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Thread: Does stopping down a lens cover edge imperfections/separation/fungus/etc.?

  1. #1

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    Mar 2017
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    Does stopping down a lens cover edge imperfections/separation/fungus/etc.?

    I've often read that this is so, but wonder if it is actually true, and if so, by how much. Do the optics bend the light rays in such a way that, even when the issue looks to be covered by the lens iris to the eye, light might still be passing through the effected portion of the lens on its way to the film?

    I ask because I have a 100mm Wide Field Ektar* with fungus etching** that looks to be completely covered by the f/11 setting, and much more than covered by f/16. I know the best way to find out the possible effects would be to use the lens, but I had back surgery a while ago, and won't be doing much for months. I'm also not very patient, and I am very curious.



    * I got the lens with a 4x5 Speed Graphic. The camera came in a Vulcanoid (I love that word) case with some unmentioned odds and ends, including this lens, which I thought was very nice, even after I found the fungal growth. I cleaned it as best I could, then set it in a sunny window for several days. It hasn't grown since.

    ** And, wow, are there a million conflicting opinions about fungus on lenses: where it comes from ('it was trapped there when the lens was made'; 'it's everywhere and always a threat'), what it 'eats' (coatings, glass, balsam), how to get rid of it, whether you can get rid of it, what effect it has on images.... It would be nice if some lens manufacturer published some actual research. If they have, I'd like to be pointed at it.

  2. #2

    Join Date
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    Re: Does stopping down a lens cover edge imperfections/separation/fungus/etc.?

    Yes light rays bend at the glass-air boundaries. Yes defects at the edge can be obscured by use of the iris.

    The surfaces of the lens are usually (very) out of focus and small defects are not usually "fatal", however when used with strong backlighting anything on the lens surface may degrade the image.

    Regarding the care feeding and elimination of fungus! The internet is a wonderful place where wisdom and folly get jumbled up and repeated and confused. I think of fungus as a something that is relatively uncommon, happens gradually, can be removed if caught early but if not treated can destroy a lens eventually. Apparently some fungus feeds on some of the materials used for the anti-reflective coating on lenses.

  3. #3
    Ed Freniere
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    Tyngsboro, MA, USA
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    Re: Does stopping down a lens cover edge imperfections/separation/fungus/etc.?

    You can test whether stopping down is helping to block the damaged parts of the lens by doing the following:

    - with the lens mounted on the camera, aim the camera at a dark background.
    - shine a directional light obliquely onto the lens, that is, from outside the field of view.
    - open the shutter
    - from the back of camera peer through each clipped corner of the groundglass in turn, looking at the lens while closing and opening the iris.

    For a better view, remove the groundglass. Any view where you can see the bad part of the lens indicates that that of the image will be susceptible to flare from the defect.

    Ed

  4. #4

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    Re: Does stopping down a lens cover edge imperfections/separation/fungus/etc.?

    Thanks for the responses, and thanks, Ed, for the practical suggestions. I'll give them a try.

  5. #5

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    Re: Does stopping down a lens cover edge imperfections/separation/fungus/etc.?

    Okay, I took the ground glass off of my Speed Graphic (it doesn't have clipped corners) and found that I couldn't see anything when stopped down to f/16, except for maybe one 'hair' of the fungus. nothing at all by f/22/

    This is really too bad, too, since the lens is pretty much pristine otherwise.

  6. #6

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    Re: Does stopping down a lens cover edge imperfections/separation/fungus/etc.?

    How big is the "hair"? Could you crop it out in printing? Fix it in Photoshop (if you scan and print like I do??) Ignore it??? I have the WF Ektar (with no fungus, fortunately) and really like it. F:22 is pretty common for LF, so maybe you can just ignore it unless you really want shallow DOF.

  7. #7

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    Re: Does stopping down a lens cover edge imperfections/separation/fungus/etc.?

    Hi Jim,

    I'm not sure the hair will have even the tiniest effect on images, so I'm actually quite glad that there's only one visible at f/16, and none at f/22. Even wider, there's not much showing. I know that it takes more than a little bit wrong with a lens to make any distortions very noticeable, so this is fine news; especially for a lens that has such a good reputation, that is not too common, and that was more or less free!

  8. #8

    Re: Does stopping down a lens cover edge imperfections/separation/fungus/etc.?

    Sounds like you have a fine lens. Use it at f22 with no reservations. I had one of these and found it to be excellent except that the shutter was a bit slow.

  9. #9
    Nodda Duma's Avatar
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    Nov 2014
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    Brookline, NH
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    Does stopping down a lens cover edge imperfections/separation/fungus/etc.?

    To affect the optical performance (i.e. MTF), ~13% of the clear aperture (surface of the lens used by the light path) must be obscured. That is a lot to cover, and fungus won't completely obscure that much area.

    However, forward scatter off the fungus will affect contrast via veiling glare. This is non-deterministic, but loosely correlated to the amount of fungus (or scratches, dust, etc).

    So I wouldn't sweat it too much...the best test is to take some photos and see what they look like.

    -Jason

  10. #10

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    Mar 2017
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    Re: Does stopping down a lens cover edge imperfections/separation/fungus/etc.?

    Yeah, I figured it was probably fine, even with the fungus, which is not terrible, even where it does exist on the lens. But it's nice to get some feedback.

    I've had a lot of my gear for quite a long time, but have not used it much. Reading and posting questions here will, I hope, give me the gumption to get out there and use what I have.

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