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Author Topic: Understanding AF on the new 650d  (Read 7346 times)

koolman

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Understanding AF on the new 650d
« on: August 05, 2012, 06:08:29 AM »
Hi,

I currently have the 550d rebel. I'm happy with it, and the IQ is very nice for the price.
My only gripe is the auto focus. I use the center point only and re-compose.
However, even this method, seems to be both somewhat slow, and not tack on accurate. I find my fast primes (50 1.4, 35 1.4) lacking in AF accuracy and speed.

My question is: Would the 650d give me any advantage here, as far as AF accuracy and speed, using its center point only ? I understand that even on the 550d the center point is extra sensitive ? On the other hand, I seem to see on some websites an explanation showing the center point of the 650d even better (like the 60d) ??

Can one of our tech oriented seasoned members clarify this for me?

Is there ANY real advantage for stills - moving from 550d to 650d (another question) ?
Jerusalem Photographer (canon t2i, 50 1.4, Tamron 17-50 non VC, canon 60mm, canon 35mm L,Samyang 14mm MF,Voigtlander 20mm MF)

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Understanding AF on the new 650d
« on: August 05, 2012, 06:08:29 AM »

Louis

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Re: Understanding AF on the new 650d
« Reply #1 on: August 05, 2012, 08:31:04 AM »
all I can say is that it isn't much a jump for you, if I was you, I would consider a 7D that way there will be a huge jump in AF performance.

neuroanatomist

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Re: Understanding AF on the new 650d
« Reply #2 on: August 05, 2012, 01:42:17 PM »
Yes, the 650D is a decent step up from the 500D/550D/600D in terms of AF for stills.  The previous recent xxxD bodies have 9 points, only the center is a cross-type, and it's a single 'hybrid' cross, with one f/5.6 line and one f/2.8 line in a '+' shape, while the outer points are f/5.6 lines.  The 650D inherits the 60D's phase AF - still 9 points, but the center point is a dual cross, an f/5.6 '+' with an f/2.8 'x' superimposed, and the outer points are all f/5.6 crosses.  The f/2.8 lines are more accurate, because they have a wider baseline.

However, I should point out that the better AF itself may not solve the problem for your f/1.4 primes, for two reasons.    First, the xxxD bodies don't offer AF microadjustment (neither does the 60D, although the 50D does, else it's 7D and up). AFMA is especially important for fast primes, where the DoF is thin. Without AFMA, the only option for a front- or backfocusing lens is to send lens(es), and possibly body, to Canon.  Second, even with a perfectly calibrated lens, shooting a fast lens wide open using focus-recompose basically guarantees you a backfocused shot, due to geometry. So, if a new 650D plays nice with your lenses (or you're willing to send to Canon), and if you use those outer cross-type points to AF, instead of focus-recompose from the center, then you should see sharp focus (as sharp as possible, at any rate - do keep in mind that lenses aren't at their sharpest wide open).

Hope that helps...
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bdunbar79

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Re: Understanding AF on the new 650d
« Reply #3 on: August 05, 2012, 01:46:35 PM »
Here's a question.  I'm a low-tech guy.  (I'm a chemist for crying out loud).  In order to enjoy the types of AF sensors and points on any camera, do you have to active them first?  In other words, can I enjoy all the cross-type sensors if I only have center AF point selected?  Or are the AF points themselves independent?  I like just doing my shooting with center AF even in sports, but I need to take advantage of the AF accuracy of the 1DX.  Thanks!
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neuroanatomist

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Re: Understanding AF on the new 650d
« Reply #4 on: August 05, 2012, 02:31:50 PM »
You only get the benefit of a specific AF point if it's selected, either manually or automatically (via expansion, zone, or full auto selection).  The only exception is that all the points work together for extreme defocus detection, but that just gets you in the ballpark when needed.
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koolman

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Re: Understanding AF on the new 650d
« Reply #5 on: August 05, 2012, 03:36:28 PM »
Yes, the 650D is a decent step up from the 500D/550D/600D in terms of AF for stills.  The previous recent xxxD bodies have 9 points, only the center is a cross-type, and it's a single 'hybrid' cross, with one f/5.6 line and one f/2.8 line in a '+' shape, while the outer points are f/5.6 lines.  The 650D inherits the 60D's phase AF - still 9 points, but the center point is a dual cross, an f/5.6 '+' with an f/2.8 'x' superimposed, and the outer points are all f/5.6 crosses.  The f/2.8 lines are more accurate, because they have a wider baseline.

However, I should point out that the better AF itself may not solve the problem for your f/1.4 primes, for two reasons.    First, the xxxD bodies don't offer AF microadjustment (neither does the 60D, although the 50D does, else it's 7D and up). AFMA is especially important for fast primes, where the DoF is thin. Without AFMA, the only option for a front- or backfocusing lens is to send lens(es), and possibly body, to Canon.  Second, even with a perfectly calibrated lens, shooting a fast lens wide open using focus-recompose basically guarantees you a backfocused shot, due to geometry. So, if a new 650D plays nice with your lenses (or you're willing to send to Canon), and if you use those outer cross-type points to AF, instead of focus-recompose from the center, then you should see sharp focus (as sharp as possible, at any rate - do keep in mind that lenses aren't at their sharpest wide open).

Hope that helps...

Thanks for elaborating Neuro !

Yes that was I was asking, basically if the 650d inherits the 60d's AF system.

I understand the problem with MF adjustments.



 
Jerusalem Photographer (canon t2i, 50 1.4, Tamron 17-50 non VC, canon 60mm, canon 35mm L,Samyang 14mm MF,Voigtlander 20mm MF)

Merlin_AZ

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Re: Understanding AF on the new 650d
« Reply #6 on: August 06, 2012, 07:46:13 PM »
Does the STM lens on a T4i focus slower than a non-T4i with a non- STM lens?
Or more clearly, how does the focusing speed on the T4i with an STM lens compare with something like the 60D with a non-STM lens?

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Re: Understanding AF on the new 650d
« Reply #6 on: August 06, 2012, 07:46:13 PM »