Alley in Pisa, Italy

by Lars Kotthoff

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In lens reviews, ability to override auto-focus manually is often referred to as a good (and important) feature.

I have rarely, if ever used that feature, so I feel I might be missing out on an important technique. I can see how it could be useful when letting the camera decide which point to focus by, but I always select the focusing point myself. Would overriding auto-focus be also useful with selected AF point?

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Have you never, ever, had occasion to shoot through a foreground element (bushes, grass, fence, etc.)? Or had your camera stubbornly use a focus reference immediately beside the thing you wanted to focus on? Playing the "mode switching" game is tiresome at the best of times. –  user2719 Apr 17 '11 at 10:59

5 Answers 5

up vote 17 down vote accepted
  1. When AF is hunting because of low light or lack of contrast in the subject.
  2. When you're doing intensive macro shooting.
  3. When you don't have enough time to change focus point.
  4. When you want to fine tune your focus.
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1  
Time is a big one - your left hand is already on or close to the focus ring, but often the joystick/pad for selecting focus points is a little out of reach for your right thumb. –  Evan Krall Apr 17 '11 at 9:03
    
People can always use focus and recompose method, but even that might not be fast enough for some situations. –  fahad.hasan Apr 17 '11 at 9:08
    
When AF is hunting because of low light, it takes at least one second before the focus stops moving and manual override is usable. Isn't it much better just to switch the lens to manual? –  che Apr 17 '11 at 9:27
    
About #1, AF hunting - shouldn't the AF system get a fix before starting to override? I remember such instruction from manual for my lens and therefore have just switched to plain MF when AF is hunting. –  Imre Apr 17 '11 at 9:29
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@che: No you don't lose focus if you tune the focus while keeping the shutter half-pressed :) –  fahad.hasan Apr 20 '11 at 4:14

I guess you are referring to the "full-time manual focus" feature which allows you to do manual focus without switching from AF to MF mode.

I find it very convenient if you combine it with back-button autofocus. Then you will never need to fiddle with the AF/MF switch. You can just leave your lens on AF all the time.

If you want autofocus, it is immediately available in the back button (just like with any autofocus lens). If you want manual focus, you can directly turn the focusing ring (as if your lens was in the MF mode).

And you can combine these any way you want. For instance, you can first AF and then immediately after that fine-tune. Or if you notice that AF starts hunting, you can simply give up with AF and use manual focus.


Of course there is a downside, too: it is easier to accidentally change the focus when you are handling the camera.

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Lens having FTM focus can be left in AF all the time no matter you combine it with back-button focusing or not. You can override the AF the moment you turn the focus ring and you can half-press the shutter to regain AF. Can you please confirm? –  fahad.hasan Apr 17 '11 at 11:02
1  
@ShutterBug: I am not saying that FTMF is useless without back-button AF, or that back-button AF is the "right" thing to do. I just posted my observation that I happen to find the combination of FTMF + back-button AF very convenient. :) One scenario that typically requires mode-switching if you don't use back-button AF is the following: you have already set up everything correctly (on a tripod) and you just want to take a picture without changing the focus. –  Jukka Suomela Apr 17 '11 at 11:17
    
I finally got your point! Yes, for composed environment, back-button AF should be more convenient. –  fahad.hasan Apr 17 '11 at 11:38

It is invaluable when accurate focusing is critical to the result as you then need not worry about the sometimes uncertain nature of auto-focus.

This applies particularly in the following situations:

  • macro photos where the depth of field is very small.
  • photos of flat objects such as documents where you want to select a focus point such that the entire field of view is sharp.
  • in some cases where the focus point is featureless, creating difficulties for the auto-focus system.
  • in low light conditions where speed is necessary the auto-focus may prove to be too slow or unreliable.

To get the best results from manual focus, you should use live view and magnify the image. It is possible then to get an exact focus (but for some applications this is rather too slow).

A useful technique is to let the auto-focus make the first selection and then to use the manual focus override to fine tune the focus.

From time to time I like to deliberately use manual focus just to keep those skills alive.

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Sometimes you must manually focus because there are foreground objects that can't be avoided and will draw off the autofocus.

You can also have a situation where the target has enough noise around it to keep the autofocus from working at all. Say a marine mammal popping out of the water in a cloud of spray.

Lastly, when you can't focus and recompose because there's nobody behind the camera--timer or remote trigger--and the auto is going to go astray for some reason. (Or my old SLR that did the focus when you started the timer rather than when it fired. If you were using that timer to get into the shot it would go astray if there wasn't something suitable there.)

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Another one: when the AF can't keep up. I regularly photograph airshows and other aircraft displays. The AF sensor simply can't focus rapidly enough on an aircraft coming in at hundreds of miles an hour, so what you do is you lock focus on a set distance and take your shot when the aircraft gets to that point. Having bright weather so you can use a deep DOF helps a lot in that, as it greatly increases the margin of error you have.

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Most cameras have AI Servo focus for this purpose. –  fahad.hasan Apr 18 '11 at 9:06

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