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New Contributor
Posts: 3
Registered: ‎02-05-2019

Back button focus

I have two cameras, a Canon 70d and a 5d Mk III.  I also have several lenses, but my 'go to' lens is my EF 100-400 4.5 5.6 L series zoom as I do a lot of aviation photography.  On each of them, I've used metering start/focus (front button) and back button focus (AF ON).  

 

My question is thus:  Why does it take several presses on the back button to achieve focus on subjects well in excess of 6.5 meters to infinity setting (ex:  A bird feeder 30 feet away).  Should it not take just a single press?  

 

I sent the lens to be cleaned and inspected by a company in California about a year ago, which claimed that the lens was working properly and adjusted to 'factory specs'.  This led me to believe the problem was with the Camera (70d).  I recently purchased the 5dM3, and the problem persists.  The batteries on both camera are fully charged, so it's not for lack of power.

Since I do not use my 24-105mm f/4 IS USM for long range photography, I cannot say if happens with that lens as well.

 

crewdog

Honored Contributor
Posts: 5,709
Registered: ‎11-13-2012

Re: Back button focus

"On each of them, I've used metering start/focus (front button) and back button focus (AF ON).  

 

My question is thus:  Why does it take several presses on the back button to achieve focus"

 

Are you saying the same thing doesn't happen if you use shutter button focus?

 

 

John Hoffman
Conway, NH

1D X, Rebel T5i, Many lenses, Pixma PRO-100, MX472, LRCC Classic
VIP
Posts: 8,039
Registered: ‎08-13-2015

Re: Back button focus

You need to press and hold BBF on AI Servo mode.
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"I don't rent software. I use Photoshop CS6, ACR 9.8 and Lightroom 6.8 ."
New Contributor
Posts: 3
Registered: ‎02-05-2019

Re: Back button focus

I will have to re-check, but looking back, it may have been the case with the front button as well.  I have never been really happy with the focus function on these cameras, as I feel that Canon camera lenses have a softer focus than others I've used.

In aviation photography of fast moving fighter aircraft, I never have the time to manually focus, so I depend on autofocus for sharp photos, and rarely is that happening.

New Contributor
Posts: 3
Registered: ‎02-05-2019

Re: Back button focus

Thanks for your reply. I believe I was using AI Servo mode, but I will be sure to check at the next opportunity. Again, Thsnks.
Highlighted
Esteemed Contributor
Posts: 3,308
Registered: ‎02-17-2016

Re: Back button focus

There is a lot more than focus for sharp images of fast moving objects:

 

IMG_6622.jpg

 

For this, I think that standard half-press focus on the center point in one-shot mode works best.

VIP
Posts: 8,039
Registered: ‎08-13-2015

Re: Back button focus

“Thanks for your reply. I believe I was using AI Servo mode, but I will be sure to check at the next opportunity.”

I like to save BBF as a custom shooting mode that uses One Shot AF. I save another custom shooting mode that the same button for [AF OFF] for action photography that uses AI Servo mode.
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"I don't rent software. I use Photoshop CS6, ACR 9.8 and Lightroom 6.8 ."
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