Jan 13, 2009

Exposure Basics - Just a drop in the bucket

Ever wonder why the subjects of your photos are too dark or too light? It all comes down to exposure. There are entire books dedicated to getting proper exposure in your photos... I will try to keep this short, but it really is a complex subject.

To sum it up quickly, your camera cannot take in and process nearly as much range of brightness to darkness as your eyeball can. If your picture has both really bright and really dark areas, one part or the other will be lost. Until camera technology gets better, you have to work within the limits of the camera to get the subject properly exposed.

Proper exposure is getting the camera to allow in just enough light to see the bright and dark areas correctly. Too much light, and you lose detail in the white or bright areas. Not enough light and you lose detail in the dark or shadow areas. When it is right, your picture will look like the scene you remember.

The exposure controls....

Aperture - How large of an opening in the lens for light to go through. If filling a bucket of water represented getting to a proper exposure, aperture would be the diameter of the water hose. Inside the lens is a mechanism to let in more or less light, sort of like a door. The various opening sizes are measured in "F-Stops" i.e. 2.8, 4, 8, 11, 16... with smaller numbers being a larger opening. (Smaller Number = Brighter)

Shutter Speed - Is how long the shutter stays open. For the bucket of water it would be how long you let the water flow. Or, if a door, it would be how long the door is open. Shutter speed is measured in fractions of a second, 1/60, 1/100, etc. (Smaller Fraction = Darker)

ISO - This is the sensitivity setting of the camera. Lower ISO numbers mean less sensitivity. ISO also affects image quality, but we will get into that another time. In the water bucket example, changing the ISO essentially changes the size of the bucket. With a higher ISO (more sensitive) you would have a smaller bucket, because less is needed to get to a proper exposure.

It takes a bit to wrap your head around those three things, and how changing any of them also affects other aspects of your image, like stopping action (shutter speed), how much of the photo is in focus (Aperture), and overall image quality (noise). For now though, just know that they all work together, and changing one can be offset by changing another. For our bucket of water... you would get the same "exposure" with either: A. Large hole, short pour time or B. Small hole, long pour time.
What Your Camera Thinks...
The camera's auto exposure modes are programmed to find an exposure that averages the light and dark areas of the photo find a happy medium. Essentially, it wants the exposure to be this.

This is great most of the time, because in most pictures, there is a nice variety of light and dark areas.

Unfortunately, the camera does not know if a scene is SUPPOSED to have more lights or darks in it, and it can overcompensate. This is why shooting into a bright sky results in silhouetted subjects, or taking a picture of the moon results in a simple white dot with no detail. If you were shooting a white wall, it would try to cut the light down (faster shutter, higher f-stop) to make it gray. A black wall? It would adjust to let enough light in (slower shutter, lower f-stop) to make it gray.

The camera doesn't care what the subject should look like. Instead, it wants a nice average. In this case, it goes with the sky, which looks nice... if I wanted a silhouette. But I don't.

Now.... what does this really do for you?

As you consider the whole picture, think about the relative brightness of the subject and everything else. If there is a good mix, then the camera should figure it out pretty well. If not, it is tell the camera who is boss.

All SLRs and many point-and-shoots have some form of exposure correction. You may have to dig up your manual to find it, but it is worth it! To make your subject look correct in the photo, you have to force the camera to let in more or less light. There is usually a scale of -3_-2_-1_0_+1_+2_+3 with negatives being less light, positive numbers, more light.

Here is a P&S camera showing +2 on the exposure compensation circled in red.

Remember, identify the subject and then you can adjust the exposure for that.... everything else is less important.
Fill Flash...
If your subject is coming up too dark because of a really bright background, simply adding exposure compensation will help, but it will also over-expose the background. (remember, the camera cannot fill even close to the same range as your eyes).
To compensate, sometimes you can add light to the subject by forcing the flash to fire. This is usually the lightening bolt button on P&S cameras. Sometimes you get too much light, so it is a bit of trial and error, but you can adjust brightness by moving closer or further from the subject. On DSLRs you can adjust how much light you get from the flash manually (called flash exposure compensation, check the manual).
This one turned out about right. (with a P&S)

On-Camera flash never looks as good as nice natural light, but it can certainly help you even out the scene to something that the camera can handle.

The reverse is also true... the camera may try to use the flash because a lot of the scene is dark, killing the subject with glaring bright light.
Turning off the flash (same lightning button on P&S cameras) and using some positive exposure compensation will help correct this. Of course, you also lose some detail in the bright areas, but that is the tradeoff you make to prioritize the subject.
OK, that is a lot of technical mumbo-jumbo, and we did not even start playing with ISO.
The good news? Knowing what the camera is trying to do will allow you to take control of the process. Now go get the camera and play with exposure compensation! This is the beginning of making the pictures that are better than the "average" that your camera wants to give you.
If you would like this broken down further, just let me know. I will be addressing the other effects of shutter speed, aperture and ISO in future posts as well. This post has gone on too long already!
don j.

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