Exposure: ok, computer

Time to get back to technology. In the last two articles we went over the various exposure settings and what they do. This was focused mainly on manual shooting (setting your camera to M and changing all the settings on your own). Sometimes, though, you don't want to have to make all the decisions. Sometimes you just want to concentrate on one setting and let the computer do the rest. So now it's time to talk about the other settings on the dark side of the mode dial and I'll start with my personal favorite: aperture priority.

Aperture priority is disguised as "Av" on Canon SLRs and "A" on Nikon and, as the name implies, gives priority to the aperture setting. All you have to do is set your camera to aperture priority, choose the aperture value you want (f number) and then shoot away. The computer will adjust the shutter speed so that you always get the correct brightness. One common place to use this setting is... well... anywhere! Most lenses have a bit of a sweet spot for sharpness. What I mean by that is for every lens there is an aperture you can set it to that will give you the best picture quality. This value can be different for each lens you put on your SLR so you either need to look it up or play around until you figure out where it is. If you know where that sweet spot is you can set your camera to aperture priority mode, set the aperture to what ever the sweet spot value is (maybe f5.6) and then shoot. You know you will always be shooting at that sweet spot and the computer will figure out the rest of the settings for you. Nice!

Next on the list is shutter speed priority, disguised as "Tv" on canon SLRs and "S" on Nikon. You may have guessed that this one gives priority to the shutter speed and that is exactly what it does. You just pick the shutter speed you want and the camera will adjust the aperture automatically to get the correct brightness. Simple. One place you might use this is at a sports match in the evening when the sun light is starting to disappear. You want your shots to be bright but you know you can't let your shutter speed open for longer than maybe 1/250 (for example). So set your camera to shutter priority, set the shutter speed to 1/250 and shoot away. Once again, the computer figures out the rest of the settings and you know your shots of the running players will not be blurry.

This next one is one that I have not actually used but may come in handy for some people. On Canon SLRs it is labeled "A-DEP". I'm not sure what it is called on other SLRs. This setting gives priority to depth of field. It uses the focus points on the camera to figure out how far away various objects are and then determines an aperture value that will make sure all those objects are in focus. The only decision you get to make in this mode is ISO since the camera needs to adjust both the shutter speed and aperture. Since the camera uses its focus points to determine where everything in the shot is, it is important to make sure that anything important in the shot is covered by a focus point.

Here's a setting that is not on the mode dial but can come in very handy. Auto ISO. This setting is usually found in your ISO selection screen if your camera supports it. When used with aperture priority mode you get a little extra protection from the camera's computer. You set the aperture value you want and then the camera tries to figure out the best ISO and shutter speed for the job. The computer knows what kind of lens you have and knows it should not open the shutter for too long to prevent camera shake. So, if the shutter speed is getting too slow it automatically turns up the ISO. It will only go beyond its shutter speed threshold if it has run out of ISO increases (mine tops out at ISO 3200 by default). I find this really handy when shooting indoors at night when the lighting is dim. I put on my f1.4 lens, turn the mode dial to aperture priority, set the aperture to f1.4, ISO to auto and I can be pretty confident that I won't get any shaky shots. I can just concentrate on focusing and framing and let the computer sort out the fine details, knowing I'll get the shot I want.

Speaking of "the shot I want"... remember how I said a while back that the computer will always give you the correct brightness? This is not always true. There is a lot dependant on your camera's metering system. I would like to talk about built in metering systems in the next article. For now, though, it is good to know that if you are using one of the modes described above and your shot looks too dark or too light (I'll talk about reasons this may happen in the next article) there is usually an exposure compensation setting you can adjust. The compensation setting modifies the computer's brightness decisions for every shot, though, so make sure you remember that you set it. For instance, if you set the compensation to double the brightness of your shots the computer will double what it thinks is the correct brightness for every shot. A very handy feature but if you forget that you changed that setting you may end up wondering why half your shots are over exposed.

So there you have it. A quick overview of some of the more advanced modes on the dial. Make sure you have a good read through your camera's manual, though. Maybe read it twice. It's good to know what the modes do but you'll need to know how to set it all up on your own camera to make use of them. In the next article "Exposure: meter, man." we will talk about how cameras decide what the right brightness is.