“Back Button” Focusing

“Back Button” Focusing

By William Lulow

“Back button focusing” is a technique that is designed to help you use your DSLR faster and easier to obtain correct focus for whatever image you’re trying to make. If you are not using it, you should be. Incidentally, it’s the only way I focus my cameras these days. I have the cameras set to “Back Button” all the time. It’s really the only way to ensure that your images will be tack sharp every time.

In essence, it allows you to focus the camera on your main subject, while enabling you to re-compose the picture the way you want. You can assign one of the buttons on the back of your DSLR to function as a focusing aid in the same way as depressing the shutter-release button half way. It activates the auto-focus mechanism and then holds the focus it acquires while you compose your image. It is very handy and has enabled me to create crystal clear images with perfect focus every time.

There are several reasons why one would want to do this:

• It is often hard to keep the shutter button depressed “half way” for any period of time
• It allows the photographer to determine exactly where, in the image, he/she wants the prime focus to be
• It locks the focus so that you can compose the picture independently
• It helps ensure that perfect focus is maintained throughout the shooting process

Each camera has its own method of assigning this function to buttons on the back of the camera, so you need to consult your manual or go online for a tutorial in how to set this up.

In addition, I have found that setting up a “spot metering” system with the camera is the best way to ensure correct exposures with any focusing system. I can then aim the camera to take a reflected meter reading of the actual “spot” in the image you decide you want to show, meter and focus.

Here is some information from Canon (Should be available from Nikon and Sony as well) available on the internet:

Which button is used for back-button AF?

Once you’ve activated this feature, you press one of two buttons: either the rear AE Lock button (marked with an asterisk or star icon), or if your camera is equipped with it, the rear AF-ON button. Either is relatively easy to reach with your right thumb on the back of the camera as you shoot.

Those cameras with the separate AF-ON button also have a Custom Function that lets you flip-flop the roles of the AF-ON button and the adjacent AE Lock button (with asterisk icon). This is called “AF-ON / AE lock button switch” in the Custom Function menu of most recent EOS models. If you find the AE Lock button easier to reach, you may want to engage this function as well.

 Activating back-button AF

Back-button AF is engaged by setting the appropriate Custom Function in your EOS camera. Remember, to use any Custom Function, your camera must first be in one of the “creative zone” exposure modes – P (Program auto exposure), Av (aperture-priority mode), Tv (shutter-priority mode), or M (manual exposure mode). Custom Functions are totally locked-out if you’re in the full-auto “green zone”, or a picture-icon setting like the Portrait mode or Landscape mode.

The particular Custom Function number varies, depending on the EOS model in question. All digital EOS SLRs, with the exception of the original, 6-million pixel EOS Digital Rebel model, have a Custom Function for moving AF from the shutter button to a back-button. Be sure to check your camera manual for confirmation on the Custom Function number for Back-Button AF in your EOS model. Here are examples of the C.Fn menu select

Back-button AF is engaged by setting the appropriate Custom Function in your EOS camera. Remember, to use any Custom Function, your camera must first be in one of the “creative zone” exposure modes – P (Program auto exposure), Av (aperture-priority mode), Tv (shutter-priority mode), or M (manual exposure mode). Custom Functions are totally locked-out if you’re in the full-auto “green zone”, or a picture-icon setting like the Portrait mode or Landscape mode.

The particular Custom Function number varies, depending on the EOS model in question. All digital EOS SLRs, with the exception of the original, 6-million pixel EOS Digital Rebel model, have a Custom Function for moving AF from the shutter button to a back-button. Be sure to check your camera manual for confirmation on the Custom Function number for Back-Button AF in your EOS model. Here are examples of the C.Fn function menu selection for recent EOS models:

EOS Rebel T3: C.Fn 7 (option 1 or 3)
EOS Rebel T3i: C.Fn 9 (option 1 or 3)
EOS Rebel T4i: C.Fn 6 (option 1 or 3)
EOS 60D: C.Fn IV-1 (option 1, 2, 3, or 4)
EOS 7D: C.Fn IV-1 (Custom Controls — Shutter, AF-ON, AEL buttons)
EOS 6D: C.Fn III-5 (Custom Controls — Shutter, AF-ON, AEL buttons)
EOS 5D Mark II: C.Fn IV-1 (option 2 or 3)
EOS 5D Mark III: C.Fn menu screen 2 (Custom Controls — Shutter, AF-ON, AEL buttons)
EOS-1D X: C.Fn menu screen 5 (Custom Controls — Shutter, AF-ON, AEL buttons)
 

Understanding the on-screen wording:

The terminology often used on the menu for this particular custom function may seem a little confusing, so an explanation is in order. In cameras without a separate C.Fn called “Custom Controls”, the function is headed “Shutter/AE lock button”, or similar wording.

