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06-16-2012, 09:36 AM   #1
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Mamiya Sekor lens

Hello Everyone, I need some help I recently purchased a Mamiya Sekor 50mm F2.8 M42 lens for dirt cheap because it was defective and wanted to dive into dismantling it. Well all said and done I think I have it in working order. However, I have an issue with it. If I shoot wide open I find that the resulting image is both soft and has a glow to it. I'm not sure if it has anything to do with my tampering or if it really is just that soft and has that glow. I noticed that if I stop it down to 5.6 and past the glow seems to disappear. But wide open its very soft. Is this normal for this lens. I've never seen anything that would do this. It seems that even my JC Penney 28mm F2.8 seems to less soft and no glow. It would seem like it gives a good soft glow for portrait like those old time filters but if not shooting portraits its like a daydream effect.

Any help on a solution to fix this would be appreciated.

06-16-2012, 12:15 PM   #2
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When you dismantled and rebuilt it, did you replace the glass elements correctly? What you've described is a symptom of a flipped rear element, as mentioned in at least one recent thread. Some portrait shootists actually prefer that effect. You may have a winner!
06-16-2012, 12:46 PM   #3
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My dismantling did not involve removing the glass elements but im pretty sure some one else did service it so maybe They messed it up. As for some portrait photographers liking that effect i think i might use it as that. Since i do have a pentax m 1.4. I need a spanner tool to get to the elements which i currently dont have.
06-16-2012, 01:19 PM   #4
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Some folks use scissors or compass dividers instead of lens spanners. I'm cowardly; I bought a 3-tip spanner set. If you do get a spanner, I recommend a multi-tip kit, to suit various retaining rings.

06-16-2012, 05:38 PM   #5
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Some 1.4 lenses can be pretty glowy at 1.4 (only), but a 2.8? No way. You've got a reversed element somewhere, assuming the glass looks clean and clear.
06-16-2012, 06:00 PM   #6
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I tried to use a compass divider as suggest by ricorico but to no avail. I will need to get a spanner tool and some friction tools to open it up. The internal elements seems to have some dust in it but nothing that can cause that much problems. I will try to take it apart later in the week and hopefully get it resolved if not i got a nice paper weight.

Has anyone perhaps taken one of these apart? Ive been stuck on how to remove the aperture ring and focus markers from the rear element. There os an auto/manual switch that doesnt seem to come off or atleast i havent figured out how yet.
06-16-2012, 07:15 PM   #7
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So i was able to take off the front part of the element and was able to access the first three elements of the lens and clean them. After cleaning them the lens seems sharper but wide open still produces some glow to it. I think the rear element must be flipped as you guys said but i am still unable to get to it from the rear of the lens. Still have to wait to get a spanner tool to be able to access it.
06-16-2012, 07:35 PM   #8
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If an element is reversed, it will never look truly sharp, even on smaller apertures although it will look sharper than wide-open and some of the glow will go away. But it will remain soft -- it will just look wrong, and you will have trouble getting focus confirmation from the body. It depends on the element how big the effect is -- some of them aren't much different reversed (which also makes them the most likely candidates for accidental reversal because they don't obviously curve one way or the other). If you are getting sharp images at f5.6 or f8, then the element is probably not reversed -- some lenses do glow a bit wide-open, but a "dreamy" effect at 2.8? Sounds unlikely unless that lens just sucks (and 2.8 for a 50 is about as slow as they get, and there are some crappy ones out there). Have you shined a flashlight through the lens just to make sure the isn't some haze or something smeared on glass in there? Oh, the other thing to look for (which you probably would have run into already in your tinkering) is a half-unscrewed *something* -- some lenses the front element will unscrew and so if it is not down tight it will produce a glow effect from being out of position.

Can you post some examples of the best quality you can achieve at 2.8, and also at stopped down some? Choose a subject with some crisp text so we can evaluate sharpness.

06-16-2012, 08:03 PM   #9
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I will post some tomorrow as all my computers are shut down and my phone doesnt read sd cards. :-(
It will also allow me to look at them on a bigger screen as ive been using the camera lcd for judgement so far.

Update:

The following links are full res straight from the camera

Wide open @ F2.8 before cleaning elements @ISO100
http://dwo.net23.net/forum/gallery/image.php?album_id=5&image_id=155&view=no_count

Stopped down @ F5.6 or so before cleaning @ISO400
http://dwo.net23.net/forum/gallery/image.php?album_id=5&image_id=156&view=no_count

Stopped down @F16 before cleaning @ISO1600
http://dwo.net23.net/forum/gallery/image.php?album_id=5&image_id=157&view=no_count

Wide open @ F2.8 after cleaning @ISO1600
http://dwo.net23.net/forum/gallery/image.php?album_id=5&image_id=158&view=no_count

Stopped down after cleaning @ISO1600
http://dwo.net23.net/forum/gallery/image.php?album_id=5&image_id=159&view=no_count

Wide open @ F2.8 after cleaning @ISO560
http://dwo.net23.net/forum/gallery/image.php?album_id=5&image_id=160&view=no_count

Best images for direct comparison is the first and last image in the set as they are of the same subject but slightly further away and different ISO. Didn't have the tripod setup and all. Hopefully you guys can help me solve this problem.

Last edited by epqwerty; 06-16-2012 at 09:36 PM. Reason: Updated with images
06-19-2012, 04:05 AM   #10
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So I saw the other post regarding the reversed element and mine doesn't seem to have that problem. Perhaps my lens is just very soft wide open and needs some more cleaning on the elements. If anyone could give a second opinion I have posted samples.
06-19-2012, 05:20 AM   #11
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They don't look bad, and are sharp stopped down. Different lenses have different looks. That lens might be a Tessar design (4 element) -- many like the "Tessar look". Kinda low contrast all-around. Typical for many older lenses, can be easily adjusted.

Try shooting a flat subject and get the camera as parallel as possible to the subject (the newspaper or brick wall test) -- you may find it is only sharp in the very center when it is wide-open.
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