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07-21-2011, 03:06 AM   #1
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AF vs MF logic and accuracy

Hello

Lately I have been using some manual focus lens. I like to shoot with Catch-in-focus technique, also known as focus trap. Many forum members use this technique.

I have learnt that you can get focus confirmation even when the subject is not completely in focus. So what I do is to press the shutter and keep it pressed while moving the fosusing ring a little bit left until the camara wont release the shutter and then a little bit right.

It takes very little time and results in several pictures from which I pick the one best focused. I use this technique both with moving and static subjects.

This method is not suitable for every situation, but at least it works for me in some cases.

When we shoot in AF mode, the camera decides that a single position in the focusing ring is a good one, or maybe the best one.

Why does the camera allow to shoot in several positions when using focus trap? Is there any way to reduce the margin of error?

Thank you.

07-21-2011, 05:21 AM   #2
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I believe there's a range that the camera considers "in focus". The AF system likely goes to the middle (or first third) of that range, and locks there. Since there is an uncertainty to any measurement (and defining the focus point is a measurement) that's how I'd do it.

In other words, there is a limit to the accuracy of your AF system, but at some point it's obvious that you're not focused anymore. Find the boundaries of your focus range, and about in the middle of that, there's your sharpest focus position. Taht's how I do it when I focus manually(and can't use liveview).
07-21-2011, 08:02 AM   #3
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QuoteOriginally posted by alvarossorio Quote
Why does the camera allow to shoot in several positions when using focus trap? Is there any way to reduce the margin of error?

Thank you.
edited.
07-23-2011, 11:57 AM   #4
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QuoteOriginally posted by alvarossorio Quote
Hello

Lately I have been using some manual focus lens. I like to shoot with Catch-in-focus technique, also known as focus trap. Many forum members use this technique.
I have never been a fan of focus trap as it involves the camera actually doing and confirming the focus which takes just too much time for fast action shots.

As the method is lengthy, here is the link I gave in manualfocus.org

Manual lens for BIF - proven viable (Page 1) - Gallery - Manual Focus Forum


Daniel

07-25-2011, 02:28 PM   #5
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Thank you for the link. I will write some comments after reading it.
07-25-2011, 07:17 PM   #6
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I use Catch-in-Focus and I have experienced like yourself some margin of error. But I believe IMHO that it is linked with the quality of the lens focus ring.

Some lenses have a very accurate focus ring with long ring rotational path like the Voigltander 120mm f2.5. With these lenses, I feel relatively very easy to be more accurate. With less accurate focus ring (eg FA31mm f1.8), I found some greater margin of error.

This is a subjective feel, but I strongly believe that the accuracy of the focus ring has a lot to do with the ease to use a MF lens: https://www.pentaxforums.com/forums/pentax-beginners-corner-q/146443-manual-f...tml.:rolleyes:
07-27-2011, 04:38 AM   #7
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QuoteOriginally posted by hcc Quote
I use Catch-in-Focus and I have experienced like yourself some margin of error. But I believe IMHO that it is linked with the quality of the lens focus ring.

Some lenses have a very accurate focus ring with long ring rotational path like the Voigltander 120mm f2.5. With these lenses, I feel relatively very easy to be more accurate. With less accurate focus ring (eg FA31mm f1.8), I found some greater margin of error.

This is a subjective feel, but I strongly believe that the accuracy of the focus ring has a lot to do with the ease to use a MF lens: https://www.pentaxforums.com/forums/pentax-beginners-corner-q/146443-manual-f...tml.:rolleyes:
Well, most of my manual primes have a nice focusing ring. But from my experience I canīt tell whether that is part of the key. I will check on that. Thank you for your contribution.
07-27-2011, 04:41 AM   #8
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QuoteOriginally posted by danielchtong Quote
I have never been a fan of focus trap as it involves the camera actually doing and confirming the focus which takes just too much time for fast action shots.

As the method is lengthy, here is the link I gave in manualfocus.org

Manual lens for BIF - proven viable (Page 1) - Gallery - Manual Focus Forum


Daniel
I have read your link. It is quite interesting, but I think that technique is not suitable for the kind of photographs I like. But for situations in which the subject is more or less predictable it seems like a very good technique, and your results prove it. Thanks for your help.

07-27-2011, 04:46 AM   #9
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QuoteOriginally posted by bdery Quote
I believe there's a range that the camera considers "in focus". The AF system likely goes to the middle (or first third) of that range, and locks there. Since there is an uncertainty to any measurement (and defining the focus point is a measurement) that's how I'd do it.

In other words, there is a limit to the accuracy of your AF system, but at some point it's obvious that you're not focused anymore. Find the boundaries of your focus range, and about in the middle of that, there's your sharpest focus position. Taht's how I do it when I focus manually(and can't use liveview).
Yes, in the end I will probably test every lens and try to get the "sweet spot" for each of them.

My experience tells me that it is not always the center of the range in which the AF considers the lens to be in focus.

That will take some time but might end in very good results.

Alvaro
07-27-2011, 06:25 AM   #10
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QuoteOriginally posted by alvarossorio Quote
My experience tells me that it is not always the center of the range in which the AF considers the lens to be in focus.
DOf calculators will show you that about a third of your "in-focus range" is in front of your focus point, and two thirds behind it. That's why I said I would probably place the AF point at the first third of the range.
07-27-2011, 09:30 AM   #11
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QuoteOriginally posted by alvarossorio Quote
Why does the camera allow to shoot in several positions when using focus trap? Is there any way to reduce the margin of error?
With "focus trap"/"catch-in focus" your focus accuracy is only as good as the underlying AF system. The precision of the focus ring is not part of the equation, though the speed with which you move the ring (moving through the zone of focus) might be.

Simply put, even the best AF systems will miss critical focus for many subjects. This is because the AF "patch" used to establish focus may or may not correspond with the actual desired subject or may be too wide for adequate precision. Factor in limited DOF with faster lenses and/or higher magnification and your result will definitely vary. A few classic cases where AF failure is higher:
  • Receding plane with limited DOF
  • Semi-profile portrait (focus should be on eye, AF chooses bridge of nose or near edge of orbit)
  • Any natural light macro, particularly of low contrast subject
If critical focus with manual focus lenses is essential, the best solution is to use an aftermarket focus screen. The focus aids (microprism and/or split image) make all the difference in the world. For macro work, a fine matte field is preferred and works best (may also be preferable for portrait work as well).


Steve
07-27-2011, 10:14 AM   #12
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QuoteOriginally posted by vrrattko Quote
I've encountered opposite problem on K10D. Since installing Katzeye, i noticed I have my focus confirmed, but split screen shows it slightly out of focus. Strange is that my focus confirmation is usually more precise than split screen. Maybe the Katzeye focusing screen is misaligned? Or I need to visit eye doctor...
The above was posted by vrrattko in the thread https://www.pentaxforums.com/forums/pentax-dslr-discussion/103297-focus-confi...-question.html

I have exactly the same problem with my K10 and Katzeye screen, so I canīt trust on visual focus confirmation.

Thanks for your help.
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