Okey dokey.
So you should've taken a few pics in aperture priority. And hopefully - because you're an observant visual person - you should've noticed a few things. The first one is slightly harder to spot than the second but I'm going to start there anyway.
A large aperture (low number) throws the background and some of the foreground out of focus.A small aperture (high number) keeps the the foreground, middle ground and background sharp.Why is this important? Because of the artistic control it allows you. (Images to follow during the week!)
The second thing should be fairly obvious:
there's a relationship between the aperture setting and your shutter speed. (
Shutter speed is how fast or how slow the shutter in your camera opens to let in light. If it opens slowly you let in more light, if it opens quickly you let in less light. Shutter speed is not assbackward like your apperture settings so a big number = a fast speed and a small number = a slow speed.)
If you've been working
in aperture priority then you should've noticed inside your camera that
each time you change your aperture there's an opposing change in your shutter speed. The relationship is a very simple one. As you let in more light through your aperture so your shutter speed will slow down to let in less light.As you let in less light through your aperture so your shutter speed will increase to let in more light.Why does it do this? Because your camera is determining the ideal exposure for you. It assumes that you'd want everything at about 17% grey. Don't ask me who decided it should do this, but I suppose they had to have some form of standard and 17% grey is what they went for.
So here's your camera with it's built in predisposition towards a light grey. When you have your camera in aperture priority it assumes that you want that photograph to be it's beloved light grey. So every time you change your aperture to let in more light, the camera will change your shutter speed to counteract the extra light you've just let in and it will increase the shutter speed effectively cutting off that extra bit of light. If you change your aperture to let in less light your camera counteracts the impending gloom by lowering the shutter speed to let in more light.
When you're in AV mode and you look at your exposure reading in your camera you'll see that it always stays right in the middle, 'perfectly' exposing your images to 17% grey no matter what changes you make.
So what's the problem.
Essentially that you're not in control of the decision making process. And that, of course, not everything in this world is 17% grey. Photograph black on black or white on white in aperture priority and you'll have an exposure that is wrong. In the case of black it will be too light; in the case of white it will be too dark.
The other problem with letting your camera determine the shutter speed is that eventually you will end up with a shutter speed that is simply too slow for either your ability to hand hold your camera or too slow to capture your subject's movement.
Most people can hand hold at a 60th to a 30th of a second (depending on the size and heaviness of their camera and lens). Very occassionally when I'm feeling particularly zen I can get away with an 8th of a second. I wouldn't recommend this if you're on a job though . . . and I have to admit to a belief that I'm shooting in between heart beats . . . and yes I'm a nut job and I did watch a Discovery showcase on Olympic ski shooters who shoot in between heart beats and the idea that I do something similar makes me feel extra special at night! ;)
But at an 8th of a second chances are your breathing and heartbeat are going to make the camera shake. At a 5th of a second, or at 3 seconds, or 30 seconds, you can absolutely depend on that happening unless it's on a tripod. If the camera shakes you don't get a sharp image - you get fuzzy crap. Fine for the occassional art piece but not fine for most commercial applications.
Also, if you want to freeze your subject in motion then you have to make sure that your shutter speed is moving faster than they are. Imagine you're trying to take a photograph of someone running towards you and you want that picture to be crystal sharp. In almost all lighting conditions you'd need to be shooting at upwards of 200th of a second to make sure that their movement is sharp.
For me, I like AV mode because it's a great way to learn what aperture does: it controls your depth of focus; and because it begins to show you how aperture relates to shutter speed. Beyond a learning tool though I've never really found it helpful and even shooting on weddings when you have to be extrememly quick in making exposure decisions I don't use it.
So the ultimate goal with these lessons is to gain control of the camera, not to let it control you. The next lessons will be on shutter speed, and then the third member of our triumvirate, ISO. In the meantime go back to shooting in AV mode and this time shoot specifically for the following:
- A high aperture where everything is in sharp focus (easiest outside with relatively bright light if you're shooting in AV mode). Whatever you decide to shoot make sure that it makes sense for the subject to have such a high level of detail.
- A low aperture where you specifically throw your background out of focus. Again, make sure that it suits your subject to not have all areas clearly defined.
- Try and shoot in dark areas and take note of what happens to your ability to hand hold your camera. (And shoot without flash!).
- Try and shoot a fast moving subject under relatively low lighting conditions - what happens to your ability to keep the image sharp? Look out for great accidents on this one, where the blur works for you instead of against you.
