Wetpixel Live: Back Button Autofocus

Back button or thumb activation of autofocus is a setting that is available on most cameras, and is often discussed on the Wetpixel forums. @Alex_Mustard and @adamhanlon chat about when they use the mode and how using or not using it helps them to get their subjects in focus.

I use Back Button Focus for several reasons. One, as Alex mentioned, I can get focused at the distance I'm shooting for and then stop focusing and just shoot, without worrying about the focusing system suddenly refocusing on (for example) a bait fish that just happens to swim by in the foreground.

But, there is another reason that you didn't mention. I have my AF-On button programmed to use use Spot Focus w/Tracking, and my AEL button programmed to temporarily (while holding it down) switch to a Focus Area of Zone and then engage the AF. In other words, 2 different buttons, so I can use either of 2 different Focus Area settings.

If there is a shark swimming in the middle of a bait ball, I will use Spot/Tracking to lock focus on the shark's eye/face and then let the camera track it while I recompose for the shot I want.

But, if I'm swimming around looking for my next shot and suddenly, just for example, a shark appears, going right past me, I don't have time to get my viewfinder up to my eye, find the subject, lock the focus spot onto the shark's face/eye, recompose, and then shoot. For that, I can point the camera in the right direction, engage the button for Zone focus area, and take a shot and all I have to worry about is hoping I got the camera pointed close enough in the correct direction.

I use the same arrangement for shooting birds perched (spot focus) and birds in flight (zone focus).

I've never tried that. Thanks for the tip.

Alex 

The New Sony A7c and A-1 cameras can be set toTracking-on + AF-On. This can be set to the AF-0n button which works with the thumb level on the back of the housing. Under focus Area Limit you can chose from one to fourteen different types of focus areas,  I have chosen three for the purpose of testing. I can then depress the FN button and set say focus wide. This gives me an array of little dancing green boxes that select something I point towards in the frame. If I want to lock focus and drag the AF point around the frame for compassion I just press the thumb focus and the tracking box locks on and can be moved around the frame. This way I can shoot using the normal preset focus for the shutter release or switch to tracking with the thumb lever. This eliminates the issue of trying to remember which trigger will acquire AF.

Like with most Sony focus updates I expect this function to be added on all A-1, A7 and A9 versions going forward. 

1 minute ago, Phil Rudin said:

The New Sony A7c and A-1 cameras can be set toTracking-on + AF-On. This can be set to the AF-0n button which works with the thumb level on the back of the housing. Under focus Area Limit you can chose from one to fourteen different types of focus areas,  I have chosen three for the purpose of testing. I can then depress the FN button and set say focus wide. This gives me an array of little dancing green boxes that select something I point towards in the frame. If I want to lock focus and drag the AF point around the frame for compassion I just press the thumb focus and the tracking box locks on and can be moved around the frame. This way I can shoot using the normal preset focus for the shutter release or switch to tracking with the thumb lever. This eliminates the issue of trying to remember which trigger will acquire AF.  

But, you still have to remember whether you want to use thumb AF or forefinger AF, right? I don't see how it changes having to remember which trigger does what. It just changes it (versus what I described) from 2 different thumb options to 1 thumb and 1 forefinger option, doesn't it?

What you're describing for how the AF-On button/thumb lever on the housing is exactly how my setup (described above) works. The camera is set to Focus Area of Spot w/Tracking, so I can do exactly as you described - press the thumb lever to lock focus and then let the AF point move around the screen as I recompose.

Or, instead of using the thumb lever, I can press and hold the AEL button with my thumb to acquire AF using Zone mode (or Wide, if I wanted to change my programming to that). With the Focus Mode set to AF-C, holding down the AEL button keeps the focus locked on whatever Zone mode decides to focus on, for as long as I hold the button down. When I let go of the AEL button, the camera goes back to Spot/Tracking mode, which I can lock on by going back to the thumb lever.

From what I can tell, what you described and what I described give the same capabilities and both require remembering which button/lever does which type of focus. But, the method you described eliminates the possibility that you could set the focus ahead of time and then hit the shutter release (multiple times even) without affecting where the focal plane is, correct?

What you gain is that you never have to worry about forgetting to focus. If you forget to press a back button, when you hit the shutter release, you'll at least get an automatic focus on whatever Wide mode grabs before the shutter releases. Whereas, if I forget to hit one of the back buttons, I'll get an image focused on whatever distance I had it focused for previously.

Trust me Stuart I have used backbitten focus for years on a number of different cameras and I am aware of the shortcomings. The difference is that the shutter depress can get you into the area you want to go very quickly and the back button can then lock onto a eye and move with that eye around the frame very quaikly. Once you have locked onto a subject the difference is that it can follow the movement not just stay focused on the same plane. So locate the shark, the lock onto the eye or head and have the camera follow the eye or head as the shark moves towards you. This is something I could not do using back botton focus. 

On 2/12/2021 at 5:03 PM, Phil Rudin said:

Trust me Stuart I have used backbitten focus for years on a number of different cameras and I am aware of the shortcomings. The difference is that the shutter depress can get you into the area you want to go very quickly and the back button can then lock onto a eye and move with that eye around the frame very quaikly. Once you have locked onto a subject the difference is that it can follow the movement not just stay focused on the same plane. So locate the shark, the lock onto the eye or head and have the camera follow the eye or head as the shark moves towards you. This is something I could not do using back botton focus. 

I am confused. I don't know about other cameras, so I'm talking about the a7rIV, which I know you shoot as well.

You're saying, well, the part that I bolded. I use Focus Mode AF-C and Focus Area Flexible Spot w/Tracking on my AF-On button, but I think you can have the same thing on the shutter half-press, can't you? Your bolded statement implies that you cannot have that Focus Area assigned to shutter half-press. 

I think I must not be interpreting what you're saying correctly.

Side note: For using the same setup for shooting Birds In Flight, I found it really useful to also add Shutter Speed to the Custom Hold Set. E.g. have the camera set to SS of 1/800 for use with the AF-On button (Focus Area Spot w/Tracking), shooting birds that are perched. Then have the AEL button set to recall Focus Area Zone (or Wide), start AF, and also change SS to 1/2500 (or whatever is appropriate for the specific environment you're in). I had very good luck with this yesterday. Shooting a perched bird, then see a different bird that happens to be flying past. Move my thumb from AF-On to AEL and instantly be "on" the flying bird and getting (potentially) some good shots. For u/w, I don't think doing a recall hold on SS is likely to be as useful.

Remove.

For Canon shooters using a WACP-1, the back button focus should be used.  The lens behind the WACP-1 is an old, clunky Canon lens (28-70mm f3.5-4.5)  which is not that fast.  I've often gotten out of focus shots when using only the shutter lever, where it couldn't focus fast enough before the shutter opened.

20 hours ago, helgromite said:

For Canon shooters using a WACP-1, the back button focus should be used.  The lens behind the WACP-1 is an old, clunky Canon lens (28-70mm f3.5-4.5)  which is not that fast.  I've often gotten out of focus shots when using only the shutter lever, where it couldn't focus fast enough before the shutter opened.

I think that is a good point. That BBF is good both in situations where the lens struggles to focus. And also with lenses that struggle to focus!