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Thread started 09 Dec 2008 (Tuesday) 17:51
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Backfocusing? Need second opinion!

 
ef2
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Dec 09, 2008 17:51 |  #1

After shooting a few test shots around my office, I'm suspecting a lens I received in a trade here is backfocusing.

First attachment is autofocus, tripod-mounted 5D + 85mm f/1.8, MLU and timer. Second is same with slight manual focus adjustment. Both are 100% crops converted from RAW.

Thoughts?


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wimg
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Dec 09, 2008 18:26 |  #2

Looks like it, unfortunately.

Kind regards, Wim


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ef2
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Dec 09, 2008 20:17 |  #3

Hmm that's not good. Of course, if I wanted people's eyes in focus, I guess I can focus on their noses?


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photoguy6405
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Dec 10, 2008 01:15 |  #4

What is "backfocusing"? I see the differences in the pics above, but what is happening to get there?


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wimg
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Dec 10, 2008 02:09 |  #5

ef2 wrote in post #6847417 (external link)
Hmm that's not good. Of course, if I wanted people's eyes in focus, I guess I can focus on their noses?

Well, lips is probably more useful, or nostrils. Otherwise the eyes are out of focus again. :D

Kind regards, Wim


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FlyingPhotog
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Dec 10, 2008 02:13 |  #6

photoguy6405 wrote in post #6848953 (external link)
What is "backfocusing"? I see the differences in the pics above, but what is happening to get there?

Backfocusing = The lens actually focuses on a spot deeper in the frame than where the electronics are showing it should focus.

Frontfocusing = The lens focuses on a spot closer in the frame...

In the OPs example, the red box should be showing that focus is on the thick black line across the middle of the chart but as you can see, the line that's approx 16-18mm behind that is actually sharp. The second frame, the OP has actually manually moved the focus ring to get the middle of the chart sharp.


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KarlosDaJackal
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Dec 10, 2008 02:22 as a reply to  @ FlyingPhotog's post |  #7

Either its back-focusing or you did the test wrong.

How did you measure the 45degree angle? Have to ask as even the manual focus shot is not correct.


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wimg
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Dec 10, 2008 02:25 |  #8

photoguy6405 wrote in post #6848953 (external link)
What is "backfocusing"? I see the differences in the pics above, but what is happening to get there?

Backfocusing is a lens or camera condition which causes the DoF-zone (Depth of Field) after AF to start after the actual point where you focused on, IOW, behind the subject you are trying to focus on.

This is what is happening in the above pictures. It was focused at the bar with the "Focus here" text, but focus ended up slightly behind that bar. This is very irritating when shooting nearby subjects, because they tend to be unsharp as a result, or have the sharpness at a different point than where you really want it. Further away this generally is less of a problem, because DoF, which is much more extended further away, then tends to catch these problems. This problem becomes obviosu nearby and with large apertures (small f-numbers); with smaller apertures (higher f-numbers) DoF grows much larger and include the OOF (Out Of Focus) area, provided the problem isn't too big.

However, in many cases this is caused by user error, as one's body always moves a little. This is why you need a good tripod, MLU (Mirror Lock Up) and a remote switch in combination with the focus test chart, or any other good (static) test subject to test whether a lens or camera is back or front focusing. Frontfocus is where after AF the focus falls in front of the subject, towards you.

If a camera front or back focuses, which is very rare, it tends to happen with all your lenses, and not, f.e., with a different camera, while for individual lenses front or back focus will be the same or similar with different bodies. This probably happens with 4 or 5 out of 100 lenses or so.

Kind regards, Wim


EOS R & EOS 5 (analog) with a gaggle of primes & 3 zooms, OM-D E-M1 Mk II & Pen-F with 10 primes, 6 zooms, 3 Metabones adapters/speedboosters​, and an accessory plague

  
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xarqi
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Dec 10, 2008 03:58 as a reply to  @ wimg's post |  #9

KarlosDaJackal wrote in post #6849151 (external link)
Either its back-focusing or you did the test wrong.

How did you measure the 45degree angle? Have to ask as even the manual focus shot is not correct.

