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05-13-2011, 01:40 AM   #1
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Measurement of lens flare (veiling glare) with and without UV filter

Hi all,
Lens flare caused by UV filters has been a hotly debated topic for a long time.
Some people say UV filters are useful to protect the lens while others say you should avoid them because they degrade the image too much.

Curious to know the truth about the matter, I set out to carefully measure how much additional lens flare (veiling glare) is caused by adding a UV filter to a lens.

How the tests were done.
The tests were conducted by measuring the edge blur of a precision back illuminated 5 degree slanted edge. The back illumination was varied from a contrast ratio of 46:1 to 5900:1. This corresponds to an exposure range of -2 EV to +5 EV, where 0 EV is mid-grey.

The tests were conducted on a Sigma 17-70mm lens (at 50mm) with and without a Kenko Pro 1D UV filter.

The tests show:
1) Veiling glare of the lens, without the UV filter, is negligible at low contrast ratios but increases quickly once the contrast ratio rises above 700:1. This is a 17 element zoom lens.

2) The UV filter contributes very little additional veiling glare up to a contrast ratio of 5900:1.

Conclusions
At normal scene lighting levels, a high quality UV filter contributes little additional veiling glare, in the measurement range of this study.

The primary component of veiling glare is caused by the lens itself. Another study with prime lenses will be necessary to determine whether a similar relationship holds.



05-13-2011, 01:07 PM   #2
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You should go and work at Lenstip.com They have done a lot of similar tests:
UV filters test - Testing procedure - Lenstip.com

Your conclusions about flare only apply to GOOD UV filters, it should be noted. The results of flare unfortunately are all to easy to see with a bad UV filter:

05-13-2011, 02:10 PM   #3
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I started my study in reaction to seeing the Lenstip study. They carefully measured UV transmission which I felt was mostly irrelevant on today's digital cameras.

The really interesting issue, for me, was whether there was significant degradation of image quality as a result of using them.

But they did not measure this, they merely showed some comparative images taken under extreme conditions that did not represent normal use.

So I set about doing some careful measurements of veiling glare so that I could answer the question whether, under normal conditions, good UV filters degraded image quality.

Yes, my results apply to good filters. It seemed a complete waste of time to test bad filters since it is a given that bad filters will give bad results.

But that does not change the central finding, that good filters introduce an imperceptible loss of image quality.
05-13-2011, 07:21 PM   #4
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QuoteOriginally posted by labnut Quote
But that does not change the central finding, that good filters introduce an imperceptible loss of image quality.
That's fair enough and makes perfect sense.

I guess the real-world issue is that for anyone to share your conclusion, they need to use a good quality UV filter in the first place. Most of the UV filters out there, alas, are probably the bad or marginal quality ones.

05-14-2011, 09:13 AM   #5
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QuoteOriginally posted by rawr Quote
I guess the real-world issue is that for anyone to share your conclusion, they need to use a good quality UV filter in the first place. Most of the UV filters out there, alas, are probably the bad or marginal quality ones.
The lenstip study is a good place to start looking for a high quality filter. My study did not try to answer that question, which filters are good.
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