# Flaws on the rear element



## sagittariansrock (Jul 28, 2014)

Hi all
I just purchased an EF 100mm Macro USM (non-L) from a member on FredMiranda. The lens has very tiny pin-point flaws on both the front and rear elements, visible when a flashlight is pointed at them. They don't look like scratches, they are too small and superficial to be scratches- and they might only affect the coatings and not the glass. There are about half a dozen, and they are midway between the center and the edge on both elements.
My question is, will these affect the image, and if so, how can I test for it? The seller has agreed to take it back for a full refund but I don't want to return it unless I really see an adverse effect. Can someone please guide me on what to look for?
Thanks in advance.


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## Valvebounce (Jul 29, 2014)

Hi sagitariansrock. 
I think I saw an article by Roger at Lens Rentals I seem to recall that small damage to the front element was essentially irrelevant, it might cause flaring if shooting towards the sun just might. Large damage i.e. a cracked front element was only slightly detrimental to images quality. Marks on rear elements were more of a problem from memory. Might be worth you searching his blog, I quickly found this,
http://www.lensrentals.com/blog/2011/08/the-apocalypse-of-lens-dust
And this
http://www.lensrentals.com/blog/2008/10/front-element-scratches
Hope this helps, try not to get lost in his blog, it is incredibly easy to burn through time there because there is so much of interest there. 

Cheers Graham.



sagittariansrock said:


> Hi all
> I just purchased an EF 100mm Macro USM (non-L) from a member on FredMiranda. The lens has very tiny pin-point flaws on both the front and rear elements, visible when a flashlight is pointed at them. They don't look like scratches, they are too small and superficial to be scratches- and they might only affect the coatings and not the glass. There are about half a dozen, and they are midway between the center and the edge on both elements.
> My question is, will these affect the image, and if so, how can I test for it? The seller has agreed to take it back for a full refund but I don't want to return it unless I really see an adverse effect. Can someone please guide me on what to look for?
> Thanks in advance.


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## Mt Spokane Photography (Jul 29, 2014)

It might be tiny air bubbles. They used to be common and considered a sign that it was top quality glass. However, on the rear element could be a issue.

Its easy to test out, but if you feel uneasy, ask the seller to take it back.


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## sagittariansrock (Jul 29, 2014)

Thanks for the posts Graham.
However, I am wondering what changes in the images should I look for, in case of miniscule flaws (or, as they say, cleaning marks). I am sure they won't be as dramatic as so much dust or a sticky!
Mt Spokane, you say it is easy to test out- my question is how do I test it? What should I look for? Thanks in advance!


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## LetTheRightLensIn (Jul 29, 2014)

are they spidery (like fungus?)


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## Aglet (Jul 29, 2014)

I went thru 4 of those 100mm macros until I found one that didn't have a lot of those sorts of flaws apparent on the internal lens elements.
Just looking into the back end of them, pointing towards the sun, I could see many such pinpoint flaws, more than could be counted, which appeared like coating flaws. Not sure if tiny bubbles as MSP suggests. They appeared considerably larger than the dust particles.
Turned out that the lens I kept was the oldest, most worn looking one I got for the cheapest from another photog. it had better contrast than the other 3 and I still have it.


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## Mt Spokane Photography (Jul 30, 2014)

sagittariansrock said:


> Thanks for the posts Graham.
> However, I am wondering what changes in the images should I look for, in case of miniscule flaws (or, as they say, cleaning marks). I am sure they won't be as dramatic as so much dust or a sticky!
> Mt Spokane, you say it is easy to test out- my question is how do I test it? What should I look for? Thanks in advance!



Imperfections on the rear lens element show up in images because they cast in focus shadows on the sensor. It could just be that the lens needs cleaning, or something more serious.

In any event, you will take a hit when you go to resell it, so get the issue resolved.

Here is a link to one tester of a lens with particles on the rear element.

http://photographylife.com/the-effect-of-dust-on-lens-bokeh


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## sagittariansrock (Jul 31, 2014)

@LTRLI: They are tiny pin points. I shall test them under a microscope, but in general they reflect light only when lit from an angle and are too tiny to photograph.
@Aglet: I do see some dust between the elements when held up to the sun, but these spots are not visible. They are visible only when light is shone on them.
@MTS: I got the lens, otherwise perfect with box and OEM hood and accessories, for $ 350. I felt that is a really good price. I also tested the images with my limited experience- I couldn't find anything obvious even when I pixel peeped. So I am wondering which way to go. 

While I am leery of buying something with a potential problem, it seems unwise to let go of a nice deal, especially when I can't visualize the problem. :-\


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