AF-On vs. Shutter button

cab.in.boston

I'm sure this has been covered before, perhaps a million times, although I didn't see anything that really answered me when I searched.

My D90 doesn't have a dedicated button, but I can program my AE-L/AF-L button to be an AF-On, and I've certainly noticed the button on other DSLRs. I played around with it a bit, and I've searched online for discussion, but have not really found an explanation that really satisfies my curiosity. So the question is, why and under what conditions is it useful or desirable to have the AF activation separate from the shutter release? I just don't really get it. Any real conditions under which you would use this would really help me understand.

Thanks!

borrowlenses.com

Here's Canon's reasons

http://www.usa.canon.com/dlc/controller?act=GetArticleAct&articleID=2286

colourbox

Someone was just posting about this the other day, it was the first time I'd heard of back-button focusing.
http://www.birchtreephotographyblog.com/2010/01/the-camera-technique-that-changed-my-life/

D'Buggs

You know...... I recently bought a new D300 and have YET figured out how to activate the shutter so that the AF 'locks' its intial focus. The damned thing wants to track and will only lock using that s t u p i d little button. Ive searched all the menue op's and can't find the "switch".

cab.in.boston

D'Buggs wrote:

You know...... I recently bought a new D300 and have YET figured out how to activate the shutter so that the AF 'locks' its intial focus. The damned thing wants to track and will only lock using that s t u p i d little button. Ive searched all the menue op's and can't find the "switch".

Is that just the difference between AF-S (not to be confused with AF-S lenses, naturally... thanks for all the acronyms!) and AF-C modes? AF-S should only focus once and lock focus as long as the button is pressed, while AF-C will continue to track. And in AF-S mode, you can't shoot unless it's locked, although I think you can change that from focus priority to release priority.

Wil Davis

D'Buggs wrote:

You know...... I recently bought a new D300 and have YET figured out how to activate the shutter so that the AF 'locks' its intial focus. The damned thing wants to track and will only lock using that s t u p i d little button. Ive searched all the menue op's and can't find the "switch".

I think what you're looking for might be explained on p.271 of the user manual:

a5: AF activation

If you set to "AF-only" the auto-focus can be performed only by using the AF-on button

(I think this is the Nikon equivalent of "back-button focusing")

HTH -
- Wil

jfriend

I love AF-ON. It took me several days of shooting to get used to it, but it's all I use now. The advantage of using AF-ON (or any button other than the shutter) for triggering focus is that you can have continuous focus or static focus merely by how you press the focus button. You don't have to switch you camera between AF-S and AF-C.

Here's an example. I'm shooting a soccer game. Clearly, I'm using continuous focus a lot to track players as they play so my camera is in AF-C focus mode.

Now, there's a corner kick. The ball is places on the ground, the player gets a running start and I want to capture the player right as they kick the ball. The best way to get this shot is to pre-focus on the ball, sitting on the ground. You know the player is going to arrive at that exact spot at the exact moment that you want the shot. It's a foolproof way to get the focus right on the shot. Ahhh, but you're in AF-C so you can't normally pre-focus because as you move the camera, focus will continue to be active. Without AF-ON or it's equivalent, you'd have to either skip the pre-focus and just use AF-C for the whole sequence or switch the camera to AF-S, pre-focus on the ball, take the shot, then immediately switch the camera back to AF-C so you can follow the rest of the action in front of the goal. Trust me, switching between AF-S and AF-C in the middle of the action means you will miss some shots.

So, with AF-ON, you don't have to change your camera settings at all. If you press and hold AF-ON, you get continuous focus for as long as you hold it down. If you aim the focus sensor to a spot you want to pre-focus on, press AF-ON, acquire focus, then release AF-ON, you are now pre-focused on that spot. You can aim the camera anywhere and focus will not change as you use the shutter. So, you can have it either way, without every changing your camera settings. Trust me, if you ever shoot action, it's great to be able to have either continuous focus or static pre-focus without changing any camera settings!

It does take a little while to get used to AF-ON. For the first several days of shooting, it took conscious thought to get focus right, to press the AF-ON button at the right time, etc... Over time, it just became automatic and I don't even think about it any more.

So, while I really like AF-ON, I am not religious about it. There are those who swear that it's the only way to get accurate focus or you can't get good shots without it. It just gives you the flexibility of having both continuous or dynamic focus merely by changing how you press the AF-ON button. If you're only shooting in continuous focus, it doesn't give better results than triggering with the shutter. If you're only shooting in static focus, it doesn't give better results than trigger with the shutter. Its advantage comes when you want to go back and forth easily.

