70D and Back Button focus

[fbarnako]

I have set up the camera for back button focus.  

HOWEVER ... when I review shots taken using BBF, in Aperture, the "Focus Point" display indicates I focused always in  the center of the screen, not off to the right or left, as I might have recomposed.  Yes, I keep the AF On button pressed.

[jrhoffman75]

Focus Point display shows the focus point that was active. If you focus and recompose you still focused with the active point.

[ebiggs1]

You must understand how and what BBF does or works.  Most of the time I use center focus point only and use BBF very rarely.  Give the center focus point, only, a try.

[fbarnako]

So, you are using the normal half-click autofocus and then follow thru, using the center focus point?

A wonderful photographer, Steve Simon (aka The Passionate Photographer) SWEARS by BBF.  Several other photogs like it very much, to judge by the tutorials on YouTube.

Do you find fault with it?

And BTW ... THANK YOU  for responding.

Frank

[ebiggs1]

"So, you are using the normal half-click autofocus and then follow thru, using the center focus point?"

Correct.

Like I say I use center point most on the time.  There are always times when something else works better but for me I see center point as the most practible.

You must remember there are folks that think they have become pros when they get a nice or high dollar DLR.  They automatically think all the new features and buttons are great and must be used.  However, sometimes the simplest way is still the best.

[amfoto1]

I don't have 70D, but have been using BBF most of the time for the past 15 years with various Canon film and digital cameras.

I'm one of those folks who swear by it!

And I would guesstimate that most people I've taught to use it over the years love it and swear by it, too.

BBF allows you to use AI Servo as your default AF mode, so is particularly popular with sports photographers or anyone else shooting action and moving subjects. I'd rather have my camera in AI Servo, since it can be used with both moving and stationary subjects. One Shot can mostly only be used with stationary subjects. So, using AI Servo as my default mode, I'm better prepared for whatever happens.

If you don't use BBF there are times that AI Servo can cause problems, such as when you use single point and a focus and recompose technique. So, that's basically why I use and swear by BBF. It puts me in more full control and allows me to have my cameras in AI Servo ready and ready to quickly get a shot of almost anything, using most any technique I wish.

But, based on your description, you might be using BBF wrong.  You wrote, "I have set up the camera for back button focus... as I might have recomposed...  I keep the AF On button pressed." In fact, this is exactly the opposite of what you need to do. What you should do when focusing and recomposing with BBF is NOT maintain pressure on the back button. 

To focus and recompose while using BBF and AI Servo: first put the AF point on the subject and achieve focus... Then release the button so the AF stays focused on the subject, while recomposing as you see fit. The subject stays in focus. (You cannot do sngle point/focus and recompose in AI Servo mode with shutter-release based AF control enabled. You have to use One Shot.)

One caveat, if using a zoom and part of your recomposing involves changing its focal length, be aware that many modern zoom lenses are "varifocal" designs that do not maintain focus when zoomed. With those, you'll be fine so long as you are using AI Servo, BBF and not recomposing (i.e, not moving the AF point off the subject)... the camera automatically corrects focus as you zoom. But if you zoom a varifocal lens at the same time or as a part of the recomposition, you need to remember to update focus on the subject after changing the focal length... else you'll miss focus. Look for info about any zooms you use, to see if they are varifocal and need refocusing after zooming... Or if they're "parfocal" type that will maintain focus during zooming.

I only very rarely take my cameras out of BBF now... Mostly just if I'm lending one to someone who doesn't know how to use it and I don't have time to give them a lesson.

BBF puts me more in control of the AF system... of exactly when and where it focuses. It gives me the best of both worlds: the speed and accuracy of AF, plus user control more like we had when we manually focused our lenses.

Like others, most of the time I tend to use center AF point only. I just don't have time to set up planned shots when shooting sports/action. Plus with some cameras and lenses the center point is higher performance. But, I see AF point selection as a somewhat separate consideration from BBF... and single point/center only works quite well in conjunction with BBF... or not.

More info about BBF here: http://www.learn.usa.canon.com/resources/articles/2011/backbutton_af_article.shtml

***********

Alan Myers
San Jose, Calif., USA
"Walk softly and carry a big lens."
GEAR: 5DII, 7D(x2), 50D(x3), some other cameras, various lenses & accessories
FLICKR & EXPOSUREMANAGER