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11-03-2010, 10:18 PM - 1 Like   #1
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Front/Back Focus issue - Is it the camera's problem or is it the lens?

Out of boredom, I decided to test all my lenses for front-focus and back-focus problems. The test did not go very far since the results were very confusing.

My test setup:
0) Camera: K-x
1) I'm using the focus chart found here focustestchart.com/focus21.pdf
2) Lighting: CFL (not tungsten)
3) AF point is center. I tried both AF-S and AF-C mode, no difference

What I observed is that all the following three lenses: FA 77mm, FA 43mm, FA 31mm are back-focused quite a bit using phase detection AF, from 6mm to 10mm. When I switched to LiveView, all the focuses are almost spot on. In LiveView, I tried two different ways of focusing: (i) zooming in and (ii) just relying on the small circle square. Both methods were consistently producing correct focus.

My questions are: Is my setup correct? Assuming it is, is it the problem with the camera? What kind of tests should I do more before contacting Pentax to have it adjusted?

I was actually quite happy with the real life results from the FA 43mm and the FA 77mm, but I wasn't as impressed with the real life results from the FA 31mm.

Update 1:
I've just adjusted the focus of the K-x to +90um to fix the back focus issue and it seems like four out of five of my lenses are focusing correctly now: FA 31mm, FA 43mm, FA 77mm, and F 100mm macro. The DA 15mm is now a tiny bit front focus, but since it starts at f4.0 and I usually use it for landscape where I shoot from f8 anyway, it's not that much of a problem. I wonder if the indoor lighting actually affects the lens' focus system? I will probably try it outdoor tomorrow to see if the results are still the same

Update 2: Took the camera and the focus test chart outside with plenty of natural light, no detectable difference in test results. All lenses are almost spot-on. My next question is: Is it better to be back-focused or front-focused? Say if I can't find a perfect setting for all my lenses (since the K-x only allows one global adjustment) and have to make some compromise between them, is it better to have FF lenses or BF lenses?


Last edited by vtqanh; 11-04-2010 at 08:49 AM.
11-03-2010, 10:42 PM - 1 Like   #2
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If I understand the internals correctly, BF or FF occurs when the distance from the back of the lens to the sensor is slightly different than the distance to AF sensor. When you use Live View you cut out the AF sensor and are only using the sensor for both focus and frame recording. Therefor LV should never have a BF or FF issue.

Your setup does not need to be fancy. I have calibrated my dozen lenses a couple of times and can do it as well with a spot on concrete as with the fancy charts. Use the widest aperture you can so that the DOF is narrow and shows up any problems. Since you cannot save a different adjustment for every lens with the k-x (correct?), the best you can probably do is find a single adjustment that at least puts the focus point inside the DOF for all lenses, even if some are more to the back or front than others.

Yes, its a camera issue, not the lenses. But if you send it in for adjustment you best send both to get best result. I would hope you can get a happy medium adjustment yourself that covers all lenses without sending all your gear away.

Hope that helps.
11-03-2010, 10:49 PM   #3
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Thanks for the explanation, now I understand why LV does not suffer from the BF/FF. I did shoot the lenses wide-open and at this point it seems like a small focus adjustment is probably good enough for all these lenses since you're correct that the K-x offers only a single adjustment for all lenses. And I believe I have to load a firmware with the debug mode enabled to have the ability to adjust the focus.

QuoteOriginally posted by imtheguy Quote
If I understand the internals correctly, BF or FF occurs when the distance from the back of the lens to the sensor is slightly different than the distance to AF sensor. When you use Live View you cut out the AF sensor and are only using the sensor for both focus and frame recording. Therefor LV should never have a BF or FF issue.

Your setup does not need to be fancy. I have calibrated my dozen lenses a couple of times and can do it as well with a spot on concrete as with the fancy charts. Use the widest aperture you can so that the DOF is narrow and shows up any problems. Since you cannot save a different adjustment for every lens with the k-x (correct?), the best you can probably do is find a single adjustment that at least puts the focus point inside the DOF for all lenses, even if some are more to the back or front than others.

