UV filters, unscientific test

Started 5 months ago | Discussions thread
sybersitizen Forum Pro • Posts: 21,337
Re: Test @ 200mm
1

Bing Chow wrote:

... I don't see a difference. This is consistent with another test that I've done, with other brands.

I have no trouble accepting that a good filter won't cause sharpness degradation on a 200mm lens. Many of the best telephotos from respected manufacturers come with high quality clear filters to help ensure that poor ones aren't used. My Minolta 200mm f/2.8 is one such, and its filter passes my tests easily.

So people who report degradation are either using REALLY bad filters, using less rigorous technique (flimsy tripod, handholding, difficult subjects, changing outdoor conditions), or they're just parroting what they read and they haven't really tested themselves.

I can't say what percentage of testers fall into each category, but I know there are filters that do cause visible sharpness degradation, and the quickest way to identify them is with longer lenses. Something a lot longer than 200mm might even show a bit of degradation with the filters you just tested.

I have an innocent-looking 67mm filter labeled as TOYO OPTICS SKYLIGHT (1A). When used on short or moderate focal lengths the effect on sharpness isn't too bad:

30mm with filter

30mm no filter

But when it's mounted on an inexpensive 500mm lens (adapted, so EXIF is absent), the degradation is so bad that I don't even have to take a shot to confirm it. I can see the blurring on the camera's LCD using focus magnification. It's shocking:

500mm with filter

500mm no filter

I assure you that the blurring is not introduced by outside factors. It's the filter doing its thing, and the effect is repeatable. I keep it only as a lens cap for that 500mm lens.

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