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Author Topic: Focus and recompose related question  (Read 4240 times)

rpt

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Focus and recompose related question
« on: May 26, 2013, 06:51:18 AM »
I have been doing focus and recompose since I started to photograph! I mean with my old Lubitel what choice did I have? Same with AE1 and the 300D. Now I always thought that the focus is done along the radius. Is my understanding incorrect? Is it done on a plane perpendicular to the ray of light going through the center of curvature of the lens? I have seen a couple of articles on this but there is no real explanation there. They just say that the focus goes off a bit. Very early on, I would shoot at f8 or f11 or even f16 so I guess I never had an issue with DOF...

If you could point me to some articles that would help.

Thanks in advance.

Rustom

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Focus and recompose related question
« on: May 26, 2013, 06:51:18 AM »

neuroanatomist

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rpt

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Re: Focus and recompose related question
« Reply #2 on: May 26, 2013, 09:36:13 AM »
http://www.visual-vacations.com/Photography/focus-recompose_sucks.htm
Thanks. So what I infer from this article is that selecting an AF point other than the center point the camera computes the angle to the subject and derives the focus plane from that. Is that a correct inference? See none of the articles talk of this...

neuroanatomist

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Re: Focus and recompose related question
« Reply #3 on: May 26, 2013, 10:15:18 AM »
Sorry, incorrect inference. There's no 'computation of angles' on a Canon camera. Using an outer AF point eliminates backfocus if it enables you to focus on your subject without recomposing, reduces backfocus if it means you don't move the camera as much to get the composition you want.

I state 'Canon camera' because some Hasselblad bodies use a gyro sensor to detect camera rotation during recomposition, and adjust focus to compensate.

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rpt

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Re: Focus and recompose related question
« Reply #4 on: May 26, 2013, 10:56:40 AM »
Sorry, incorrect inference. There's no 'computation of angles' on a Canon camera. Using an outer AF point eliminates backfocus if it enables you to focus on your subject without recomposing, reduces backfocus if it means you don't move the camera as much to get the composition you want.

I state 'Canon camera' because some Hasselblad bodies use a gyro sensor to detect camera rotation during recomposition, and adjust focus to compensate.
See, the Hasselblad thing makes sense to me. Surely there has to be some computation of angles internally to identify the focusing plane in order to eliminate the back-focus.

Thanks for the inputs.

neuroanatomist

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Re: Focus and recompose related question
« Reply #5 on: May 26, 2013, 11:44:13 AM »
Surely there has to be some computation of angles internally to identify the focusing plane in order to eliminate the back-focus.

Why does there have to be?  The point is, with a fast lens shot wide open, don't focus/recompose. Use an outer focus point so you don't have to move the camera.  With narrow apertures and/or distant subjects, the deeper DoF masks the backfocus, so focus/recompose is ok.
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Pi

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Re: Focus and recompose related question
« Reply #6 on: May 26, 2013, 12:01:39 PM »
F&R is usually not a problem with longer lenses at not extremely close distances. It is a problem with 24/35 in most real world cases, and sometimes with 50mm.

One technique that works quite well is to use the rear AF button in AI Servo mode, and the outer AF points. This increases their precision tremendously, at least on my 5D2. The precision is similar to the center AF this way, with apertures like f/1.2-f/1.4.

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Re: Focus and recompose related question
« Reply #6 on: May 26, 2013, 12:01:39 PM »

rpt

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Re: Focus and recompose related question
« Reply #7 on: May 26, 2013, 12:49:52 PM »
Surely there has to be some computation of angles internally to identify the focusing plane in order to eliminate the back-focus.

Why does there have to be?  The point is, with a fast lens shot wide open, don't focus/recompose. Use an outer focus point so you don't have to move the camera.  With narrow apertures and/or distant subjects, the deeper DoF masks the backfocus, so focus/recompose is ok.
:)

Sorry, I don't doubt that it needs to be done. The programmer in me is curious about how it is implemented thats all.

Sporgon

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Re: Focus and recompose related question
« Reply #8 on: May 26, 2013, 01:35:27 PM »
I know this is a little off topic, but I have always found the single plane sensitive AF points (as found on the 'outer' points of many SLRs)  quite beneficial. Over the years I have often found situations where the x type sensor can become confused, yet find a vertical line ( or horizontal depending on the AF point you've chosen) and the camera can nail focus.

So I'm quite happy that the 6D along with the 5D mk1 and mk2 has this type of outer sensors, and am quite happy to use them for accurate focus, and to avoid 'focus and re compose'. It just means you must choose the appropriate line to focus on.

alexanderferdinand

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Re: Focus and recompose related question
« Reply #9 on: May 26, 2013, 01:41:07 PM »
Although the 5d2 had a great IQ, the not so good outer AF- fields and focus/recompose sometimes drove me nuts.....

neuroanatomist

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Re: Focus and recompose related question
« Reply #10 on: May 26, 2013, 02:17:23 PM »
Surely there has to be some computation of angles internally to identify the focusing plane in order to eliminate the back-focus.

Why does there have to be?  The point is, with a fast lens shot wide open, don't focus/recompose. Use an outer focus point so you don't have to move the camera.  With narrow apertures and/or distant subjects, the deeper DoF masks the backfocus, so focus/recompose is ok.
:)

Sorry, I don't doubt that it needs to be done. The programmer in me is curious about how it is implemented thats all.

Maybe I'm misunderstanding - are you suggesting that when an off-center AF point is selected on a Canon dSLR, the camera is performing some sort of algorithmic focus correction for the point being off-center?  I don't understand...
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chauncey

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Re: Focus and recompose related question
« Reply #11 on: May 26, 2013, 06:01:57 PM »
That focus point represents a set distance...anything the same distance will be in focus...anything of a different distance will not be in focus.         ;)

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Re: Focus and recompose related question
« Reply #12 on: May 27, 2013, 05:18:21 AM »
http://www.visual-vacations.com/Photography/focus-recompose_sucks.htm

+1 Big THANK to you Neuro! Illuminating article! Enjoyed to read the content!/All the Best to you Neuro!/C
Truly the light is sweet, and a pleasant thing it is for the eyes to behold the sun!

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Re: Focus and recompose related question
« Reply #12 on: May 27, 2013, 05:18:21 AM »

sanj

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Re: Focus and recompose related question
« Reply #13 on: May 27, 2013, 06:47:12 AM »
When you focus and re-compose, you change the focus distance and focus goes off slightly.
Also, how does one use 'focus and recompose' when tracking focus?

It is so important for all who photograph moving objects and large apertures to become totally at home with focus points and back focus. I am getting many more keepers now.

Regards

Singsling

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Re: Focus and recompose related question
« Reply #14 on: May 27, 2013, 07:05:35 AM »
This Guys red line/green line theory is correct, but focus the intended area first i.e. her face and then recompose to include the dress. Result pin sharp where you want it to be on her face with the dress slightly out of focus. In other words put the cart before the horse.
« Last Edit: May 27, 2013, 07:09:47 AM by Singsling »
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Re: Focus and recompose related question
« Reply #14 on: May 27, 2013, 07:05:35 AM »