Auto-focus-ON Button: On a Canon 5D MkIII camera there is a button on the top rear that is marked "AF-ON." The manual says only that it functions the same as pressing the shutter...

DavidT Loc: Maryland

On a Canon 5D MkIII camera there is a button on the top rear that is marked "AF-ON." The manual says only that it functions the same as pressing the shutter button half way to set auto-focus and exposure. Then, why is there a need for this separate button? Does anyone in UHH use this button, and, if so, under what circumstances would it be more useful than pressing the shutter button halfway?

Darkroom317 Loc: Mishawaka, IN

Yes, back button focus is awesome. When you use shutter button focus the camera with refocus when you press the shutter. This can cause things to be out of focus if moving. With back button focus, you set your focus independent of the shutter. You can focus on a moving object and your camera will keep the same plane of focus until you refocus with that button. It gives you greater control of your focus. I always use it. It is great for sports. I noticed my number of in focus shots went way up after I made the switch.

Erv Loc: Medina Ohio

Hi David. I shoot Nikon and it has this too. And I have the focus turned off on the shutter button. It is good to frame a shot. You can hold the focus button down and get your subject in focus and then reframe for the shot. It can also help with BIF. Helps keep focus while tracking the bird. Take a while to get use to it, but once you do. You will never go back.:) Oh, the only down side is if you give your camera to someone else. I can guaranty every shot will be blurry.:) I have a custom menu setup for Janny. When she wants the camera I just switch to it.:):)
Erv

EstherP

I also find that back-button focus is faster than the 2-step focus-shoot. When using the AF-ON button, your focus remains the same till you change it. So you can keep shooting at the same focus and with just one push on the shutter button. I find this feature very helpful for example, when the kids are jumping on the trampoline or are busy in some other activity that keeps them at an even distance from the camera.
EstherP

St3v3M Loc: 35,000 feet

It is for an option called Back-Button Auto Focus primarily used in Sports or action shots.
The articles below should explain, but there are several good YouTube videos too.

AE Lock and AF-ON Buttons http://learn.usa.canon.com/resources/photos/articles/back_button_af/back_button_focus_af-and-ae_buttons_2010.shtml
- click the Technical Info tab
AF-ON/AE Lock button switch http://learn.usa.canon.com/resources/photos/articles/autoexposure_ae_lock/using_ae_lock_eos50d_cfnIiv2_af-ae-switch2010.shtml

Back-Button Auto Focus Explained http://learn.usa.canon.com/resources/articles/2011/backbutton_af_article.shtml

nikonshooter Loc: Spartanburg, South Carolina

St3v3M wrote:

It is for an option called Back-Button Auto Focus ... (show quote)

Try it - you will cease using the shutter to acquire focus if you do.

DavidT Loc: Maryland

Thank you all. I think I understand the concept, but need to try it out. I do mostly studio work of products and macro photography (e.g., flowers, insects) and haven't done much sports shooting since my son graduated high school and college. I'm starting to get interested in bird photography so your advice will definitely come in handy. I'll have to play with this feature some more to fully understand it. Thanks again!

BillyDuds Loc: North Carolina

Here's how I understand it: When you press and hold the AF-On button with stationary subjects (AF-S in Nikons) the focus remains fixed, allowing you to reframe and press the shutter at your leisure. If you press and hold the AF-On button while using AF-C for moving subjects, focus will continue to readjust until the instant the shutter button is pressed. This is particularly useful for rapidly moving subjects, such as sports, BIF etc. (This assumes you've disabled focusing with a half press of the shutter button, so that focus is only activated by the AF-On button.) Am I correct?

nikonshooter Loc: Spartanburg, South Carolina

BillyDuds wrote:

Here's how I understand it: When you press and hol... (show quote)

With one caveat.....Vibration Reduction can only be initiated when using the SHUTTER. This is not a problem with me as I do not like VR. It slows focusing, drains battery life, and with high ISO abilities with my cameraa, using fast shutter speeds renders it useless. (if you take issue with this, try this....if you can adjust your ISO so that you can shoot at least twice (shutter speed) your focal length - then try shooting with the VR off for the first half of an event, then turn the VR on and shoot the second half. Check the infocus shots and compare. If your images cannot tolerate the ISO's required to shoot at 2x focal lenght, then VR is for you....as it can give you a stop of forgiveness, but at a cost.)

Also, AF-On works with both Servo and Continuous......for me, using continuous for sports works well....often my subject is out of focus but a distant or closer subject may be tack sharp, I can sometimes crop out a usable picture. Also predictive focus works with AF-On or Half Shutter in continuous....again great for sports or anything moving object.

DavidT Loc: Maryland

These are all great comments. There's so much that isn't mentioned in instruction manuals.

nikonshooter Loc: Spartanburg, South Carolina

nikonshooter wrote:

quote=BillyDuds Here's how I understand it: When ... (show quote)

I should have added, some of my best sports shots are not of my target, but as luck would have it, something else in the frame, that I did not see, steals the show and is in focus when my target may not have been....... I therefore crop, post, print and try to sell that one. When I get the question....how in the world did you see that, I act real humble and say...."guess it's just years of practice. How little do they know."

But to be sure, Luck is a lot of clicks!

Erv Loc: Medina Ohio

It will work great with macro Too. If you try it, give it a few days to get use to it.
Erv

DavidT wrote:

Thank you all. I think I understand the concept, but need to try it out. I do mostly studio work of products and macro photography (e.g., flowers, insects) and haven't done much sports shooting since my son graduated high school and college. I'm starting to get interested in bird photography so your advice will definitely come in handy. I'll have to play with this feature some more to fully understand it. Thanks again!

RocketScientist Loc: Littleton, Colorado

DavidT wrote:

On a Canon 5D MkIII camera there is a button on the top rear that is marked "AF-ON." The manual says only that it functions the same as pressing the shutter button half way to set auto-focus and exposure. Then, why is there a need for this separate button? Does anyone in UHH use this button, and, if so, under what circumstances would it be more useful than pressing the shutter button halfway?

That is a decent feature. On lesser Canons you need to go into the Custom Functions and swap functions of AE Lock button with the shutter halfway down if you want back button focus.

mborn Loc: Massachusetts

Here is a tutorial on Rear Focus http://www.birdsasart-blog.com/2011/09/13/rear-focus-tutorial/

I use it all the time

Shine11 Loc: London UK

Erv wrote:

Hi David. I shoot Nikon and it has this too. And I... (show quote)

Why don't you keep both on?