Back button focusing...

Started 10 months ago | Discussions thread
Doug Pardee
Doug Pardee Veteran Member • Posts: 9,436
Mirrorless AF is different

alfaholic wrote:

I am experiencing some weird behaviour with my BBF setup, at least compared to what I am used to with Nikon.

STOP! Mirrorless AF operates very differently from SLR/DSLR AF. If you try to treat it the same, you won't get what you expect. (Been there, done that.)

Mirrorless cameras fundamentally use contrast-detect AF, perhaps with on-sensor phase-detect AF as an acceleration feature that can be used in some conditions. The X-Trans II and X-Trans III Fujis all have OSPDAF capability, as does the X-A5. But there are situations where OSPDAF isn't usable and the camera uses contrast-detect. By the way, the OSPDAF has no cross-type AF points -- it's only sensitive to vertical lines, plus or minus maybe 45 degrees.

A mirrorless camera has to use the same image-sensor data for viewfinder, metering and auto-exposure, and auto-focus. This requires that the camera do quite a bit of juggling of sensor setup prior to shutter release. You'll find it behaves differently before and after half-press, before and after AE-Lock, and before and after AF-Lock (which in turn can differ between AF-S and AF-C). On a DSLR, the AF sensors are single-purpose, but with a mirrorless the availability of AF data is limited by the needs of the viewfinder and metering/AE systems.

The problem occurs after I press the shutter half way down, it does something with the AF and I hear the noise, no matter I turned off Shutter AF in settings menu.

If Shutter AF is turned off for the AF mode that your front switch is set to, the camera isn't doing anything with AF when you half-press. However, the camera is reconfiguring in preparation for image capture on half-press, including stopping the lens down. You're hearing the lens iris and (on the 18-55) perhaps optical image stabilization activating.

You may also see some changes in the viewfinder, as the camera reconfigures the sensor in preparation for image capture.

Another thing is hunting with 56mm f1.2 with BBF and Continuous AF. Somehow it can not lock the focus and stop hunting so I can release back button, but it always hunts

You have the camera in AF-C. That means the camera is continuously refocusing. When an AF cycle finishes, the camera checks to see if the shutter release is depressed. If not, it immediately starts a new AF cycle to refocus on the (possibly moved) subject.

it is very hard to tell when to press the shutter and take the shot.

The camera will not release the shutter in the middle of a focus cycle. What you need to do is just press the shutter release and wait for the picture to be taken. Yes, this introduces a bit of shutter lag, especially when the camera is using contrast-detect AF.

The focus/release priority options mean something different than on a DSLR. "Release priority" means that after the current focus cycle is complete, the camera will release the shutter regardless of whether AF was successful or not. It does not mean "release the shutter immediately."

it should be able to lock the focus and stay there, for example on the eye.

Not in AF-C. It won't "stay there" because the camera needs to refocus in case the subject moved.

The reason why I used AF-C always earlier is because it is very easy to change between AF-S to AF-C, actually you do not need to change anything, just release back button after it locks the focus to simulate AF-S, or keep it pressed for AF-C.

Don't do this on Fuji. When you let off the button, the Fuji will immediately stop focusing. In AF-C, that's almost always going to be in the middle of a focusing cycle, when focus hasn't yet been achieved. WIth AF-S you can let off the button once the focus cycle is done, but with AF-C you need to keep the button held down until the shutter has actually released (which only happens when the AF cycle has completed, before starting the next AF cycle).

Another thing not to do on Fuji: don't turn on the AF+MF option "just in case" you ever want to touch up the focus. This option overrides the protections against out-of-focus captures, because it assumes you're always going to be tweaking the focus. And you don't need it if you've disabled Shutter AF.

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