D-5100 Back Button Focus

[Tommy49]

I was wondering if this was typical? Sometimes after I focus using BBF and then maybe recompose and try to BBF again, the camera won't re-focus unless I half depress the shutter button first, then it will re-focus. FWIW, I have my shutter priority set to "Release" mode.

Any thoughts on why this is happening and is it normal?

[NorthernFocus]

I'm not familiar with the back button on the 5100. Is it an AF button or is it AE/AF-L button? If the latter, the -L stands for "lock". So it locks exposure and/or focus until you depress the shutter. In the higher end cameras you can customize the button function but as I said I'm not familiar with the 5100. Check the manual.

[Tommy49]

I'm not familiar with the back button on the 5100. Is it an AF button or is it AE/AF-L button? If the latter, the -L stands for "lock". So it locks exposure and/or focus until you depress the shutter. In the higher end cameras you can customize the button function but as I said I'm not familiar with the 5100. Check the manual.

It's an AE/AF-L button but I have it reassigned to "AF-On" to perform the back button focus option.

[nimitzbenedicto]

Hi Tommy,

Just tested my D5100. Set its AE/AF-L button to "AF-On".
AFC- Priority to release.
AF-A AF area mode - Single. ( same results with AF-C )

While holding down ""AF-On" button, it can continuously focus from near to far.
When I focus on something and release the "AF-On" button, it can refocus again using
the AF-On. But it won't focus even if I half-depress the shutter button.

HTH

[Dave Humphries]

I think you need to be in AF-C (continuous) mode to get the behaviour you (and I) expect - I assume you are, but if not, try this first.

Beyond that ...
Does the D5100 have a Custom menu option called "Focus tracking with Lock-on"? (it is a3 on D7100)
It has Off (instant response) and 5 other steps between Short (1) and Long (5), this will insert a delay in refocusing when re-composing if not set to Off and it may even when re-pressing (the button set as) AF-ON.

EDIT: actually not too sure either of those is relevant

To rule out other causes; is the;
light good?
subject static?
good contrasty edge?
single focus point selected?