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Author Topic: Autofocus selection points...  (Read 1618 times)

Scarpz13

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Autofocus selection points...
« on: December 01, 2012, 05:42:11 PM »
Sorry if this is a re-hash of an old topic; I was wondering what people general use for auto-focus points.
I have a 60D, and love to use my 50mm 1.4 and 85mm 1.8 for potraits and indoor photography. With most of my lenses I use the center point and recompose.
I know this isn't ideal with such a shallow depth of field. Especially when taking pictures of 3 or more people who are standing in a group, I find inevitable at least one person is out of focus.
Is there any value in selecting ALL autofocus points and letting the camera choose? is this better or worse when using one of the above lenses wide open, or stopped down to F2 or so?

thanks for your help!
5D MkIII  |  60D  |  EF 17-40 F4L |  EF 24-105 F4L IS  | EF 70-200 F4L IS  | EF 40 f/2.8  |  EF 50 f/1.4 |  EF 85 f/1.8  | 580EX II

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Autofocus selection points...
« on: December 01, 2012, 05:42:11 PM »

neuroanatomist

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Re: Autofocus selection points...
« Reply #1 on: December 01, 2012, 06:13:36 PM »
The only time to let the camera pick the AF point is during AI Servo tracking (and even then, you're picking the starting point).

Center point focus-recompose = backfocus with a fast prime shot wide open.

If your subject is off center (mine usually are), select the closest AF point to minimize (or eliminate) recompose.

If you have multiple people in a shot, you almost always need to stop down a bit for deeper DoF.
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PeterJ

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Re: Autofocus selection points...
« Reply #2 on: December 01, 2012, 06:34:04 PM »
It sounds a bit like you want the A-DEP (automatic depth of field) feature that selects an aperture so that near and far objects on the focus points are all in focus. The 450D used to have it but it seems to have been removed from more recent cameras (or maybe XXXD still have it?) but either way it's not available on the 60D.

Scarpz13

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Re: Autofocus selection points...
« Reply #3 on: December 01, 2012, 07:10:10 PM »
The only time to let the camera pick the AF point is during AI Servo tracking (and even then, you're picking the starting point).

Center point focus-recompose = backfocus with a fast prime shot wide open.

If your subject is off center (mine usually are), select the closest AF point to minimize (or eliminate) recompose.

If you have multiple people in a shot, you almost always need to stop down a bit for deeper DoF.

Thank you Neuro- yes I quickly discovered centre point focus- recompose resulted in backfocus. I will take the time to select the closest AF point from now on.

As a follow up- how much does one typically have to stop down for a group shot? I imagine F2.8 gives you a reasonable balance between DOF of low light performance... otherwise why would wedding photogs lug around those huge 2.8 lenses all day- most of the time there at least two people in their shots!
5D MkIII  |  60D  |  EF 17-40 F4L |  EF 24-105 F4L IS  | EF 70-200 F4L IS  | EF 40 f/2.8  |  EF 50 f/1.4 |  EF 85 f/1.8  | 580EX II

neuroanatomist

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Re: Autofocus selection points...
« Reply #4 on: December 01, 2012, 07:26:43 PM »
DoF is affected by aperture (wider = shallower), focal length (longer = shallower), and subject distance (shorter = shallower).  So...50mm f/2.8 at 8' may be fine for two people, but a shot framing just their heads at 85mm f/2.8 at 6' may be too shallow.  There are online DoF calculators (google DoFmaster), iPhone apps, etc., to play around with. After a while, selecting the aperture that's just narrow enough for your subject(s) becomes intuitive. 
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PeterJ

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Re: Autofocus selection points...
« Reply #5 on: December 01, 2012, 07:33:27 PM »
A lot depends on your framing / distance to subject. For example according to a DOF calculator a 50mm on a crop at f/1.4 at 5 meters has a DOF of 0.43m which is ample for two people. At a wedding you often want full length shots that have a lot of background context. Plus for tighter shots you can often get the couple in pretty good alignment with each other.

But yeah for small groups (fairly well in line) and couples f/2.8 is probably a good range, but probably f/4 if you have a few people at the front/back etc. Neuro just posted back while I was typing but that is a good suggestion to have a play with a DOF calculator.

Scarpz13

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Re: Autofocus selection points...
« Reply #6 on: December 01, 2012, 07:36:47 PM »
DoF is affected by aperture (wider = shallower), focal length (longer = shallower), and subject distance (shorter = shallower).  So...50mm f/2.8 at 8' may be fine for two people, but a shot framing just their heads at 85mm f/2.8 at 6' may be too shallow.  There are online DoF calculators (google DoFmaster), iPhone apps, etc., to play around with. After a while, selecting the aperture that's just narrow enough for your subject(s) becomes intuitive.

Ah! there you go- I knew wider aperture affected depth of field, didn't realize subject distance changed it. Explains why I have the problem more with close up "head shots" on my 50 1.4, then on full body shots.

I will check out DoFmaster for sure... thanks for all your help!
5D MkIII  |  60D  |  EF 17-40 F4L |  EF 24-105 F4L IS  | EF 70-200 F4L IS  | EF 40 f/2.8  |  EF 50 f/1.4 |  EF 85 f/1.8  | 580EX II

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Re: Autofocus selection points...
« Reply #6 on: December 01, 2012, 07:36:47 PM »