Author Topic: Mark on lens  (Read 1663 times)

Mark on lens
« on: April 15, 2018, 07:30:23 pm »
I've got a lens with a couple of grey marks on the rear element about 2mm across. They seem to be damage to the surface of the glass.



They are at the edge of the lens, does that fact they are where they are affect the image less than if they were at the centre?

I think those spots wouldn't even end up on the image anyway because my camera is a crop sensor dslr and even on full frame the edges of the circle won't end up on the rectangular image.

Re: Mark on lens
« Reply #1 on: April 15, 2018, 10:55:25 pm »
No, the full lens can project to each part of the image, although it may be aperture dependent. Otoh, it probably won’t have much (or any) visible impact - ‘though rear element damage can be worse than front iirc

Mike

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Re: Mark on lens
« Reply #2 on: April 15, 2018, 11:07:48 pm »
It may be fungus.

Best way to see the impact is to photograph some 1mm graph paper at all apertures, then some plain white paper.  It may not be all that bad. You may not notice it.

What lens is it? What camera is it used on?
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Re: Mark on lens
« Reply #3 on: April 15, 2018, 11:30:22 pm »
Cracking sunspots!  :thumbsup:
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Re: Mark on lens
« Reply #4 on: April 16, 2018, 12:13:51 am »
I've only just got it and I don't want to say too much as I might want to return it. It's a M42 zoom, I think it's a retailer/distributor brand, made in Japan.

The postage was more than the lens!

I haven't tried it yet as I haven't got a M42 to Pentax K adapter. There wasn't any fungus when I got it. I've read that fungus can etch the glass. If it was caused by fungus, it was cleaned off before I got it.

Aunt Maud

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Re: Mark on lens
« Reply #5 on: May 24, 2018, 08:51:00 am »
You're right that the image on the sensor will only be a portion of the total circular image projected by the lens. It's how tilt and shift lenses work, except they project a larger image circle than a normal lens.

Why don't you give it a try and see if there are any blurry spots on the photograph ?

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Re: Mark on lens
« Reply #6 on: May 24, 2018, 09:56:07 am »
I haven't tried it yet as I haven't got a M42 to Pentax K adapter. There wasn't any fungus when I got it. I've read that fungus can etch the glass. If it was caused by fungus, it was cleaned off before I got it.

If it was fungus and was simply wiped off it probably wasn't eradicated and could grow back again. I would look for some kind of optics-friendly fungicide and try to kill it, because there's a faint possibility that it could migrate to any other optics put in proximity to it.
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Re: Mark on lens
« Reply #7 on: December 03, 2019, 03:03:38 pm »
Apparently fungus dies in UV light, so you might try and treat it by leaving the lens in the sunshine. That won't remove the marks but it should inhibit growth.

This lens rental company discovered a dead fly inside their lens and then shot some resolution charts to see if it was perceptible. The fly's body affected images only when the lens was stopped down at short focal distance. The fly's legs (which had become detached) didn't show up. The lesson is not to worry about small particles in a lens because It takes something relatively large to create visible problems.

Re: Mark on lens
« Reply #8 on: December 03, 2019, 04:28:37 pm »
Photographers tend to fret over the slightest marks on lenses but in practise you might be surprised how big something needs to be before it can be detected in images. I'd be surprised if those marks on your lens cause a problem.

However, as others have said, hard to tell from here, but that does look a bit like fungus. If it is and the lens is returnable, I'd return it. The problem is, unless you can kill it, there's a chance it can spread if, say, you keep it in a bag with other lenses in a damp environment (like all of the UK). If you leave it on a windowsill in dry air and bright daylight for a week or two, that can work, but it's hard to know for sure and do you want the aggro?

This is one reason why buying used lenses on ebay from far-eastern dealers that look like a great deal sometimes turns out not to be.