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10-24-2014, 08:23 AM   #1
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Question on SMC Pentax-A 35-105 (Diaphragm)

Hi,
i recently acquired an SMC Pentax-A 35-105 mm. It seems in very good condition, no dust marks, focus and zoom rings are rather smooth, no signs of heavy misuse or damage. It seems to take farirly good pictures as far as i can tell, as it is the only copy of this lens i've ever owned.

By visual inspection, i noticed a strange behavior of the diaphragm blades, and i was wondering if it is normal or not. I attached some pictures to explain what i believe i'm seeing, forgive me for the awful quality as they were taken with my cell phone.

The first one is a picture taken through the front of the lens, wide open (manually setting the aperture ring at f/3.5) and zooming at about 50mm focal length. I see a perfectly circular shape as i expect.
However as the focal length decreases towards 35mm (second attachment), i start to see some asymmetry, as if the iris blades do not completely retract out of the optical path when fully open, as they are supposed to be. In the third picture it appears that they are actually protruding out more than i would expect. This behavior does not change if i set the lens to A mode ( or any other setting) and manually operate the aperture control lever to open the diaphragm.

My questions are: Is it normal behavior ? Does this suggest a mechanical malfunction of the aperture mechanism ? I note that apart from that the aperture ring seems to operate smoothly and there are no signs of grease or stuck blades.

Thanks in advance for the help.


Last edited by simon_199; 09-04-2015 at 01:23 AM.
10-24-2014, 08:42 AM   #2
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It's normal.
10-24-2014, 12:57 PM   #3
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QuoteOriginally posted by simon_199 Quote
i start to see some asymmetry
This is not abnormal and many, many lenses show some asymmetry on examination.


Steve
10-24-2014, 01:03 PM   #4
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Normal. Just checked mine.

10-24-2014, 05:20 PM   #5
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Normal. Mine does the same.

This asymmetry with this lenses (and others) can have a subtle effect on how brighter out-of-focus areas look sometimes, but 99.5% of the time you will never see anything.

If everything works fine with the lens, enjoy it.

I wish Pentax would re-issue that lens one day, with AF and the latest lens coatings, maybe with a future full-frame. Or do something similar like a 24-105 f4.
10-24-2014, 05:57 PM   #6
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QuoteOriginally posted by rawr Quote

I wish Pentax would re-issue that lens one day, with AF and the latest lens coatings, maybe with a future full-frame. Or do something similar like a 24-105 f4.
I would buy it in a heartbeat. I love my A35-105
10-25-2014, 01:00 AM   #7
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Thanks for your replies, so it's common for this lens, nothing to worry about.
I've taken some random shots in the garden and it indeed seems pretty good...its downside is that it is heavy.
It was called a "stack of primes" but a bag full of primes would actually be lighter :P
10-25-2014, 01:42 AM   #8
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QuoteOriginally posted by simon_199 Quote
its downside is that it is heavy.
True. Lots of lenses of that period share that feature. They are built to last.

But while the 1980's era A 35-105 weighs 615 grams, it's 2014 equivalent (the Sigma 24-105 f4) weighs even more, at 885 grams. And some modern [non-Pentax] primes like the Sigma 35 f1.4 also weigh more (665 g) than the A 35-105.

The 35-105 is also a complex construction (15 elements in 13 groups), so there's a lot of glass in there.

10-25-2014, 02:55 AM   #9
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You are not alone. My A35-105 also has a slightly asymmetrical diaphragm.
If you have some lens flare where the asymmetrical aperture is apparent in the photo, use a hood or don't point it at the sun.
10-25-2014, 03:30 AM   #10
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Actually my copy came with a short screw-on metal hood that the original owner had made by modifying a 35mm super takumar hood ( so says the lettering on it ) which seems to work pretty well.
Moreover when the diaphragm is stopped down i can see no signs of that, only at the maximum aperture setting asymmetry manifests itself...so i believe the lens is fine.
10-25-2014, 11:35 AM   #11
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I count my A 35-105 as one of the best lenses I own. (Shamefully, I succumbed to an extended bout with LBA as can be seen in my sig.). It is very sharp and has excellent contrast and color. Images captured with this lens require virtually no post-processing as they look wonderful straight out of the camera.

Last edited by ripper2860; 10-26-2014 at 09:17 AM.
10-27-2014, 04:58 AM   #12
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QuoteOriginally posted by simon_199 Quote
My questions are: Is it normal behavior ?
It is normal. It is largely an optical illusion, if you look from the back of the lens the aperture is actually circular wide open.
10-27-2014, 10:26 AM   #13
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Yes there is a very narrow angle of view (alonge the optical axis) such that, from the back of the lens at least, it looks circular. I still wasn't completely convinced because i never noticed something similar with my other lenses...but this probably depends on the relative position of the iris blades with respect to the other elements in this particular construction....

So far my untrained amateur eyes are pleased with the results, even wide open straight from camera.

Last edited by simon_199; 09-04-2015 at 01:23 AM.
11-01-2014, 12:20 AM   #14
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This much asymmetry is fine, if it gets more than this than you can have sticky aperture blades, which needs cleaning. This is best walk around lens for the price, and if you are fine with manual focus
11-02-2014, 08:49 AM   #15
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My 50mm f/1.7 has this strange (not sure if I should call it downright unpleasant) "clawed" bokeh at f/2... it gets better when stopped down and wide-open...



It only happens at f/2, so not much of a worry
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