How do you get exposure just right? Consider the image to the right. The "correct" exposure is in the middle; there is detail both in the blacks and whites of the puffin's feathers. Images to the right are too bright; images to the left are too dark.
In order to control the brightness of an image, you need to understand how the exposure meter in your camera works. The camera tries to make everything it sees a nice shade of gray. The exact shade of gray is something called 18% reflective gray, and you can buy gray cards that meet this standard. Theoretically, if you expose a picture so that the gray comes out gray, whites will end up white, and blacks will be black.
The problem comes when your camera's meter isn't looking at gray. In the picture above, if you meter off the white feathers, the camera will try to make the white gray. It won't let as much light hit the film, and the overall image will be dark, like the one at the left of the middle row. Now, if your camera metered off the black feathers, the camera would sense a dark image and try to lighten things up by a longer shutter speed, a wider aperture, or both. The image would resemble the one at the right of the middle row. On the other hand, if your camera metered off the gray stone in the background, the grays would come out gray, the whites white, and the blacks black. Your camera is happy in dealing with gray.
Where does this leave you? Well, sometimes you have to outfox your camera. There are a number of ways to do this:
If your camera has a spot meter feature, you can meter from a white area, then from a black area, and average the exposures. Some cameras can store these readings and do the averaging for you.
Bracket tricky exposures by 1/2 stop in either direction. Some cameras have an autobracketing feature to help you do this.
What does it mean to overexpose or underexpose by a stop? A half stop? Lets say you are shooting the puffin above and your meter tells you to take the picture at 1/250 of a second at f/16. To overexpose by one stop you could either double the amount of light reaching the film (go to f/11); or you could double the length of time that light hits the film (go to 1/125 sec). To underexpose by one stop you could either go to f/22 or 1/500 of a second. A half stop overexposure would mean f/13 or 1/180 sec; one half stop underexposure would be f/19 or 1/350 sec. The table below gives some examples:
In practice, this is much easier to do with your camera since all you need to do is remember to click the shutter speed, aperture, or exposure compensation dial a click or two in the appropriate direction.
If you really want to learn about exposure, however, I recommend reading some of the materials nationally renowned bird photographer Art Morris has put together. These can be seen in a booklet, Bird Photography Pure and Simple, or a gorgeous book, The Art of Bird Photography, or on his website (which has information on how to get the books. Art has a very clear way of explaining exposure.
Finally, a note about exposure and Photoshop:
Lets look at those puffin pictures again:
These 3 histograms are from the middle row, the pictures with the "correct" contrast (the middle column has the "correct" brightness). Looking at the histogram, you can see that the middle picture has pixels in all of the 255 brightness levels, while the light and dark images are missing pixels at either end. Although it's hard to tell on these small images, this means that there are no pure whites in the dark image, and no pure blacks in the light image. When you are adjusting an image in Photoshop, be sure to check the histogram when you are done. Sometimes when you are trying to salvage a poorly exposed (or very contrasty) image it will be necessary to leave the whites or blacks clipped like those shown above. Other times, however, you can expand the range of pixels in your image. To do this, use the eyedroppers in the Image:Adjust:Curves section:
Simply click on the white dropper (the one to the right in the circled area) and then click on the whitest part of the image (white arrow). Then click on the black dropper and then on the darkest part of the image (black arrow). This will allow the program to reset all the pixels and spread out their values.
While I really like this puffin picture, it was slightly overexposed. The detail is gone from the whitest area of the feathers, and nothing in Photoshop will bring it back. Moral: Make a good exposure the first time; don't count on saving the exposure on the computer.