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09-16-2010, 01:15 PM   #1
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What's wrong with my FA* 28-70?

I have never been too impressed with my FA* 28-70. It just lacks sharpness and the bokeh is not that good either. But from the outside I cannot see anything wrong with the lens.

However, when I take photos of OOF lights I always get the following:



The OOF discs are always the same shape and this does not change while zooming. It looks as if one of the elements is broken inside, but I really cannot see anything like that.

I asked a friend to do the same with his FA* 28-70 and he got this, a perfect OOF circle:



Any ideas?

09-16-2010, 01:19 PM   #2
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There could be large dust particles inside inside your lens, i.e., dust particles near the aperture diaphragm.

Identifying dust location on SLR camera and Digital SLR (DSLR)
09-16-2010, 01:24 PM   #3
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QuoteOriginally posted by asdf Quote
There could be large dust particles inside inside your lens, i.e., dust particles near the aperture diaphragm.

Identifying dust location on SLR camera and Digital SLR (DSLR)
Thanks! I initially thought the same but I cannot see anything like that. Also, that won't explain the large and perfect 'cut' in the OOF disc I think?
09-16-2010, 01:27 PM   #4
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QuoteOriginally posted by Asahiflex Quote
Thanks! I initially thought the same but I cannot see anything like that. Also, that won't explain the large and perfect 'cut' in the OOF disc I think?
Maybe, it's hair or some other obstruction. ...but you're saying that you looked inside there but didn't see anything, eh?

09-16-2010, 01:29 PM   #5
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QuoteOriginally posted by asdf Quote
Maybe, it's hair or some other obstruction. ...but you're saying that you looked inside there but didn't see anything, eh?
Yep. I know I'm very picky when it comes to glass, but I don't see anything (not even under a strong lamp). Of course I see some very small dust particles but nothing like a hair or something like that. So that makes me wonder where this perfect cut comes from...
09-16-2010, 01:44 PM   #6
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QuoteOriginally posted by Asahiflex Quote
Yep. I know I'm very picky when it comes to glass, but I don't see anything (not even under a strong lamp). Of course I see some very small dust particles but nothing like a hair or something like that. So that makes me wonder where this perfect cut comes from...
Check out...

https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/81530-lens-bok...arch-help.html

There is some discussion about what could cause the light to be obstructed inside the lens (fungus, dust, element separation, etc.).
09-16-2010, 01:46 PM   #7
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I would think that an imperfection in the glass would have to cause this. Maybe an internal element is defective?
09-16-2010, 01:53 PM   #8
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A defect would be unfortunate because the lens is really minty... But I'm very interested in the source of the problem and will go look at the link that asdf provided. Thanks asdf!

09-16-2010, 05:21 PM   #9
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Any UV or clear filter mounted on this lens.
If so, do you get the same result without it on?
09-16-2010, 10:29 PM   #10
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QuoteOriginally posted by mlatour Quote
Any UV or clear filter mounted on this lens.
If so, do you get the same result without it on?
Thanks for the idea, but I never mount filters on any of my lenses.
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