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Justin Hancock Jun 28, 2006 11:36 PM

So tonight I decided to test my kit lense for my Rebel XT at various f-stops and with/without my Canon UV filter.

The first/left image is WITH the UV filter while the second/right image is WITHOUT the UV filter:

It appears that through the different f-stops, the image WITH the UV filter comes in clearer.

My assumption was that the UV filter, if anything, slightly degrades the image quality as it is adding another piece of glass between the object and the sensor. Why does it seem to improve image quality?

slipe Jun 29, 2006 12:38 AM

The one on the right looks to not be in focus. Not sure why you can't focus without a UV filter.

If you set the focus with the filter before taking all the shots you might have to re-focus without the filter.

Justin Hancock Jun 29, 2006 12:49 AM

That could be, here was my routine:

UV filter on...
Autofocus then set to manual focus for the following set of shots...
Shot at f/3.5 through f/22 - every 2 stops

Took UV filter off (ruined focus)
Autofocus then set to manual for rest of shots...
Shot at f/3.5 through f/22 - every 2 stops

Perhaps the focus simply wasn't spot on after I took off the UV filter, which then threw off all the shots without the UV filter on.

Am I correct in the thinking that UV filters should, if anything, degrade image sharpness?

slipe Jun 29, 2006 1:50 AM

I doubt a good quality coated UV filter would degrade anything that you would notice. Most digital cameras filter UV so it is just a lens protection. My personal preference is to not have the extra two air/glass surfaces but I might change my mind on that if I ever scratch an expensive lens coating.

tclune Jun 29, 2006 8:41 AM

As a general rule, you are correct that a filter that is supposed to be transparent will degrade or, in the limiting case, not affect your photo. In the examples you posted, the suggestion that the focus was wrong in the no-filter case is very probable.

However, not all filters that are billed as "UV" are truly invisible to your camera's sensor. Some of them would be more accurately called "haze filters." They extend down into the visible range a bit. In that case, you may find the photo improved under the circumstances that the filter was designed for -- if you are shooting a hazy scene the image should look sharper than without the filter. That doesn't apply to your test, but it is worth keeping in mind that some of these filters are actually useful as filters under some conditions. BTW, such filters would be detrimental under other conditions -- they would skew the chromatic content of the scene.

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