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06-03-2015, 11:27 AM   #1
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Please teach me about the AF button

I am quite new to DSLR. I used film SLR (amateur models, not pro models) before, from manual focus to auto-focus era.
On SLR with auto-focus, mostly the focus locks (also simultaneously AE locks) with shutter half-press. Some SLR had AE-lock button, which is understandable.
But what is that "AF button" for? I cannot figure out why one would need or want to press (and hold) that AF button, recompose, then press the shutter release.
Please explain > when to use AF button? What situation that AF by halfpress shutter does not work?

06-03-2015, 12:00 PM   #2
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Here's one explanation. One key reason is to avoid refocusing with every shot (and recomposing can make sense: for example, if AF doesn't find the subject you want in focus because it's not centered, or if something in the foreground you want out of focus keeps grabbing its attention).
06-03-2015, 12:04 PM   #3
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For when you want to separate the shutter release from AF. You can turn off AF on the shutter release and use AF button instead. There may be other use cases, but this is mine.

Regards,
Dan
06-03-2015, 01:05 PM   #4
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The problem with focusing and recomposing with the shutter button is that if your finger lets off pressure on the button, then the camera will re-focus on some other point in the image when you go to fully press the shutter button. To make matters worse, Pentax DSLRs don't give a tactile response upon half-press, i.e. you can't "feel" when you've half-pressed the shutter.

Disabling the focus on half press function, and using the back button means that once you release the back button, the camera will not try to refocus, even from shot to shot, unless you tell it to.

06-03-2015, 01:22 PM - 1 Like   #5
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Set the camera to center focus point, AF.C mode (continuous).
Use "back button" AF button to acquire focus, and keep focus. Hit the shutter trigger button when you want exposures, while maintaining focus on your subject.

My biggest problem with back button is that I tend to smudge my glasses with my thumb.
06-03-2015, 01:29 PM   #6
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The button can be programmed. By default, you can hold it and the camera will not refocus when you press the shutter button. But you can give it other functions as well. I think the manual covers these, as does that linked thread. Its a useful little button. I have it set like the guys above, so that camera will only AF if I press this button. This means I don't waste time focusing when I don't need it. It gives you some more control, but it depends on your uses, techniques.
06-03-2015, 02:43 PM - 1 Like   #7
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I highly recommend using the rear AF button and turning off the shutter half-press. It's a fantastic feature. Here's what I like about it with my K-5:
  • Focus-and-recompose. Press the AF once to focus, release, then recompose and press the shutter.
  • Focusing on small foreground objects is easier. Sometimes the camera has a tendency to lock onto the background and it takes a few AF tries to get the correct object. With a half press the camera tries to refocus every time and you repeat the AF pain each photo. With the rear AF, get focus correct once then take multiple images without refocusing.
  • Jumping between AF.C and AF.S doesn't require any setting changes. Tap the AF button to get AF.S. Hold the button to get AF.C.
  • Quickshift lenses. You can manually focus and not have to worry about AF kicking in when you press the shutter. There's no need to flip the focusing switch to MF.
If you make the change, it may feel a little unnatural until you adjust. Allow a few days of shooting so don't reprogram the camera just before an important photo event.
06-03-2015, 03:22 PM   #8
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QuoteOriginally posted by DeadJohn Quote
I highly recommend using the rear AF button and turning off the shutter half-press. It's a fantastic feature. Here's what I like about it with my K-5:
  • Focus-and-recompose. Press the AF once to focus, release, then recompose and press the shutter.
  • Focusing on small foreground objects is easier. Sometimes the camera has a tendency to lock onto the background and it takes a few AF tries to get the correct object. With a half press the camera tries to refocus every time and you repeat the AF pain each photo. With the rear AF, get focus correct once then take multiple images without refocusing.
  • Jumping between AF.C and AF.S doesn't require any setting changes. Tap the AF button to get AF.S. Hold the button to get AF.C.
  • Quickshift lenses. You can manually focus and not have to worry about AF kicking in when you press the shutter. There's no need to flip the focusing switch to MF.
If you make the change, it may feel a little unnatural until you adjust. Allow a few days of shooting so don't reprogram the camera just before an important photo event.
I learnt this technique a few weeks ago from another post on this forum. After years of "1/2 press" it felt a bit strange at first but is now standard operating procedure.

06-03-2015, 04:55 PM   #9
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QuoteOriginally posted by DeadJohn Quote
  • Jumping between AF.C and AF.S doesn't require any setting changes. Tap the AF button to get AF.S. Hold the button to get AF.C.
  • Quickshift lenses. You can manually focus and not have to worry about AF kicking in when you press the shutter. There's no need to flip the focusing switch to MF.
These are my biggest reasons as well. The MF trick comes in handy rarely but when it's useful, it's needed, like when you want to focus halfway between two objects--but there's nothing but air there.
06-03-2015, 07:15 PM   #10
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Thank you very much for all responses. Now, I understand that AF button has a useful function for serious users / pros.

