Using Aperture Priority on a DSLR

Aperture is the term used to describe the size of the adjustable opening is that lets light through your lens. Aperture is measured using the term f-stop. There’s a unintuitive inverse relationship between the f-stop value and the degree to which your aperture is open. So if you’ve got your lens aperture set to f/2.8, which is a low f-stop number, that means the aperture can open “wide,” letting lots of light in. Silly, eh? While an f-stop value of f/16 means the aperture is really tiny, just letting a little light through.

Using Aperture Priority Mode to Control Depth of Field

Turn your DSLR to Aperture Priority Mode (“A” on Nikon, or “Av” on Canon) and try taking two close-up shots of the same flower. First try it with a relatively high f-stop number, like f/11. Now, try turning it down as low as it will go, like f/2.8 if you’re using the “Thrifty 50″ Nikon 50mm f/2.8. You’ll see a shallow depth of field (the background is blurry) when you’ve got the aperature down at f/2.8. At f/11 you’ll have both the foreground and background in focus.

Back in the day, newspaper photographers were encouraged to, “Set the camera to f/11 and be there,” which meant you’d be relatively certain to capture the whole scene of what was going on – that the depth of field wouldn’t be too shallow with a blurry background.

When Should I Use Aperture Priority Mode?

• Use Aperture Priority Mode whenever you want to control depth of field.
• Use the lowest f-stop number to achieve shallow depth of field, like if you want to bur the background and keep the subject in focus.
• Use a low f-stop number if you’ve got limited light.
• Use a high f-stop number, like f/16, if you’ve got plenty of light and want to keep both the foreground and background in focus.
• Use Aperture Priority Mode all of the time, if you like. It’s the “default” mode that I walk around with.