Like I said in my last post I walk around with my camera set to Aperture Priority, well, almost all the time. I do this so that I don’t miss any images that come along unexpectedly. With my camera set to Aperture Priority I have already chosen the f-stop that I want to use and the camera chooses the corresponding shutter speed needed to get an exposure that should be acceptable. Why did I say should be acceptable? Well, the camera’s light meter can be fooled (more on this in a later post about metering).
Your camera has two different ways of controlling the amount of light needed to make a good exposure. The aperture controls the amount of light that enters the camera and the shutter controls how long the light comes into the camera. The aperture is expressed as an f-stop (also referred to as an f-number). The f-stop will tell you the exact size of the aperture if you are willing to do a little math. To make it easy lets say I’m using a 50mm lens and I set my f-stop to f8. All you need to do at this point is divide 50 by 8. Here is the math: 50 ÷ 8 = 6.25. So the aperture, the size of the hole that lets light through the lens, is 6.25mm in diameter. Simple, now that you know that you can mystify your friends at your local pub the next time you stop in for a pint!
Besides controlling the amount of light that passes through the lens, the aperture also controls the depth of field in your image. This is the main reason that I leave my camera set to Aperture Priority mode. When I’m walking around I want to have a pretty good idea of the amount of depth of field that I will be getting in my images. I usually stay right around f8 for a moderate amount of depth of field (look for more on depth of field in a future post). With my camera set to Aperture Priority I’m off to make photos!