But >>> as a very casual photographer (ie. shooting 400 - 500 pictures per year on vacations only), with DSLR and compact cameras, using the "basic" procedure of halfpress shutter to AF + AElock is OK, am I correct ?
06-03-2015, 09:00 PM   #11
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QuoteOriginally posted by Poompat Quote
Thank you very much for all responses. Now, I understand that AF button has a useful function for serious users / pros.

But >>> as a very casual photographer (ie. shooting 400 - 500 pictures per year on vacations only), with DSLR and compact cameras, using the "basic" procedure of halfpress shutter to AF + AElock is OK, am I correct ?
You don't need to press the AELock in the basic procedure.
06-03-2015, 09:20 PM   #12
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AE only locks with AF if you have the camera configured like that.

You can try it yourself:
aim camera at a darker subject around you and half press the shutter; check the reading in the viewfinder
keep the shutter half-pressed and aim the camera at a lighter subject (e.g. the sky) and see how the readings change
06-05-2015, 08:59 AM   #13
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Good Lord - here we go again.

Please people do a search before asking years old questions.

https://www.pentaxforums.com/forums/38-photographic-technique/236970-perfect-...rytime-11.html

QuoteOriginally posted by PDL Quote
Here are links (again) to several reasons to use "Back button Focus".

10 Autofocus Tips - PentaxForums.com
K10 autofocus question - PentaxForums.com
Auto-focus: how do you use it? - Page 3 - PentaxForums.com
Talk me out of selling all my pentax gear - PentaxForums.com
Focus lock on subject - PentaxForums.com
If you can't focus with your Pentax... - PentaxForums.com
2 questions... - PentaxForums.com
Manual focus retouching and Hyperfocal - PentaxForums.com
Do you use the AF button? - Page 3 - PentaxForums.com

Read more at: https://www.pentaxforums.com/forums/6-pentax-dslr-discussion/43995-who-uses-a...#ixzz3Uf67z0zY

and comment #87 on this thread which shows

I have not been commenting on much lately, but here goes.

About using the AF button and de-coupling AF from the shutter button. The following is a list of posts/threads that I have commented in over the last few years. This is not a new subject and it is a revelation only to the uninitiated. Maybe this subject should become a sticky.

Do you guys fire more often in autofocus or manual focus?
https://www.pentaxforums.com/forums/58-pentax-beginners-corner-q/61808-af-opt...you-use-2.html
https://www.pentaxforums.com/forums/58-pentax-beginners-corner-q/75115-do-you...gs-camera.html
Rant about Pentax's One Legged AF and a weird quirk that I wish would go away

quote from this thread
"To those of you using the AF button to - disable AF.

Just who is in control of when the subject is in focus?
Shutter button ==> the camera.
Shutter button -> AF button ==> the camera until the human intervenes - but still the camera.
AF Button ==> the human.
manual ==> the human.

Personally I prefer the last two - I am in control - not the camera."


Another quote from the thread:
"I have attended two photo workshops sponsored by National Geographic. At each one the guy(s) from NG have de-coupled AF from the shutter button - so THEY decide when the object is in focus - not the camera. In fact, one of them (Michael Melford) said that on his Canon MkIII something or another - the only two custom functions he sets up are to display the histogram on the LCD and to de-couple AF. He has at least 5 covers."

AF button on K20
tips on AF-ON usage

Contains a long rant a duplications of threads:
Who uses the Auto Focus button?
How do you focus?
If you can't focus with your Pentax...
Back button Focus

Enjoy -


Read more at: https://www.pentaxforums.com/forums/38-photographic-technique/249785-back-but...#ixzz3Uf72YcFP


Just look around - the answers to your questions are here.
The elitist - formerly known as PDL
06-06-2015, 05:55 AM   #14
mee
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QuoteOriginally posted by PDL Quote
Good Lord - here we go again.

Please people do a search before asking years old questions.

https://www.pentaxforums.com/forums/38-photographic-technique/236970-perfect-...rytime-11.html

ALL of the beginner help questions are years old questions, no need to get ugly about it.

And, to answer the question, half press on the shutter is FINE for your usage, Poompat. I still use the default setup myself as moving the focusing to another button did NOT function well for me. Once you get a feel for the shutter button it isn't an issue, much like driving and getting a feel for the speed pedal -- you can keep your vehicle steady at any speed without even thinking about it.
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