Focus tracking

Started 1 month ago | Questions thread
OP DavidWright2010 Senior Member • Posts: 1,737
Re: back button focus

John_A_G wrote:

DavidWright2010 wrote:

Yes, I've come to the same realization.

When the next bird comes by I will try AF.C mode, TAv , and back-button focusing, and see what happens.


David - I think people suggest back button focus without understanding what it does and doesn't do. It will have zero impact on the camera's ability to track a moving subject.

But when I hold down the BBF button and pan around, the cam re-focuses continuously. Doesn't that save some time - I then just press the shutter when the BIF looks good?

It can be very useful when you're WAITING for action at a given point. When in AF-C with focus controlled by the shutter button - when you press the shutter button the camera will start to focus. If you're waiting for a bird to take off (without the need to track AFTER it takes off) if you have focus control on the shutter button and you take your finger off while you wait - when you see the bird take off and press the shutter the camera will focus again - maybe you've moved it and it focuses on the wrong thing.

With back button focus, you can focus with your thumb on the bird, take your thumb off and wait. When the moment comes you press the shutter with your finger and the camera won't re-focus.

I will try that. Up til now I haven't noticed that this is what happens.

Then, should you want to start tracking you can simply push back button with your thumb. Push back button with thumb early and you have same risk - camera will refocus (maybe you've moved point and it focuses on the wrong thing).

So, back button focus can be useful in very limited circumstances. HOWEVER, if you want to use it I suggest you keep it on all the time so you build up muscle memory and don't get confused over whether to use finger or thumb for focusing. But, just make sure you understand what the feature does and doesn't do.

Yes, good advice and thanks for the extended discussion.

Switching topics, the last 2 in-focus perched birds were inadvertently taken at 210 mm, and my working hypothesis from that was that the 70-300 mm lens was a little soft at 300 mm. A series of pix from a tripod showed a similar result. (But 1 exposure at each different camera setting.)

Then I took the series below just now. Aperture priority (f/7.1), the left 4 taken at 260 mm and the right 2 at 300 mm. There's no correlation with FL. The upper right is good focus, and so is the upper center (different FLs). The others are OK not quite as sharp. (Except the lower right which is clearly OOF)  Each time I focused on the bird with a half-shutter, then took the exposure.

This series make me think that AF is just not that reliable for this cam/lens combo.

Same, shown at 200%:


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