Aperture Priority Mode

Aperture Priority Mode gives you the most control over your depth of field

Now that we have the basic concepts concerning exposure, balancing exposure and exposure compensation in our heads, we need to look a little more closely at how these concept translate into the mechanics of taking a picture.  We need to consider the "shooting modes" or "exposure modes" on your camera that you can use get the image you want.  On your digital SLR, you'll notice a dial near the top, and among other things, you'll see a series of letters.  On a Canon, those letters will be

P  Tv  Av  M

On a Nikon or Sony, they will be:

P  S  A  M

These are all shooting or exposure modes that you can use.  In the grand scheme of things, it makes no difference which one you choose.  As long as you get the shutter speed, aperture, and ISO you want, the shooting mode has accomplished its purpose. But some modes are more useful for certain purposes.   It's simply easier to get what you want if you're shooting in the appropriate mode for the situation you're in.  In this post, we'll look at Aperture Priority Mode, labeled either Av or A.  I shoot probably 90% of my photographs in this mode.  It's the mode that matches best the way I think about photography.

Most SLRs either have two dials for exposure or they use the same dial in two two different ways:

1. Two Dials: If your camera has two dials for exposure (highly recommended--it's much easier), one dial (usually by the shutter) is used to dial in the f/stop you want to use.  That puts you in control of the aperture and depth of field of your photograph.  The camera then chooses what it believes the proper shutter speed should be to go along with that f/stop.  The second dial (on Canon dSLRs, this is on the back of the camera), sets your exposure compensation.  If you need to underexpose your image, simply turn the dial until the scale in the viewfinder reads "-1."  
2. One Dial:  If your camera only uses one dial, that dial (usually by the shutter) is used to dial in the f/stop you want to use.  That puts you in control of the aperture and depth of field of your photograph.  The camera then chooses what it believes the proper shutter speed should be to go along with that f/stop.  If you press a button and turn the same dial, that will set your exposure compensation.  If you need to overexpose your image by 1-2/3 stop, simply turn the dial until the scale in the viewfinder until it's on the second tick between +1 and +2.  Consult your camera user manual to figure out exactly how to do this with your camera.

This is my default mode.  When shooting in this mode, you're concerned primarily with the depth of field  you want in your composition.  As you dial in larger f/stops, you'll get more and more depth of field, but you'll also notice that your shutter speed will decrease.  Be watchful of what shutter speed your camera sets for you.  If it becomes too slow, you can compensate for that by raising your ISO or by going back to a lower f/stop.   Also, if you're shooting still subjects, use a tripod.