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DSLR hint: what’s that AF-ON button for anyway?

By Noah | October 6, 2010

Somewhat to my surprise, the photography postings in this blog continue to be by far the most popular.  So, here’s another.  This time I want to share some thoughts about careful focusing, and in particular, how and why to use the AF-ON button that’s on some advanced DSLR cameras.  If the camera is capable of focusing automatically as you release the shutter, why would you set set it to use a separate focus button?  It took me awhile to figure that out, but now AF-ON is one of my favorite features.

As with the earlier postings, I refer directly to settings on the Nikon D300, but many other Nikon cameras such as the D300s, D700, D3 variants, etc. do AF-ON in a similar way.  Advanced Canon cameras sometimes call the feature AF-ON, and sometimes Back-Button Autofocus; I don’t have my own Canon to try, but at least the general advice here should still apply.

What does the AF-ON button do?

That’s simple.  Assuming you set the camera in the obvious way, the AF-ON button focuses the camera.  We’ll see later that, on Nikons at least, you’ll want to change some menu settings to keep the shutter release button from undoing the focus you set with AF-ON, but first let’s consider why you’d want to use a separate button for focusing at all.

Why bother to use AF-ON?

Here is a list of some of the reasons I’ve found that AF-ON can be useful:

No doubt there are other good reasons, but that’s a pretty interesting list.  In general, AF-ON is tempting in the situations where you would use manual focus, but for some reason you need the help of the camera to lock in.

Overall, when I’m not in a hurry, I find that focusing as a separate step makes me more thoughtful and deliberate as a photographer.  More on that below, but first, here’s what you’ll need to know to get the camera settings right.

Setting up the Nikon D300 to use the AF-ON button

If you set the camera to its default settings and press AF-ON, the camera will autofocus, but there’s a problem: as soon as you press the shutter release, the camera will refocus.  So, to use the AF-ON button well, you’re probably going to want to change some menu settings.  If you’re nervous about getting your old settings back, read the posting titled:  Nikon D300 hint: Saving your Settings — it explains ways of reseting the camera, or of getting your favorite settings back.

There are various combinations of settings that make AF-ON usable, but here’s what I’ve settled on.  On the front of the camera, set focus mode to AF-S (the one that locks focus); on the back, set the AF area mode switch to Single Point.

The most obvious change you’ll want to make is to tell the camera not to refocus when you press the shutter.  To do this, find the menu item named A5: AF Activation and change it to “AF-ON only”.  Now try pressing the shutter button, and you’ll see that the camera does not autofocus;  press the AF-ON button and it will.

You might think you’re done, but there’s a bit of a problem: if you use AF-ON to focus on something, then point the camera somewhere else, it probably won’t take pictures at all!  Why?  Go to the menu item named: A2: AF-S Priority selection.  The default for that is “Focus” priority.  This setting tells the camera not to take a picture unless the object under your chosen focus point (usually the center of the image) is in focus.  In fact, if you hold the shutter release down and swing the camera back to point to the original object, it will probably fire off as soon as it passes an object that is in focus.  (That trick is called trap focusing, and indeed it can be very useful for things like getting a race car as it crosses the finish line — it’s mostly not what you want for typical uses of AF-ON).

To fix this lockout problem, change the A2: AF-S Priority selection to “Release priority”.  That tells the camera: shoot even if what’s under the focus point is out of focus.  Of course, you probably do want your subject in focus, but with AF-ON you tend to get that set long before you release the shutter, and when recomposing, the camera’s selected focus point may wind up on something else.

I put these A2 and A5 menu settings in a separate D300 Control Bank that I name AF-ON, which happens to be Bank B.  To use the camera normally, I let the camera default to Bank A.  When I want AF-ON only, I just select Control Bank B, and both settings A5 and A2 get reset.  I can go back and forth anytime.  If you use AF-ON rarely, just change the two settings manually when you need them, and then reset them when you’re done.

A bit more about low light, AF-S, AF-C, and flash

Expert readers will note that AF-C mode works with AF-ON too.  When you do this,  the camera will focus and refocus as necessary as long as AF-ON is pressed.  What’s more, because the default for a1: AF-C priority selection is “Release”, you don’t have to override that.  In most situations, there’s absolutely nothing wrong with using the camera this way;  I occasionally do, though I usually prefer seeing the camera lock focus, as it does with AF-S.

