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Strange spots behind front lens of EF17-40mm

Started Jan 20, 2020 | Questions thread
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.

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