The angle is not at all critical. 45° just makes the marks on the chart actually represent the stated distances, an interesting datum perhaps, but of negligible practical significance where a gross error is apparent.




  
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nightcat
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Dec 10, 2008 04:54 as a reply to  @ xarqi's post |  #10

This is usually a lens problem, not a camera problem. Is this a Canon lens? Is it still under warantee? If so, send it in for adjustment.




  
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ef2
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Dec 10, 2008 07:06 |  #11

It's pretty consistent between my 5D and 1D. It's the Canon 85mm and probably not under warranty. I hope the person I traded my 100mm with will take it back. He said it works fine with his camera.


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Sigma 50mm f/1.4 EX

  
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ito4u
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Dec 10, 2008 09:00 |  #12

You should try to get your old lens back. You could always ask another photog to test it out and see if they have the same issue.


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ef2
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Dec 10, 2008 11:40 |  #13

All the photogs I know use Nikon.

Here's one from an XT here at work that looks slightly better. Sorry for the white bal


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Canon 580EX
Canon 16-35mm f/2.8L | 24mm f/3.5L | 50mm f/1.4 | 100mm f/2.0 | 100mm f/2.8 Macro | 135mm f/2.8 SF | 70-200mm [COLOR=black]f/2.8L IS | 70-300mm f/4.5-5.6 DO
Sigma 50mm f/1.4 EX

  
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photoguy6405
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Dec 10, 2008 12:04 |  #14

FlyingPhotog wrote in post #6849117 (external link)
Backfocusing = The lens actually focuses on a spot deeper in the frame than where the electronics are showing it should focus.

Frontfocusing = The lens focuses on a spot closer in the frame...

In the OPs example, the red box should be showing that focus is on the thick black line across the middle of the chart but as you can see, the line that's approx 16-18mm behind that is actually sharp. The second frame, the OP has actually manually moved the focus ring to get the middle of the chart sharp.

wimg wrote in post #6849162 (external link)
Backfocusing is a lens or camera condition which causes the DoF-zone (Depth of Field) after AF to start after the actual point where you focused on, IOW, behind the subject you are trying to focus on.

This is what is happening in the above pictures. It was focused at the bar with the "Focus here" text, but focus ended up slightly behind that bar. This is very irritating when shooting nearby subjects, because they tend to be unsharp as a result, or have the sharpness at a different point than where you really want it. Further away this generally is less of a problem, because DoF, which is much more extended further away, then tends to catch these problems. This problem becomes obviosu nearby and with large apertures (small f-numbers); with smaller apertures (higher f-numbers) DoF grows much larger and include the OOF (Out Of Focus) area, provided the problem isn't too big.

However, in many cases this is caused by user error, as one's body always moves a little. This is why you need a good tripod, MLU (Mirror Lock Up) and a remote switch in combination with the focus test chart, or any other good (static) test subject to test whether a lens or camera is back or front focusing. Frontfocus is where after AF the focus falls in front of the subject, towards you.

If a camera front or back focuses, which is very rare, it tends to happen with all your lenses, and not, f.e., with a different camera, while for individual lenses front or back focus will be the same or similar with different bodies. This probably happens with 4 or 5 out of 100 lenses or so.

Kind regards, Wim

Thanks for the answers. I learned something new today. Now I can go back to bed. :lol:

Anyway, sometimes I have issues with focusing close-up and have always blown it off as me and my eyes. Are these focusing charts easy to obtain?


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FlyingPhotog
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Dec 10, 2008 13:57 |  #15

photoguy6405 wrote in post #6851298 (external link)
Thanks for the answers. I learned something new today. Now I can go back to bed. :lol:

Anyway, sometimes I have issues with focusing close-up and have always blown it off as me and my eyes. Are these focusing charts easy to obtain?

"Focusing Close Up" .. Maybe you're up against the MFD (minimum focus distance) for the lens in question?


Jay
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"If you aren't getting extraordinary images from today's dSLRs, regardless of brand, it's not the camera!" - Bill Fortney, Nikon Corp.

  
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