What the heck is back button focus & why do I need to use it?

When most people make a photo with their camera, they aim, press the camera's shutter release button, wait for the camera to focus, then press the button the rest of the way. Whammo, the image is made.

While that's easy and direct, if you're using a DSLR (Digital Single Lens Reflex) or MILC (Mirrorless Interchangeable Lens Camera) camera, it may not be the best way to focus and shoot. Until 1989, it was the only way to activate autofocus and make a photo with an SLR (Single Lens Reflex) camera, the film based forbearer of today's DSLR and MILC cameras.

It wasn't until 1985 that autofocus was even available. That year, Minolta, having purchased Leica autofocus technology, released the first commercially successful autofocus SLR, the Maxxum 7000. Minolta configured the Maxxum autofocus system to work as most photographers know it today. Autofocus was activated by pressing the shutter release button.

Four years later, in 1989, Canon introduced back button focus. It removes the autofocus function from the shutter release button and assigns it to a button on the back of the camera. Hence the name, back button focus.

AE, AF, FV locks demystified

While many digital cameras today don't have an FV lock, virtually all have an AE and AF lock. Unfortunately, few digital camera owners know what these locks are, nor that their cameras have them. It's unfortunate because they are extremely useful camera features.

Here's a basic description of each lock:

• AF-L or AFL — Autofocus Lock: When using auto focusing, this locks focus even when the camera is moved to recompose the photo, prior to making the photograph.
• AE-L or AEL — Autoexposure Lock: When using any of the automatic or semi-automatic exposure modes, this locks the exposure settings, even if the light changes or the camera is moved, prior to making the photograph.
• FV Lock —  Flash Value Lock: When using an automatic setting for your built-in or external flash, this locks the power output level of the flash, even when camera is moved, to recompose the photo, prior to making the photograph.

I use these locks often, especially the AF Lock. I consider the AF Lock as essential as the ISO, Shutter Speed, and Aperture exposure controls.