light meter because it measures the light that has been reflected into it by the subject. Consider this lighting diagram.
Now that we understand how our camera’s meter sees light, we have to also know what it does with it. The meter gives the camera settings including ISO, shutter speed and aperture that render an image of that area that is about 12.5% reflectance. That’s a fairly dark gray. Our mind tells us that taking the reading of a white mouse on a white background will give an exposure that results in a white mouse on a white background. If only. Here’s what actually happens.
Getting a correct exposure
12.5% gray is about visual middle gray, or halfway between black and white. If the subject is white the meter in the camera delivers an exposure that will make it that middle gray. Pure white is actually three stops brighter than middle gray. We can see this with a little arithmetic. Add one stop of light to the exposure the meter provided and the reflectance goes from 12.5% to 25%. A one stop increase doubles the amount of light. 25% is twice as bright as 12.5%. Add a second stop and the reflectance doubles again from 25% to 50%. Do this a third time. The light doubles again and 50% moves up to 100% or pure white. White is 8 times brighter than middle gray. White with detail, like the shingles on the white washed buildings in the first photo of this post, is 2 and 1/3 to 2 and 2/3 brighter than the meter reading.
Any automatic exposure setting on a camera will always turn a white subject into a 12.5% gray one. The key to getting to a correct exposure is to use the reflectance chart above to set the Exposure Compensation on the camera to add 2 and 2/3 stops of light for every photograph. Once the EC is set, make the photograph. The white mouse is finally white and it has detail!
The dark side…
Black subjects are treated the same way by the camera’s reflected light meter. The black subject will be 12.5% middle gray too. Here’s the arithmetic for them… The reading results in 12.5% gray. Reduce the light by half (one stop.) 12.5% gets darker to 6%. Rounding is OK. Reduce the light by another stop. 6% becomes 3% reflectance. Reduce by another stop to go from 3% to 1.5%. Black with detail is two stops darker than the exposure. Almost true black happens at four stops darker. Below is the whole reflectance chart.
Exposure Compensation redux
A black subject requires less light than the camera’s meter reads. Black with detail is usually two stops darker than the reading or 3% reflectance. This is a quarter of the light the meter say it is. Set the EC at minus two stops.
Do try this at home
Get a white towel and an egg. Get a black towel and the eight ball from a pool table. Take it all outside. Lay the towels out side by side. Put the egg on the white towel. Put the eight ball on the black towel. First, set your camera on P for program. Now, fill the frame with only the white towel and egg. Take the photo. Do your best to not look at the camera’s monitor. Repeat for the black towel and the eight ball. Next put the EC on plus 2 and 2/3 stops, photograph the white set. Now set the EC on minus 2 stops and shoot the black set. Step inside. The first two photographs are middle gray even though the first is white and the second is black. The next two photos will be white and black respectively. Magic? No. It’s simply understanding how the camera meter sees and compensating for it.
There is just one more thing…
The bride is wearing her white gown. The groom in a stunning black tuxedo stands next to her. White and black are in the same shot. What’s a photographer to do? Think about it for a moment. If the exposure is set for the white of the wedding dress, how will the black tuxedo look? It’s going to look black like it is supposed to look. Why? The correct exposure for the dress is two and two thirds stops brighter than the reading of only the dress. A correct exposure reveals the subjects true tone. The trick is to lock the exposure down by setting the camera on Manual so the settings don’t change with the composition. When the bride and groom stand in the same light, exposure for the dress will reveal its true tone and the tux’s too. Go back to the two towels. Set the camera on manual. Set the Exposure Compensation for plus two and two thirds stops then read the white towel. Take the photo. Don’t change a thing. Shoot the black set. Sure enough the black towel and eight ball are black. The key here is keeping the white settings from changing by having the camera in Manual. In Program, the exposure would change to the wrong one as soon as the meter sees the black towel.