I walk around with my camera set to Aperture Priority 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 depending on the scene you are photographing(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 through the lens and the shutter controls how long the light is allowed to enter 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 opening 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).
Why Aperture Priority and not Manual mode? Constantly changing lighting conditions and subject matter are two of the reasons. When lighting conditions change you need to be ready to change the settings on your camera when you are in Manual mode. Lets say you were making images of some graffiti on the side of a building in and alley. There is no direct sunlight hitting the graffiti so you are working with indirect lighting. Once you walk out of the alley you see fantastic sunbeams behind a building and you see a small cloud getting ready to pass in front of the sun. You maybe have half a second to get the shot. If you were in this situation, with your camera set to Manual mode, do you think you can get that shot? What would be your best guess for an aperture and shutter speed combination for this scene? With your camera set to Aperture Priority mode you will have a better chance at getting an acceptable image. Your camera’s meter will react to the change in lighting conditions and select an appropriate shutter speed. Manual mode is great and I use it quite a lot but I’m happy to let the camera do some of the split second decision-making for me so that I get images that I might otherwise have missed.