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08-10-2013, 05:56 PM   #1
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28mm A Lens Question

Hi all,
I just received a 28mm F2.8 A lens and the lens is in great shape except for two things, one is that on the rear lens element there's a slight
scratch which I think it's a coating scratch, I took a few pictures tonight and so far I don't see anything amiss no flare or other distortion. The next
thing is the aperture ring is stiff, which isn't a problem for me because I'm using it on my K20D, any thoughts on this.

P

08-10-2013, 06:12 PM   #2
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Small scratches often don't cause too much problem, if you don't notice a problem I would not worry about. I have read about people using black marker to cover the scratch and reduce flare. But I have never tried it so do some research before you do anything.

If you are using the lens in 'A' mode then stiffness in the aperture ring is immaterial. I've had lenses that it was very difficult to turn the ring but they worked loose as you used them. I would suspect dried grease. But leave it on 'A' and don't worry about it. The focus helicoid on the other hand should work smoothly or I would not accept keep it.
08-10-2013, 06:17 PM   #3
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The Focusing ring is smooth as silk That's what I love about the Pentax lenses, I don't know about black marker, I have done that on the black metal parts not on the lens itself.


P
08-10-2013, 06:30 PM   #4
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QuoteOriginally posted by jatrax Quote
Small scratches often don't cause too much problem, if you don't notice a problem I would not worry about. I have read about people using black marker to cover the scratch and reduce flare. But I have never tried it so do some research before you do anything.
I have also read about the black marker on scratches - but only on the front element and not the rear element. It may work on the rear element too, but - I don't know. I firmly believe in - it there isn't a problem, don't create one.



08-10-2013, 07:03 PM   #5
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The problem of stiff aperture ring on A28/2.8 is quite common.

Underneath the mounting flange, there is a plastic ring that's coupled to the aperture ring. For some reason, this ring becomes mis-shapen and becomes too tight to move within the retaining groove. As it's plastic, it shouldn't need greasing and I've tried it to see if it helps but it doesn't. The only solution I found is to grind or sand the ring carefully.

As others have said, this shouldn't be a problem as most people use the 'A' setting on the aperture ring and leave it there.

If you do attempt to repair it yourself, just be aware that when you take off the mounting flange, there are some small springs and other parts that can fly away and get lost quite easily. Trying to put the springs and the small contacts back together can also be a challenge and requires skill and patience.
08-10-2013, 07:59 PM   #6
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QuoteOriginally posted by interested_observer Quote
only on the front element and not the rear element. It may work on the rear element too, but - I don't know. I firmly believe in - it there isn't a problem, don't create one.
Agree. And not sure I would do it myself even on the front element.
08-10-2013, 08:40 PM   #7
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The aperture ring on my F28 2.8 is terribly hard to move, you have to use so much force that when it does move it will jump two or three stops, it's been like that for years. When I first got it, it used to bother me but since I always leave it in the 'A' position now it's not a big deal (I still feel the urge to move it now and then, I don't know why) I have fixed one or two in the past that were extremely loose, but as Tom1803 stated, if you do that be careful as there are small springs and the little ball bearing itself that is very easy to use.

As for the scratch on the rear element, if one was going to show up on your pictures one on the rear would be more likely to show than one on the front, if its not noticable in your pictures I would not worry bout it. If its just a scratch in the coating itself it will probably never be seen.
08-10-2013, 09:19 PM   #8
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QuoteOriginally posted by jatrax Quote
Agree. And not sure I would do it myself even on the front element.
Evening, In theory the reason why you would take a black marking pen and fill in the scratches on the front element is so that the light coming through the front element will not flare as it passes through the scratches or chips. The scratches form another surface that diverts the light off at an angle - possibly flaring (depending on the relative angles involved). With the black in the scratches, it just does not pass through, thus no flare, and the "marks" don't show up on the image.

That's the theory - I have never have had to try it - yet. I did buy an A* 300/4 where the seller indicated during the sale that he had just happened to see several very light scratches (and I did get a discount). I have looked and have yet to see them - my front element appears to be pretty perfect.




Last edited by interested_observer; 08-11-2013 at 09:07 AM.
08-11-2013, 09:05 AM   #9
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QuoteOriginally posted by interested_observer Quote
In theory the reason why you would take a black marking pen and fill in the scratches on the front element is so that the light coming through the front element will not flare as it passes through the scratches or chips.
That's what I have read as well. Fortunately never had the occasion to test the theory
08-11-2013, 04:07 PM   #10
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Tested the 28mm today, had Sun most of the day, so far there's no problem with the lens.

P

Last edited by P Shooter; 08-11-2013 at 05:47 PM.
08-16-2013, 06:39 AM   #11
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Well I decided to send it back, between the stiff aperture ring and the scratches on the rear element,
I'll wait for something better to come along.

P
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