[alexeyga]

Got a silly question up here... was testing an older prime lens the other day and wide open it was showing some pretty bed purple fringing... Then, just for the sake of it i've mounted a UV filter on it and the images came out much cleaner...

I'm not a big fan of filters, but i'm wondering if it does actually help to limit the purple fringing effect?

[Just1MoreDave]

Until I had a similar experience, I would have said that you must be completely mistaken. However, I noticed that a polarizer filter really helped fringing on my Tokina SD 400mm f5.6. I noticed it while testing something else. I have it on my list of things to retest when it gets warmer.

[Digitalis]

I'm not a big fan of filters, but i'm wondering if it does actually help to limit the purple fringing effect?

not a silly question, it's an interesting one. It is possible that a UV filter or Polariser could cause this. I surmise that the reduction of contrast that a filter causes could influence this. But other than that, I haven't the faintest Idea.

[atiratha]

I wonder, could it be the MC coating of the filters?

[blende8]

You are right.
Lenses are not well corrected at the borders of the visible spectrum, especially the UV end, starting at ca. 410 nm.
Those color fringes occur at hard contrast edges. This is mainly UV light reaching the sensor. Due to imperfect chromatic correction fringes appear at the border.
The digital sensors are very insensitive to UV light, fortunately, but at such strong contrasts the light is intense enough so that a little fraction is reaching the sensor.
One can reduce this by using a strong UV filter. A good one is the B&W 420, which is blocking everything below 420 nm.

NB: With the UV filter you can of course only reduce the UV light.
You cannot reduce purple fringing that originates from the addition of visible red+blue light.

[estudleon]

Thank you Blende 8. Very informative and instructive too.

[Digitalis]

I seem to recall that glass isn't a very good transmitter of ultraviolet light. Correct me if I'm wrong, I seem to recall that Standard glass will absorb as much as 70% UVA and UVB light. Hence, the use of specialised quartz lenses for ultraviolet photography. And then you have to consider modern multi coating techniques, I'm pretty sure they would have UV inhibiting coatings on most lenses. I have my doubts that much ultraviolet light gets through lenses these days...may I remind you how long it takes to sun bleach a radioactive Takumar...depending on the severity of the yellowing, it takes up to several weeks.


besides the ultraviolet spectrum, like infra-red does not focus on the same plane as visible light. So even if it is getting through to the sensor, the effect it would have would be an insignificant variable.

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Last edited by Digitalis; 03-15-2009 at 06:10 AM.

[alexeyga]

Thanks for the info guys.

But it kinda sucks, I'm a bit of a purist and I trust the statement that one's glass is as good as the filter mounted on top of it... thus, I don't use any at all (occasionally I can mount a polariser, but that's as far as it goes...)

Well, I guess it might actually be a good Idea to snap a good UV at lest on fast prime that are used wide open most of the time...

[Digitalis]

I'm the same Alexyga, Large format photographers never put filters on their lenses (excepting centre ND grads to counter heavy vignetting on extreme wide angle lenses) neither do medium format photographers, it seems to be a 35mm photographer phobia of getting fingers on their lenses..besides the lens selection I use there are some of the sharpest lenses ever made...the last thing I want is for a filter to make a mess of it all.

Sure, I'll use polarisers, but I don't leave them on any of my lenses..I don't like rely on them to make an image 'work'...or overuse them like so many photographers these days do. I only occasionally use grads, and the ones I use are premium filters made my Lee. Who use the highest quality coated optical glass in their filters, I don't trust cokin.

[Parallax]

...............one's glass is as good as the filter mounted on top of it... .

Absolutely true. I think if you stick with top end filters such as the B+W F-Pro you won't ever see any negative effects.

[Ben_Edict]

There was a lengthy discussion and thread, accompanied with many example images (very well-made!) in the Tamron forum, where the thread-opener (sorry, but forgot his name) showed very clearly, that a front-mounted UV-filter can improve image quality in older lenses. He used a Tamron 300/2.8 (60B) lens as an example whichz he tested against a much more modern and expensive Olympus lens. The Tamron first was good, but very much outperformed by the Oly lens. When he added the front filter to the older Tamron, the lens performed much better - wide open, that is. The improvement was only significant wide open, I must emphasize that.

