Focus with back button.

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donaldjledet

Senior Member
For the first time the other day I set the ae-l af-l button to focus.
Had on the 18-270 or the 70-300. Can't remember. So took a few shots
from different ranges and some came in focus and some not.
So what did I do wrong?
Do you need to press button every time you shoot a photo,
know matter what focal lent you use?
Thanks
 
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donaldjledet

Senior Member
DSC_4318.jpg
 
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DraganDL

Senior Member
It has nothing to do with what button has been chosen for AF (either a shutter release button which is a factory default or a back button) - in each case you've got to press (some) button when you (re)focus...
"No matter what focal lent you use?". But, of course you need to press it, otherwise, how would you "persuade" your camera to activate the AF (since it is no more activated by half-pressing the shutter button)?
 
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DraganDL

Senior Member
@the second photo@This is not an AF related problem - it is a camera shaking (too much vibration, exposure time being too long for the specific focal length)... On the other hand, if you had used a tripod (with VR set to "off") the tripod might not have been stable enough.
 
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kluisi

Senior Member
donaldjledet said:
For the first time the other day I set the ae-l af-l button to focus.
Had on the 18-270 or the 70-300. Can't remember. So took a few shots
from different ranges and some came in focus and some not.
So what did I do wrong?
Do you need to press button every time you shoot a photo,
know matter what focal lent you use?
Thanks
Click to expand...

Don't think of it as using a button to turn AF on or off, but more like the camera will only auto-focus during the time when the button is depressed. Once the button is released, the camera will stay at the last focused point (or revert to manual focus).
 
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DraganDL

Senior Member
The whole idea is to decouple the shutter releasing (actuating) from the AF - you press (and hold) the back button and you turn the AF "on". Once you stop pressing it, the achieved focus remains where it is, even if you zoom and/or change composition, and the shutter button only actuates...the shutter (and the metering, if you want it that way - most people do).
Word of advice: when using telephoto lenses (or zooming into the telephoto range with lenses like yours) for landscape photography and/or "regular" portraiture, it is often better to rely on a manual focusing (especially when you have plenty of time for doing this, like when you took this second photo - unless you took it from within the vehicle in the motion!?).
Also, remember that these lenses of yours are relatively slow (on any Nikon) at AF, so you better wait a little while the AF mechanism sets the focus "dead on"...
 
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nickt

Senior Member
There are some times whens you will not press the back button for each shot. Like when you focus and recompose. Focus on your subject's eyes with the back button, then recompose and snap as many shots as you want as long as your distance to subject does not change. No need to go back and focus on the eyes each time. Another time is anticipated action... focus on basketball hoop with back button. As long as you don't move, you are ready to catch the next shot passing through the hoop. Camera response will be quicker because it will not attempt focus.
 
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