Where AF-C doesn’t work at all is if you want the aid of the focus assist light, either from the camera, or from the infrared on the SB-800.  I find that to be tremendously valuable, and it’s available only with AF-S.  You also have to be sure to select the center focus point, or else the assist light will stay off (I wish Nikon let you override that).

Anyway, I prefer using AF-S and a control bank to override both A2 and A5, but you may prefer AF-C.  Of course, if you’re shooting sports or other scenes where focus is changing rapidly, AF-C is likely the way to go, whether you use AF-ON or not.

Exposure and AF-ON

If you recompose your picture after focusing with AF-ON, keep in mind that the spot-metering mode sets exposure based on the selected focus point. Unless you also use the AE-lock function, metering will be done just as you press the shutter release. So, if you’re spot metering, you can’t forget about where the focus point is just because you’ve already focused with AF-ON.

Using spot metering with AF-ON may only come up occasionally, but many photographers will use flash with AF-ON. I can’t prove it, but my impression is that D300 flash metering also pays attention to the focus point. So, even if you use AF-ON to lock focus, when you’re actually shooting, try to have the focus point over whatever you want the flash to expose correctly (you may want to set an off-center focus point to make this convenient, and then use that with AF-ON).

In praise of thoughtful focusing

(OK, I’m going to run on here a bit, and if all you want is to learn to set up AF-ON, you can stop reading.)

What I’d like to do in closing is to emphasize a bit more the rewards of more thoughtful and deliberate focusing, however you do it. Before autofocus became widely available in roughly the 1980s, learning to focus was a skill that photographers would practice for years.  Some of that was out of necessity: taking sharp pictures in a hurry required that skill.  Some of it was the satisfaction of being good at the craft of photography and using your tools well, but ultimately it was a reflection of the fact that carefully selecting focus and controlling depth of field is as fundamental to each picture as choosing an appropriate film, composing properly, and exposing correctly.  Focusing manually makes you conscious of focus.  Do want the front eye or the ear to be sharp in that portrait?  Are you happy with the depth-of-field?  Etc.

Manual focus also has a different rhythm.  Using it encourages you to slow down, concentrate on the image you’re getting ready to shoot, maybe even think harder about other settings like exposure, Picture Control (if using a Nikon to shoot JPEGs), etc.

Of course, choosing focus points carefully is especially important when you are using wide apertures or long lenses, where depth of field is small.  Indeed, many photographers don’t realize that for a portrait taken at F2.0 with a 85mm lens on a D300, sharp focus might extend only an inch or two in front and behind the actual focus point (see the very handy online depth of field calculator).   If the eye is in focus, the nose and the ear almost surely aren’t.   Using AF-ON or manual focus can help you top stop and look at the results of your focus choice before you go ahead and shoot.

What about true manual focusing vs. AF-ON ? When I’m working slowly (and even sometimes when I’m working fast), I prefer true manual to any form of autofocus, but not on my D300.  I’ve focused SLRs manually for over 40 years, and the D300 is one of the few that just makes it hard:  the focusing screen is not tuned for it.   Even with wide aperture primes it can be a struggle to really see when focus is perfect, and with 12 megapixels, one can blow up to the point where focus has to be perfect.  (When you’re working really slowly, especially on a tripod, one way around this is to use Live View, and zoom in — that’s my preferred way of manually focusing the D300 on a tripod.)

Sometimes,  the AF-ON button can get you the best of both worlds:  the thoughtful control of confirming focus carefully, with the assistance of the camera to actually do the focusing.  Especially given the challenge of manually focusing cameras like the D300, it can be a very helpful compromise.

Still, if you get nothing else out of this post, I hope you’ll think about the pleasures of composing and focusing slowly and thoughtfully, whatever means you use.

If you found this post useful, you might want to look at or subscribe to some of the other Arcane Domain photography postings.  They’re available from the Arcane Domain Photography Feed.

Topics: Photography | 9 Comments »

9 Responses to “DSLR hint: what’s that AF-ON button for anyway?”

  1. Teresa Says:
    October 18th, 2010 at 11:38 AM

    I am going to bookmark this. What I really need to do is save my current settings, create this new setting and save it also then I can move between them. Unfortunately I haven’t had much time for shooting at all lately and so I’m just happy when I can pick up my camera and have a bit of time to grab some shots.

