IS UV filter really necessary? with AR or without AR?

Started Jul 8, 2010 | Discussions thread
Karl Gnter Wnsch Forum Pro • Posts: 11,408
Re: Yes (it depends) and yes!

Daniel Lee Taylor wrote:

Right off the bat I don't believe you've done your homework here because you cite a characteristic which, by nature, cannot be affected by a filter. Bokeh is a characteristic of the lens design and would not be altered in the least by a filter.

Wrong, the bokeh was visibly and drastically altered by a high grade filter I got when I bought my 70-200 f/2.8L USM used...

And while I can't discount the possibility that a filter could affect AF accuracy,

It can and it does. I myself had the same experience as the previous poster. So have others, down to the point that lenses went to service multiple times without the filter as that was removed for transport to the service center and always came back the same back- or frontfocus because the first action of the complainant was to reattach the filter - until one time he went there in person and had the technician check the lens in front of him (it had escalated to the point that the service center offered this) and the proof was

it would have to be one terrible filter to do so.

I had the best filter money could buy, so this would infer that the cheaper ones are worse (you get what you pay for).

I would guess the visible distortion would have to be significant enough to be observed with the naked eye.

If you look at the glass under polarized light you may see the culprit: The glass on my filters (I checked several brands) was invariably "stressed", either while cooling or cutting or mounting the glass was put under varying pressures and thus the structure changed enough to cause all the mayhem I saw.

I've never even heard of this happening.

You must have been living under a rock then...

But I do have two small scratches on the filter protecting my 17-40L. But for the filter, those would be on the front element. So you can bet that filter is going no where unless I decide to replace it with another.

If the scratches were anywhere impacting the image quality then you should be replacing the filter right away. If they don't there wouldn't be anything to worry if that was the front element. But the image degradation of the filter is always there and can ruin the shot of a lifetime.
Karl Günter Wünsch

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