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In the photography forum [link] I've noticed several posts about exposure lately by people who apparently don't understand how the meters in their cameras work. Someone suggested that I write a tutorial and so here it is.

Most cameras have SOME kind of meter these days. These can be averaging meters, center-weighted meters, spot meters, or any of several other kinds. Most cameras have averaging meters. One thing they all have in common is that they will all give you an exposure recommendation based on a shade of gray called "18% gray." That is to say that they assume that anything you point the camera at is supposed to be this shade of gray. If it isn't, they will recommend exposures that will MAKE IT so. Actually, it is a LITTLE more complicated than this, since some manufacturers (like Minolta) set their meters for 13% gray (the approximate value of Caucasian human skin). This is close enough though. It will be within your sensor’s or film’s exposure latitude, so you will get a usable exposure, and I will be calling it 18% throughout this tutorial for the sake of simplicity.

For example, and again this is just for the sake of simplicity, let's take a hypothetical situation in which you are shooting two different nudes, identical blonde twins (pretty much the same color value all over), each of which has a different colored backdrop behind her. One nude is standing in front of a white backdrop. The other is standing in front of a black backdrop.

So let's start with the nude with the white backdrop. You take the photo, develop the film and make prints. You will find that you have a photo with a gray background and the model is far too dark. Your film has been underexposed and you can't figure out why. You did what the camera said to do and it still came out too dark. What happened?

Well, when you turned on your camera's light meter what the meter saw was the whole composition. It's just a machine that measures reflected light. It can't recognize objects or what it is looking at. It didn't differentiate between the model and the background, but what it did do -- immediately -- was decide that the composition as a whole did NOT average out to 18% gray. Even though Caucasian human skin is roughly the right value, it also took the background into account. This made the AVERAGE value of the whole composition much lighter. It recommended an exposure setting that would make the background and subject darker, in order that the composition as a whole would average out to that 18% gray it wants. This results in underexposure.

Now let's photograph the model with the black background. Again, you aim the camera, adjust the exposure as the meter tells you to do, and take the photo. You develop the film and make prints. You are perplexed when you find that the background is still gray, but this time the model looks snow-white. Your film was badly overexposed. Again you are left scratching your head, and now you're wondering if the camera is broken.

The camera and the meter are both working fine. What happened is that the meter looked at your composition and found that, once again, it didn't average out to 18% gray. This time it was too dark (because of that black background), and so it recommended an exposure that would make it average out to the "right" shade of gray. It overexposed your film, to make the composition lighter.

As you have probably figured out by now, the camera will only give you an exposure recommendation that is absolutely correct when the composition really is 18% gray. Unfortunately, in real life, this doesn't happen very often. What you have to do is recognize that this is happening and take it into account. You have to make adjustments for it. As you gain experience you will find that you can do this by eye, more or less. Fortunately for the beginners, there are a few relatively easy ways to get around this without relying on their nonexistent experience: spot metering, incident metering, and gray cards.

In spot metering the meter only looks at one small area of the composition. All you have to do is find an area in the composition that is close to 18% gray and meter the light on that. In our hypothetical nude photography scenario, the paler areas of the model's skin are pretty close to 18% gray, so this won't be a problem. You’d just pick an area in medium shadow and meter on that. If you don’t have a spot meter, you can simply walk up to your model, until the area you want to meter fills the viewfinder, set your exposure, and back off. What do you do if there is nothing in the composition that is the right shade of gray though? What if your model is an albino? What if she's black?

Well, there are two more ways of getting the right exposure. One is to use a handheld incident meter. Instead of measuring the light reflected from the model toward the camera, these measure the light that is falling on the model directly. Instead of standing by the camera and metering the model, you stand by the model and point the meter at the light. This way there is no possibility of the meter getting confused by different colored backgrounds.

By far the most common solution though, is to use a gray card. A gray card is simply a piece of pasteboard that is colored 18% gray. They're one of the few things in photography that are cheap. A gray card gives you something that you can hold in front of the camera that actually is the right shade of gray. You set your camera on manual, hold up the gray card (in the light, at the same distance from the light as the model), set your exposure, then set the card aside, focus, and shoot, confident that you will get a good exposure.