The reason was the poorer colour correction of the old Tamron, which lead to more frequencies being not optimally focused.

So my conclusion would be, that a UV filter can reduce colour fringing (which is a sign of chromatic aberration). At the same time, this would improve resolution and contrast.

Ben

[estudleon]

There was a lengthy discussion and thread, accompanied with many example images (very well-made!) in the Tamron forum, where the thread-opener (sorry, but forgot his name) showed very clearly, that a front-mounted UV-filter can improve image quality in older lenses. He used a Tamron 300/2.8 (60B) lens as an example whichz he tested against a much more modern and expensive Olympus lens. The Tamron first was good, but very much outperformed by the Oly lens. When he added the front filter to the older Tamron, the lens performed much better - wide open, that is. The improvement was only significant wide open, I must emphasize that.

The reason was the poorer colour correction of the old Tamron, which lead to more frequencies being not optimally focused.

So my conclusion would be, that a UV filter can reduce colour fringing (which is a sign of chromatic aberration). At the same time, this would improve resolution and contrast.

Ben

I think that the filter (UV, SKYLIGHT, POL., etc) can improve the color transmission of the lens (specially in oldest cold lens like tamrons make the image more warm) but NEVER can improve the resolution.

Be sure that if you put any filter over your lens, the l/mm (or PPI) and the contrast will decrease, except the appearance of more contrast due to the transmission (don't be equal to contrast) improved.

The POL is another history, and it can be necessary in many times.

[mithrandir]

Blende8 brings up an interesting point. Is there such a thing as a purple filter? Maybe a small reduction in red and blue light (on the fringe) would be useful to correct for purple fringing.

[Digitalis]

So my conclusion would be, that a UV filter can reduce colour fringing (which is a sign of chromatic aberration). At the same time, this would improve resolution and contrast.

many years ago I did a test of filters,polarizers and UV filters using an old 50mm f/1.4 Pentax Takumar. And I have never seen an increase in resolution brought about by simply adding a filter, at any f stop. I have certainly seen the opposite where fine resolution was oblitherated by the use of a poor quality filter. I have seen improvements in contrast because of the filters coatings helping reduce flare, that's one of the best ways to improve an older lens,but modern lenses really don't need them. UNLESS you really need to protect your lenses against dust or water. And in those cases only use the best you can get.

[Ben_Edict]

many years ago I did a test of filters,polarizers and UV filters using an old 50mm f/1.4 Pentax Takumar. And I have never seen an increase in resolution brought about by simply adding a filter, at any f stop. I have certainly seen the opposite where fine resolution was oblitherated by the use of a poor quality filter. I have seen improvements in contrast because of the filters coatings helping reduce flare, that's one of the best ways to improve an older lens,but modern lenses really don't need them. UNLESS you really need to protect your lenses against dust or water. And in those cases only use the best you can get.

I am completely with you. I did not say, that all lenses can be improved by using a front filter. I simply quoted a real life example of one particular lens. Even if the old Tamron 60B uses ED glass, it is a very old design and will profit from a UV filter, because of its longer focal length. Short fl lenses will usually exhibit less chromatism anyway, as the different foci for different wavelengthes are not that far spread out, as is the case (naturally) with longer and fast lenses. Also, the measurements (which where very thorough) showed clearly, that the advantage of the filter vanished, when stepping down the lens.

I think that the filter (UV, SKYLIGHT, POL., etc) can improve the color transmission of the lens (specially in oldest cold lens like tamrons make the image more warm) but NEVER can improve the resolution.

Be sure that if you put any filter over your lens, the l/mm (or PPI) and the contrast will decrease, except the appearance of more contrast due to the transmission (don't be equal to contrast) improved.

The POL is another history, and it can be necessary in many times.

The UV filter in my example (Tamron 60B) does NOT improve colour transmission. It reduces the amount of far Blue and near UV light. The improvement in resolution is a natural and quite logical result of using the filter as you remove "colours" from the image, which cannot be brought to the same focal plane, as the image forming visible part of the spectrum. Just simply physics and no guesswork needed.

Ben