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Strange spots behind front lens of EF17-40mm

Started Jan 20, 2020 | Questions
Fotookes New Member • Posts: 22
Strange spots behind front lens of EF17-40mm

Hi Everyone,

I recently pulled out my venerable EF17-40mm wide angle lens from storage and was a bit shocked to notice multiple small spots behind the front lens element! This is not mould nor dust, but seem to be consist of 'flakes' of some kind. Could it be that the coating of the front lens or the one behind it (from the optical design it seems like there are two elements in the first, front, group) has started to break up and that these flakes are the result?

Usually a little dust wouldn't bother me much, have some really old Pentax lenses with a lot of dust inside which doesn't harm at all, but if the coating of the big front lens element is breaking up, especially on a wide angle (tele would be less of a problem?)

For your reference: there is one internet posting with pictures of the problem you can easily find under the name "strange spots on the front lens of my ef17-40mm"!

The person who posted it didn't know what the cause was, he just illustrated his attempt (failed!) to disassemble his lens in order to clean it, but had to give up because the front element was clearly part of a group and could not be cleaned individually.

For those wondering, I did shoot a few shots to test, but do not have anything big screen with me on this trip, so cannot really judge much if anything is bad, image quality wise!

Thanks for any helpful feedback!

ANSWER:
This question has not been answered yet.
WindwardHaole Senior Member • Posts: 1,550
Re: Strange spots behind front lens of EF17-40mm

Don't bother showing IQ impact of the spots, show the spots themselves. If the front element is part of a group it may be involved in centering, if so, send it to Canon. If you first want opinions, send pics of the spots to Lensrentals.com, they very well may give you some advice.

John_Hoffman
John_Hoffman Senior Member • Posts: 2,245
Re: Strange spots behind front lens of EF17-40mm

https://youtu.be/ZCxeG3IQFTU

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John Hoffman
Conway, NH

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RCicala
RCicala Contributing Member • Posts: 820
Re: Strange spots behind front lens of EF17-40mm
2

John_Hoffman wrote:

https://youtu.be/ZCxeG3IQFTU

First, anyone who wants to do that, good for you; but I believe the front of a 17-40 f/4 is a centering element. If you don't have the ability to recenter the lens you could really screw up the IQ taking it out and putting it back together.

Plus, like so many youtube experts he doesn't show you that you need to mark the lens to be sure you line it up the same way when you put it back: the rest of the elements are adjusted to it being in the proper rotation. So yes, you can take that out and put it back and you'll maybe or maybe not have decent IQ.

As to the spots, I'm not certain what they are without seeing them in person, but it doesn't look like a coating issue to me, those tend to be blotchy and never bright white.

WindwardHaole Senior Member • Posts: 1,550
Do as RCicala advises

No text.

OP Fotookes New Member • Posts: 22
Re: Strange spots behind front lens of EF17-40mm

Thanks for the input (all!)

It seems like this one will remain a mystery? Don't feel like 'just shipping it of to Canon' because of the cost most certainly involved...

I did some more checking and found a the same question being asked about this lens problem on the forum here and it created the (un)necessary confusion!

One guy did come up with an interesting theory and that was something called Schneideritis! This phenomenon really describes the spots in my lens perfectly.

It also explains why these spots exist in between two elements of one group (read: lens elements which are 'glued' together and cannot be separated in the practical sense!)

This is why sending it in to Canon will surely cost me, because there will be more than just some cleaning needed for fixing this one...

Anyway, will try an email Canon here in Japan with the question if they are aware of this kind of problem (cement holding lenses in a lens group together starting to cdumble) with their EF17-40mm...

To be continued?

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