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:
- Working faster: you’d think that having a separate step for focusing would slow you down, but not necessarily. If you’re going to take multiple pictures of the same subject, and the desired focus point isn’t going to change, then it can be faster and more reliable to set focus with AF-ON, and then fire off as many shots as you need. There will be zero autofocus delay, and no risk that the camera suddenly decides to focus on something different as you’re trying to shoot. Shooting a baseball batter with a long lens? Just use AF-ON to lock in on the plate once, and fire away.
- Avoiding repeated recomposition: everyone has the experience of focusing, holding the shutter button half way down, recomposing, shooting, then going back to do it again for a second shot, and a third. It’s slow, distracting and error-prone. Even if you take the trouble to choose an off-center focus point (using the thumb wheel rocker), there’s still a tendency to have to move the camera a bit after focusing. AF-ON tends to avoid repeating that.
- Being sure the focus is where you want it: slowing down can be a good thing. Using the AF-ON button gets you to stop and think about focus before you shoot.
- Low light: this is one of the main reasons I use AF-ON. When shooting in really low light, the camera will sometimes hunt for focus, and it can be way too dark to see for manual focus. Flashes like the SB-800 have infrared focus assist lights that will come on as long as you are in AF-S focus mode and using the center focus point. A great trick is to use AF-ON to get focused on something you know is about the right distance. It can be a person, a point on the floor, whatever. You can do this with the assist light if you like, or else by just looking carefully if the camera is locking focus without it. Then, even in the dark, you’ll know the camera is focused there. Even better, the camera will not go hunting to focus just when you fire off a shot. I use this technique all the time when shooting dancers at parties or in dark clubs: I just pick a point where the dancers are likely to be, choose an aperture that gives a few feet of depth of field, AF-ON to focus, and then snap pictures just as the dancers hit the pose I want.
- Panoramas: if you’re taking multiple shots to stich into a panorama, you’ll want the focus (and exposure!) consistent on all of them. Use AF-ON, or focus manually.
- Using manual override: many modern lenses let you refocus the lens manually, even without setting the focus mode on the camera to M. Without AF-ON, though, the camera will override your manual choice as soon as you press the shutter. With AF-ON, you can mix-and-match. With a thumb on the AF-ON button, and your other hand ready to grab the lens focus ring, you can switch quickly from auto to manual, or use manual to tune after autofocusing. Do be careful not to hold the ring while the camera is trying to turn it though!
- Preference: some photographers just don’t like the camera focusing as they take a picture.
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.