Front-Back Focusing Revisited


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kayheem

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Jun 11, 2004
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Ok, I would like to bring up this topic again (sorry!). I have a Canon 350D and a new Sigma 70-200mm F2.8 EX DG with a back-focusing problem.

Canon probably won't touch the Sigma and Sigma tells me to check the camera out first. My point is this: Is the problem with the lens or camera or both? Why is it they say that the lens must be calibrated togehter with the body? Shouldn't they follow the same autofocus protocol and the same reference specs/signals?

What if you have more than 2 bodies and 2 lenses? How do they calibrate all of them? If you sell off the lens, would the next buyer have to calibrate the lens again to his body?

When I brought my lens to Sigma, they asked me to bring along the body for testing. According to the tech (Harry), he said my lens contained a new re-writeable chip (after checking the serial number). After doing some testing, he managed to recalibrate the lens for me after about 1 hr using their new equipment.

I was fortunate in that I bought the lens with the original sigma warranty i.e. non-grey set. This meant I got the recalibration for free.

Do other 3rd party lenses (e.g. Tamron and Tokina) suffer from these front/back focus issues? How about other bodies e.g. Nikon, KM, Pentax etc?

Any comments?
 

fWord

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Jun 23, 2005
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A third possible reason for subpar results from a new camera/ lens is the photographer's error. This isn't always the case, however I've had a fair share of scares with a new lens some months back. I brought it out for testing in orchard road, shooting without a tripod (!) in less than ideal lighting (!) and less than ideal shutter speed (! again). In the end, camera shake, coupled with not using the AI focus for moving subjects led to horrible results. Subsequently I got over the learning curve and the lens was amazing.

There is a theory stating that back-focusing/ unsharp copies of lenses is a problem that plagues the inexperienced user, and for my case it was certainly true.

On the other hand, I've also read reports of people who genuinely state that they did obtain a bad copy, and after exchanging it, got a copy which worked perfectly. Some users have had to send their lenses back to technical support, as you have, to have it recallibrated. It certainly proves that such issues do occur, and it is known that Sigma has to continually rechip their lenses in order to maintain compatibility with newer Canon bodies. My assumption is that they were manufactured with reverse-engineering techniques, the same way their 500DG Super flash was made.

To be frank, I haven't read of Tokina or Tamron producing poor copies, but I'm sure that they must. With mass-production in place, some things that come off the line will be duds...like my laptop computer of late. :rolleyes:
 

dundee

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Sep 12, 2004
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kayheem said:
Ok, I would like to bring up this topic again (sorry!). I have a Canon 350D and a new Sigma 70-200mm F2.8 EX DG with a back-focusing problem.

Canon probably won't touch the Sigma and Sigma tells me to check the camera out first. My point is this: Is the problem with the lens or camera or both? Why is it they say that the lens must be calibrated togehter with the body? Shouldn't they follow the same autofocus protocol and the same reference specs/signals?

What if you have more than 2 bodies and 2 lenses? How do they calibrate all of them? If you sell off the lens, would the next buyer have to calibrate the lens again to his body?

When I brought my lens to Sigma, they asked me to bring along the body for testing. According to the tech (Harry), he said my lens contained a new re-writeable chip (after checking the serial number). After doing some testing, he managed to recalibrate the lens for me after about 1 hr using their new equipment.

I was fortunate in that I bought the lens with the original sigma warranty i.e. non-grey set. This meant I got the recalibration for free.

Do other 3rd party lenses (e.g. Tamron and Tokina) suffer from these front/back focus issues? How about other bodies e.g. Nikon, KM, Pentax etc?

Any comments?


Try to figure out if you actually HAVE focus issues in the first place. Sometimes your camera focus sensors may not be accurate enough, or it may be user error.

(get a tripod , protractor and a metal ruler, shoot the ruler at 45deg.
It MUST be 45deg exactly or your results will be off.)

shoot at 20mm intervals (70, 90, 110, 120, 140, 170) at f/2.8. the degree of backfocus may be different at different focal lengths.

Due to wear and tear, lenses need to be calibrated after every hundred thousand shots or so, exp. if you are using a consumer zoom (like the 18-55mm or the 28-80mm) dont sweat it. At f/2.8, afew mm off that ruler will not really matter in real life shooting. The main problem crops up when doing macro or using f/1.0 - f/1.4 lenses.
 

gama

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Feb 6, 2006
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Interesting thread.

I just bought 30D and Sigma 30/1.4 lens and found it to be front-focused rather badly. I am not sure if I tested correctly but when I take normal photos on tripod with the lens wide-opened (1.4) and focus on a subject using center focus point (single shot), I found that the subject in front of the one indicated by the center focus point was always in focus.

I went back to the shop and they told me that it was the body problem and not the lens. But I have tried Canon 50/1.4 and found no problem. Eventually managed to get it exchanged with what looks like a better lens.

I will try what Dundee suggested (any specific advise for testing prime lens?) but I am still thinking of bring it in to Sigma workshop if it only takes 1 hour and is covered by warranty.
 

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