Back button focus

[crewdog]

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

[jrhoffman75]

"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?

[crewdog]

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.

[kvbarkley]

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

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

[Waddizzle]

You need to press and hold BBF on AI Servo mode.

[crewdog]

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.

[Waddizzle]

“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.