    I wish I could do the manual focus, but my eyes aren’t all that good and when I have tried manual focus it’s only worked in a couple of instances. (water reflection pictures)

    I’m still getting used to the differences between my D80 and my D300. LOL. Ah to have the time to putter about with my camera. One day maybe. In the meantime I grab it when I can and enjoy the time I do have to take pictures.

  2. Noah Says:
    October 18th, 2010 at 8:43 PM

    Teresa wrote:

    > What I really need to do is save my current
    > settings, create this new setting and
    > save it also then I can move between them.

    One good way to do that, as I sort of implied, is to start by switching to a new Control Bank (maybe B instead of A). Now, make the changes, which will be associated with Bank B, and will come back whenever you switch to B. When you switch back to A, you’ll get whatever you started with. Remember though: these focus settings are in the >control< bank; thinks like jpeg vs. raw, iso, etc., are in the >shooting< banks. If you find that you're really using that B bank a lot, there's an option in the menus to give it some useful name, so you'll remember what it's for in a few weeks. For just playing around, don't bother. Good luck! Noah

  3. Jeannie Says:
    January 28th, 2011 at 11:52 PM

    More, more! I enjoyed reading your d300 tips on your blog. After a year of owning my D300 with no previous DSLR experience, I still have much to learn about my camera and passion for the art of photography. Your tips were very helpful! Please keep posting!

  4. Noah Says:
    January 29th, 2011 at 11:23 AM

    Jeannie, thank you so much!

  5. CalOldBlue Says:
    March 28th, 2011 at 8:24 PM

    Thank you thank you thank you!

    You have just cemented some thoughts I’ve been having over the past few days. I’ve been shooting 35mm for almost 50 years now. Argus C4 (sic) to Mamiya-Sekor 500DTL to N6006 to D70.

    I’m finally pulled the trigger on a D700 two weeks ago, and have been pondering the myriad focusing options I now have to choose from.

    I was reaching the conclusion I’ve gotten lazy in my shooting habits, and the D700 was making it worse, then I started to read about the AF-ON button.

    You are absolutely right; letting the camera (smart as they are getting) choose the focus point is abandoning a key element of composing a great photo (aperture/shutter speed being the other we tend to let the electronics take over).

    Thank you for putting it so cogently.

  6. Noah Says:
    September 22nd, 2011 at 1:39 PM

    Jason Odell has a nice post on using AF-On in his Luminescent Photo Blog.


  7. youcan Says:
    February 19th, 2013 at 6:13 PM

    Very useful post, thank you very much!

  8. max Says:
    April 16th, 2013 at 9:59 PM

    very useful guide, it helped me a lot

    there is one thing that is not very clear to me, and I’m probably missing something: what’s the point of setting the AF-S priority to release? wouldn’t be the same result to just set the camera to AF-C?
    by using AF-C you’d have the option to chase a moving subject simply holding the AF ON button.

    In my opinion if you change the AF-S priority you lose the only remote utility of the AF-S in AF ON mode which is to act as a ‘Focus trap’. Perhaps you do this to be able to use the AF assist light?

    I can’t choose the AF-S priority on my D80, so I am keeping it in AF-C so that I can recompose.

  9. Noah Says:
    April 17th, 2013 at 10:09 AM

    Max: thank you for the comment.

    As I said in the section “A bit more about low light, AF-S, AF-C, and flash”, using AF-C is a fine choice, and many people do use that with AF-ON. I don’t know about the D-80, but on the D-300, there are some pros and cons, which I think I discuss there.

    First of all, whether you’re using AF-ON or not, AF-S tends to give you a focus lock indication, and once focus is locked it doesn’t move. So, you can press AF-ON, watch where the focus locks, make sure you’re happy, and then shoot.

    The other reason I gave, which is very important for some of the work I do indoors, is that with the D300 and SB-800 flash, the focus assist infrared beam is turned off when you’re in AF-C. For work in really low light, sticking with AF-S let’s you use the assist beam to focus, then shoot in near darkness.

    Overall, the way I think about it is this: there are situations, such as when subjects are moving, that AF-C is the right mode to use, with or without AF-ON. Use it. In situations where the subject is not moving (landscapes, still lifes, many portraits, etc.), I tend to prefer AF-S, whether or not I’m using AF-ON. With AF-C, you don’t get to check the choice made by the camera until after the shot.

    Still, I understand that many people just use AF-C for everything. No reason not to if you prefer that.


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