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09-30-2011, 06:26 PM   #1
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fa77mm strange behaviour

I recently had the front assembly changed because a pin came loose in an aperture blade. I thought I may have to readjust the focus. The results were strange and can be seen here kaedepics: Galleries: Digital Photography Review

What I find strange is that the focus point is the same upto f3.2 but at f3.2 and smaller, the focus point moves to front focusing. Also the lens seems slightly softer at every aperture than before the repair.

Can anybody tell me what replacing the front assembly entails and what could cause the change in focus point.

Any comments would be welcom, thanks

09-30-2011, 10:49 PM   #2
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I think, that variable focus (focus shift) is quite common to some degree. I had a DA 40 that was dead on at f/2,8 but had a back focus at f/4. Got adjusted to do the opposite, great at f/4 portrait distance but way off at close distance 2,8. So i sold the lens. Pentax stated that the lens was within factory spec regardless of adjustment.

I think many lens design has this "feauture" to some degree, i have not seen it as a serious problem on my FA 77 though. I mostly blame my camera and myself when the FA 77 miss focus

About your images. If you really see that they are softer than before, try to get the lens exchanged.

Imho, you pictures in that gallery looks great!
10-01-2011, 01:18 AM   #3
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Original Poster
Thanks for the comment. I didnít realise this focus shift was a common problem.

At first I thought the tripod had moved but after running through the apertures several times I new it wasnít due to camera movement and it consistantly shifts after f3.2. The difference in sharpness is slight. Before I could see surface detail of the paper wide open but now it canít be seen until f4.

I canít exchange it but I could get it adjusted under warranty but I am reluctant to do this as it has been in several times for focusing inconsistancy. Before this recent repair, front assembly replaced because of a loose pin in an aperture blade, the lens had been in to have the elements realigned. This fixed the focusing inconsistancies and I was very happy with the quality of the images.

That is why I was wondering what replacing the front assembly involved. Would glass have been changed. If not then the lens is set up is off and I have to return it. Or maybe the paper I used this time is better quality thereforeÖÖ. and it is my mistake and the lens is ok. I hope so.

Thanks again
10-01-2011, 04:30 AM   #4
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Sorry to hear all this, its so boring to go through all this when all you want is to shot with the damn lens.

I dont know the answer to your questions, but asking Pentax or who ever made the repair would give you these answers I hope....

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aperture, f3.2, focus, front, k-mount, pentax lens, slr lens
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