Description
In the photography forum [link] I've noticed several posts about exposure lately by people who apparently don't understand how the meters in their cameras work. Someone suggested that I write a tutorial and so here it is.

Most cameras have SOME kind of meter these days. These can be averaging meters, center-weighted meters, spot meters, or any of several other kinds. Most cameras have averaging meters. One thing they all have in common is that they will all give you an exposure recommendation based on a shade of gray called "18% gray." That is to say that they assume that anything you point the camera at is supposed to be this shade of gray. If it isn't, they will recommend exposures that will MAKE IT so. Actually, it is a LITTLE more complicated than this, since some manufacturers (like Minolta) set their meters for 13% gray (the approximate value of Caucasian human skin). This is close enough though. It will be within your sensor’s or film’s exposure latitude, so you will get a usable exposure, and I will be calling it 18% throughout this tutorial for the sake of simplicity.

For example, and again this is just for the sake of simplicity, let's take a hypothetical situation in which you are shooting two different nudes, identical blonde twins (pretty much the same color value all over), each of which has a different colored backdrop behind her. One nude is standing in front of a white backdrop. The other is standing in front of a black backdrop.

So let's start with the nude with the white backdrop. You take the photo, develop the film and make prints. You will find that you have a photo with a gray background and the model is far too dark. Your film has been underexposed and you can't figure out why. You did what the camera said to do and it still came out too dark. What happened?

Well, when you turned on your camera's light meter what the meter saw was the whole composition. It's just a machine that measures reflected light. It can't recognize objects or what it is looking at. It didn't differentiate between the model and the background, but what it did do -- immediately -- was decide that the composition as a whole did NOT average out to 18% gray. Even though Caucasian human skin is roughly the right value, it also took the background into account. This made the AVERAGE value of the whole composition much lighter. It recommended an exposure setting that would make the background and subject darker, in order that the composition as a whole would average out to that 18% gray it wants. This results in underexposure.

Now let's photograph the model with the black background. Again, you aim the camera, adjust the exposure as the meter tells you to do, and take the photo. You develop the film and make prints. You are perplexed when you find that the background is still gray, but this time the model looks snow-white. Your film was badly overexposed. Again you are left scratching your head, and now you're wondering if the camera is broken.

The camera and the meter are both working fine. What happened is that the meter looked at your composition and found that, once again, it didn't average out to 18% gray. This time it was too dark (because of that black background), and so it recommended an exposure that would make it average out to the "right" shade of gray. It overexposed your film, to make the composition lighter.

As you have probably figured out by now, the camera will only give you an exposure recommendation that is absolutely correct when the composition really is 18% gray. Unfortunately, in real life, this doesn't happen very often. What you have to do is recognize that this is happening and take it into account. You have to make adjustments for it. As you gain experience you will find that you can do this by eye, more or less. Fortunately for the beginners, there are a few relatively easy ways to get around this without relying on their nonexistent experience: spot metering, incident metering, and gray cards.

In spot metering the meter only looks at one small area of the composition. All you have to do is find an area in the composition that is close to 18% gray and meter the light on that. In our hypothetical nude photography scenario, the paler areas of the model's skin are pretty close to 18% gray, so this won't be a problem. You’d just pick an area in medium shadow and meter on that. If you don’t have a spot meter, you can simply walk up to your model, until the area you want to meter fills the viewfinder, set your exposure, and back off. What do you do if there is nothing in the composition that is the right shade of gray though? What if your model is an albino? What if she's black?

Well, there are two more ways of getting the right exposure. One is to use a handheld incident meter. Instead of measuring the light reflected from the model toward the camera, these measure the light that is falling on the model directly. Instead of standing by the camera and metering the model, you stand by the model and point the meter at the light. This way there is no possibility of the meter getting confused by different colored backgrounds.

By far the most common solution though, is to use a gray card. A gray card is simply a piece of pasteboard that is colored 18% gray. They're one of the few things in photography that are cheap. A gray card gives you something that you can hold in front of the camera that actually is the right shade of gray. You set your camera on manual, hold up the gray card (in the light, at the same distance from the light as the model), set your exposure, then set the card aside, focus, and shoot, confident that you will get a good exposure.