Yes, its a camera issue, not the lenses. But if you send it in for adjustment you best send both to get best result. I would hope you can get a happy medium adjustment yourself that covers all lenses without sending all your gear away.

Hope that helps.
11-03-2010, 11:23 PM   #4
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I've just adjusted the focus of the K-x to +90um to fix the back focus issue and it seems like four out of five of my lenses are focusing correctly now: FA 31mm, FA 43mm, FA 77mm, and F 100mm macro. The DA 15mm is now a tiny bit front focus, but since it starts at f4.0 and I usually use it for landscape where I shoot from f8 anyway, it's not that much of a problem. I wonder if the indoor lighting actually affects the lens' focus system? I will probably try it outdoor tomorrow to see if the results are still the same

11-03-2010, 11:29 PM - 1 Like   #5
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QuoteOriginally posted by vtqanh Quote
I've just adjusted the focus of the K-x to +90um to fix the back focus issue and it seems like four out of five of my lenses are focusing correctly now: FA 31mm, FA 43mm, FA 77mm, and F 100mm macro. The DA 15mm is now a tiny bit front focus, but since it starts at f4.0 and I usually use it for landscape where I shoot from f8 anyway, it's not that much of a problem. I wonder if the indoor lighting actually affects the lens' focus system? I will probably try it outdoor tomorrow to see if the results are still the same
It's been my experience that indoor lighting (tungsten) does affect the AF performance on some cameras. I know it did on my K10d. On the K20d it was hit or miss as it is on the K7. Each has improved, the K10d was by far the worse in that situation. I would try everything in natural lighting before going any further with calibrations. You May find now that your FA limited lenses are now off outdoors.

11-03-2010, 11:54 PM   #6
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QuoteOriginally posted by JeffJS Quote
It's been my experience that indoor lighting (tungsten) does affect the AF performance on some cameras. I know it did on my K10d. On the K20d it was hit or miss as it is on the K7. Each has improved, the K10d was by far the worse in that situation. I would try everything in natural lighting before going any further with calibrations. You May find now that your FA limited lenses are now off outdoors.

Thanks for the input, that's what I suspected, too since I've been VERY pleased with the performance of the 43mm Ltd for real-life shooting and such a large amount of back-focus came as a big surprise for me. I'll try again tomorrow morning outside and see if the adjustments still hold true.
11-04-2010, 08:48 AM   #7
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Took the camera and the focus test chart outside with plenty of natural light, no detectable difference in test results. All lenses are almost spot-on. My next question is: Is it better to be back-focused or front-focused? Say if I can't find a perfect setting for all my lenses (since the K-x only allows one global adjustment) and have to make some compromise between them, is it better to have FF lenses or BF lenses?
11-04-2010, 11:06 AM   #8
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QuoteOriginally posted by vtqanh Quote
Is it better to be back-focused or front-focused? Say if I can't find a perfect setting for all my lenses (since the K-x only allows one global adjustment) and have to make some compromise between them, is it better to have FF lenses or BF lenses?
I would venture it may depend on shooting style and subject matter. Personally, I adjust mine for 1/3 of the DOF in front and 2/3 behind the focus point on the assumption that I am focusing on eyes and therefore there will be more subject I want inside the DOF behind than in front. It would be interesting to hear from more experienced users on this too.

03-09-2011, 03:01 PM   #9
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Interesting ! I just downloaded the focusing chart. Printed it out & set aperture to f1.8 on my FA31mm. I found my K-x needed +70 !

Reason I checked is that I was noticing that face shots focused on the eyes were blurry with 5cm behind their face in focus. You could see part of the hair in focus.

Last edited by tux; 03-25-2011 at 04:17 AM.
09-26-2012, 12:02 AM   #10
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Question:
- Presuming I've messed things up, in order to reset to factory settings, do you simply set "FocusCorr" to 0 or you have to do a "Reset" from the MENU ?
Thanks!
09-26-2012, 04:58 AM   #11
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QuoteOriginally posted by imtheguy Quote
Yes, its a camera issue, not the lenses.
Others would beg to differ:

LensRentals.com - Notes on Lens and Camera Variation
LensRentals.com - “This Lens Is Soft” and Other Facts
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