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When I take a portrait with settings which result in a DOF shallow enough that the subjects' movements risk missing focus on the eyes, should I use continuous AF mode - is the continuous AF fast enough to compensate little movements of the model/photographer or should I stick with the AF-S mode?

  • Please define your understanding of tiny DOF. Because for example mine is < 0,1 mm – Romeo Ninov Dec 9 '15 at 7:45
  • I had no certain value in my mind - I would say 3 cm. – sid_com Dec 9 '15 at 10:02
  • In such case IMHO its not need to use continuous AF, the movement of model will be not so much :) – Romeo Ninov Dec 9 '15 at 11:31
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    I think for this purpose, a reasonable definition would be "shallow enough that the subjects' movements risk missing focus on the eyes". – Please Read My Profile Dec 9 '15 at 21:23
  • Changed accordingly to mattdm's proposals. – sid_com Dec 10 '15 at 5:01
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With a depth of field of 3 cm/1+ inch, there isn't much room for error. Since you say you're shooting a portrait I'll assume this is a portrait of a person and not a static subject. (If it were a static subject, though, I would tell you to lock the camera down on a tripod!) But, on the topic of a portrait: what kind of results are you after? With a DOF of 3 cm you are able to get your subjects nose in focus. DOF won't reach to their eyes or ears.

But, ignoring specific depth of field for a moment and trying to better answer the question of continuous or single AF mode: yes, continuous mode can be very helpful in achieving focus lock when using shallow depth of field. Most specifically, look at sports photography: shooters are after slim DOF to highlight one or a few players and the players are often moving fast. Continuous AF is often a good way to follow them to get an in-focus shot (or, more correctly, continuous AF is an important part of following them to get an in-focus shot).

There's more than one way to create the shallow depth of field that is often associated with portraits, however. We most typically talk about using a large aperture to do it, but another thing to consider is subject-distance relationships: the farther away the background, the more out of focus it will be. Depending upon lens focal length and aperture, f5.6 or f8 can create the effect of a shallow DOF if the background is far enough away. Likely helpful reading: What exactly determines depth of field?

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  • +1 for ignoring the specific DOF, but I think the last paragraph could use an edit. Smaller aperture always creates larger DOF, and changing the camera-subject-background distances is just a way to get a similar bokeh effect with that larger DOF. You can't get eyes in focus with blurry ears with smaller aperture unless your client is Helen Parr. – Caleb Jan 8 '16 at 22:22
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I would not rely on continuous AF for that purpose.

You have a choice about how much DOF you use. There is an unhealthy obsession these days with extremely small depth of field in the mistaken belief that extremely blurred backgrounds are always desirable. In fact you just need some background blurring to create a visual separation, but not extreme amounts. Good composition and lighting will do the rest.

Too much background blur is just as distracting as too little.

So if "tiny DOF" is causing you a problem with focus, the solution is simple : stop using it !

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  • The question isn't about how much DOF is appropriate -- that's an aesthetic choice for the photographer. The important gist of the question is whether single or continuous AF will better help deal with subject movement, and that's a valid question whether you're shooting a face at f/1.2 or a race car at f/4. – Caleb Jan 8 '16 at 22:14
  • It is not purely an aesthetic choice, it directly affects the precision required by the AF system as small DOF drastically increases the accuracy the user is demanding from the AF system. Like all things in photography the user has to balance what is technically practical with what is their own personal desire. You cannot simply make arbitrary choices of settings based only on aesthetic choices. I am suggesting the choice of aperture should be broadened to allow for an improved chance of getting good focus, rather than refusing to do this and having lower odds of an in focus shot. – StephenG Jan 9 '16 at 1:55
  • I didn't say it was a purely aesthetic choice, but all the support for your answer is aesthetic: unhealthy obsession, mistaken belief, you just need some background blurring, too much background blur is just as distracting. None of that explains why you wouldn't use continuous AF for a narrow DOF shot, it only explains why you don't think the OP should take a narrow DOF shot in the first place. – Caleb Jan 9 '16 at 3:05

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