FYI, I find I like it for macro on a tripod too. I can hit and release the AF-ON button to acquire focus and then decide if I want to manually tweak the focus a bit by rotating the lens. With the shutter trigger, I was sort of stuck in either manual focus or auto focus and couldn't easily go back and forth.

D'Buggs

Thanks, cab.in.boston. Thanks, Will Davis.

Yup, the directions haven't been consulted yet. I thought I could figure it out from my familiarity w/ the D80 - Obviously, not a chance. And according to jfriend's post, I think I need too read up on it as I can see a benefit of the 'back button' now.


Hopefully I didn't derail this thread too bad.....

cab.in.boston

Thank you, John. That gives me a better idea of why and when you would want to use this than the various internet searches (including Canon's explanation) has done.

I will definitely play around with this some more, although I don't think I could dedicate myself to this unless and until my wife and I aren't sharing a camera anymore. She would go nuts if I set up the D90 to focus with the AE-L/AF-L button and forgot to change it back.

jfriend

cab.in.boston wrote:

Thank you, John. That gives me a better idea of why and when you would want to use this than the various internet searches (including Canon's explanation) has done.

I will definitely play around with this some more, although I don't think I could dedicate myself to this unless and until my wife and I aren't sharing a camera anymore. She would go nuts if I set up the D90 to focus with the AE-L/AF-L button and forgot to change it back.

You are correct with that observation. Taking the focus off the shutter button makes it hard to pass off your camera to someone else. I regularly have that problem when I just want someone else to take a family snap with me in it.

I don't know about the D90, but on the D300, you can set up shooting banks so you can quickly switch to a shooting bank that uses the shutter to trigger focus again, but even that's a pain. I don't regularly share my camera with anyone else so I live with that slight inconvenience.

cab.in.boston

jfriend wrote:

I don't know about the D90, but on the D300, you can set up shooting banks so you can quickly switch to a shooting bank that uses the shutter to trigger focus again, but even that's a pain. I don't regularly share my camera with anyone else so I live with that slight inconvenience.

I'm almost positive that the D90 does not have that ability. I am in lust mode for a D300s and so I saw the shooting banks on that body, but I either haven't looked hard enough on the D90, or it doesn't have it.

One other question. When using AF-S mode and the AF-ON button, once you acquire focus lock, do you have to continue to hold down the button after you recompose in order for it to fire, or does it then just let you shoot away, even if you've moved the focal point away from your subject. I believe on the D300/s, you can set AF-S mode to be release priority, so maybe you've programmed your camera to allow anytime shooting, but I find it very annoying when the camera won't let me shoot because it thinks it's not in focus.

jfriend

cab.in.boston wrote:

I'm almost positive that the D90 does not have that ability. I am in lust mode for a D300s and so I saw the shooting banks on that body, but I either haven't looked hard enough on the D90, or it doesn't have it.

One other question. When using AF-S mode and the AF-ON button, once you acquire focus lock, do you have to continue to hold down the button after you recompose in order for it to fire, or does it then just let you shoot away, even if you've moved the focal point away from your subject. I believe on the D300/s, you can set AF-S mode to be release priority, so maybe you've programmed your camera to allow anytime shooting, but I find it very annoying when the camera won't let me shoot because it thinks it's not in focus.

You would have to be in release-priority for it to allow you to a shot when you've moved away from what the camera thinks it wants to focus on.

I don't have a D90 or the manual for it. This article makes it sounds like the D90 is focus priority only for AF-S and release priority only for AF-C with no way to override either one, but I can't tell for sure. You can set both to either on the D300. You will probably have to confirm this for yourself either in the D90 menus or the manual.

One thing to remember is that once you're using a different button to trigger focus, you NEVER use AF-S again because you get the same functionality in AF-C by just pressing the focus button and then releasing it. At that point, you have acquired focus, but you are free to reframe however you want and press the shutter anytime. Since that's what you use AF-S for, there is no need to ever use AF-S again.

D'Buggs

Good thing I tuned in. I'm meeting w/ clients this evening and while waiting, I had too get my dgrin fix ...... The manual is NOW in my jacket pocket.

CRS seems to be problematic, lately.
Now lets see if I remember to read it.

D'Buggs

D'Buggs wrote:

Good thing I tuned in. I'm meeting w/ clients this evening and while waiting, I had too get my dgrin fix ...... The manual is NOW in my jacket pocket.

CRS seems to be problematic, lately.
Now lets see if I remember to read it.

Got it aaall sorted out now and for the next lit'll bit, I'm going to be a "back-button" kinda guy . WTH,,,, I'll give 'er a go and see if it sticks.

Thanks for the help, guy's.