What this means is that anything before the slash mark refers to how the shutter button will behave. Anything after the slash tells you how the rear button will work if that option is selected. Using the popular EOS 50D as an example, here’s what you see on-screen, and here’s what it means:

0: Metering + AF start (note: there’s no slash here)
Factory-default setting. You activate camera’s meter and AF by pressing shutter button half-way down. Rear AF-ON button also does same if it’s pressed, so you don’t get the benefits of removing AF activation from the shutter button when this option is set.

1: Metering + AF start / AF stop
AF is still at shutter button. Pressing the rear button will actually LOCK the focus; potentially useful if you shoot a lot of moving subjects in AI Servo AF and prefer to activate AF with a conventional half-press of shutter button. Focus is unlocked by removing thumb from back-button.

2: Metering start / Meter + AF start
Back-button AF activation. Shutter button no longer activates AF, but of course fires the shutter. Metering is continuously updated — if you shoot a sequence of pictures, the camera takes a fresh meter reading for each one. There’s no locking of exposure, unless you separately press the AE Lock button (this last item is not possible on some EOS models).
 

3: AE Lock / Metering + AF start
Back-button AF activation. Difference between this setting and option 2 directly above is that when you press the shutter button half-way, your exposure is locked and won’t change until you pull your finger off the button entirely. Thus, if you shoot a sequence of pictures in any auto exposure mode, the exposure setting used for the first shot is used for each subsequent shot. Can be useful if you were using back-button AF to easily lock focus and shoot a series of portraits, where you wouldn’t expect lighting to change.

4: Metering + AF start / Disable
Similar to setting “0” above, but now, the camera’s rear AF-ON button is disabled. AF activation is at the shutter button. Convenient if you’re worried about accidentally pressing the back-button and don’t want to use back-button AF.

Summary:

It can take a little practice to get the hang of back-button AF, but we suggest giving it a try if you haven’t done so already. Even if at first its operation seems unorthodox, in fact it can simplify certain types of shooting and allow you to work more quickly with fewer missed shots. Back-button AF was first suggested to Canon back in the late 1980s by sports photographers who saw the need for some way to be able to start and stop AF without interfering with shooting continuous pictures. The feature is now available on all current and many previous EOS models. It’s no longer just for pros — any photographer can experiment with it and benefit from it in certain conditions.

Finally, remember that like any Custom Function, you can always return the camera back to factory-default operation by returning that Custom Function to option “zero”, or returning the Custom Controls C.Fn for the shutter button, AF-ON, and/or AEL button to the first of each button’s available options.

I now use this method exclusively whenever I am shooting anything. Here’s a shot of Beale Street in Memphis I shot last year. I was shooting a panorama with my 20mm lens so I wanted the entire image to be sharp. My meter and focus spot was set for the center in the camera and I aimed it about where the woman in the red dress was in the middle of the picture. I then made two overlapping exposures that I knew I would be able to merge in the final photo. I then kept my thumb on the back button to ensure that my focus would remain the same for both exposures and this was the result:

Now, whenever I am shooting anything, I decide what part of the image I want to be in sharpest focus and set the camera for that spot holding the back button down. Then I compose the image the way I want and that spot will always be sharp. With a wide-angle lens, most of the image will be sharp anyway because of the increased depth-of-field. But with longer lenses it even works better. Here Is a shot I did last year of singer/songwriter Paula Cole in concert:

This image was shot with my 85mm lens, ISO2000, shutter speed 1/100th of a second, aperture approximately f/5.6.. The focus was on the singer’s face. Look how sharp all parts of the image are. Same “back button” technique. I use it all the time.