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Filters and Chromatic Aberration
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PostPosted: Wed Sep 07, 2011 5:07 pm    Post subject: Filters and Chromatic Aberration Reply with quote
After noticing the CA produced by the Argus Cintar - I was wondering would using a UV filter might reduce fringing when using color film?

Does anybody have experience with this? I have a set of filters for the C3 so I might try it next time I put film in it.

I assume that using a red filter with B/W film would also reduce fringing.

Any experience with this? Google has lead me to some opinions - but I'm hoping this being a forum just for lenses that maybe somebody's tested this out, or at least has some first hand experience.

Thanks!


PostPosted: Wed Sep 07, 2011 7:10 pm    Post subject: Reply with quote
Hi,

I have never tried it on film, but i can ensure you that a good UV filter (B+W, in my case) reduced the purple fringing affecting my EBC Fujinon 135/2.5 when used wide open (tested on the same scene at a few secs. interval - the time to screw the filter on the lens) ...

I suppose thus that the effect would have been the same if i had used a film camera instead of my K10D ...


PostPosted: Wed Sep 07, 2011 9:46 pm    Post subject: Reply with quote
Opinion (personal theory): A UV filter makes no change whatsoever to the lens color corrections which reduce CA. Reduction of purple fringing by UV filter may be related to the reduction in UV.

Looking forward to more experiences! Very Happy


PostPosted: Wed Sep 07, 2011 10:34 pm    Post subject: Reply with quote
visualopsins wrote:
Opinion (personal theory): A UV filter makes no change whatsoever to the lens color corrections which reduce CA. Reduction of purple fringing by UV filter may be related to the reduction in UV.

Looking forward to more experiences! Very Happy


Well that is what I mean. Will it remove light that adds to fringing - enough so to have any noticeable effect?

I guess I'll try it and see. Thanks for answers!


PostPosted: Thu Sep 08, 2011 8:18 am    Post subject: Reply with quote
I guess this trick is only applicable to B/W photography.

For color photography adding an UV filter will not help to reduce the CA (at least not in the visible light range).


PostPosted: Thu Sep 08, 2011 5:39 pm    Post subject: Reply with quote
visualopsins wrote:
Reduction of purple fringing by UV filter may be related to the reduction in UV.


Well, it seems a quite valid explanation ... Nevertheless, i must confess that it was not my idea (using the UV filter on that lens), i read it while browsing some Google searches on "EBC Fujinon 135 2.5 test review", at least another user has done it before me ...

Anyway, it worked for me: the purple fringing in high contrast areas was strongly reduced. It didn't totally disappear, but it was reduced to acceptable levels (will post some sample pics as soon as i will have the time to browse my archives) ...


PostPosted: Thu Sep 08, 2011 6:10 pm    Post subject: Reply with quote
I tried a UV on the Tamron SP 300mm and it made no difference at all.


PostPosted: Thu Sep 08, 2011 9:38 pm    Post subject: Reply with quote
martinsmith99 wrote:
I tried a UV on the Tamron SP 300mm and it made no difference at all.

I can't see why it would. UV light is invisible!

Not only that, but CA occurs because the lens is focussing different wavelengths at different distances, with green in the centre. Purple fringing occurs because the film/sensor plane is at an average distance from the lens, (ie where the green light focusses), which leaves the red and blue out of sync. UV light (if we could see it) focusses even beyond the blue and IR light beyond the red, which is why some lenses have an IR mark on the focus dial.

courtesy of Wikipaedia:


PostPosted: Thu Sep 08, 2011 9:47 pm    Post subject: Reply with quote
peterqd wrote:
martinsmith99 wrote:
I tried a UV on the Tamron SP 300mm and it made no difference at all.

I can't see why it would. UV light is invisible!

[...]


But not all UV wavelengths are invisible to sensor/film...I'm thinking these wavelengths which are not well corrected are reduced by UV filter, much like a "Haze/UV" filter reduces haze which cannot be seen with eyes, but is seen on film and digital sensor.


PostPosted: Thu Sep 08, 2011 9:52 pm    Post subject: Reply with quote
visualopsins wrote:
But not all UV wavelengths are invisible to sensor/film

I was just a poor pun! Rolling Eyes


PostPosted: Thu Sep 08, 2011 10:29 pm    Post subject: Reply with quote
The highest resolution using lenses can be achieved using monochromatic light (and very short UV wavelenghts). 1200 lpm and more may be done.

In Astro there are special filters like the Baader "fringe killer" that does exactly that, clipping from teh end of the spectrum the "unwanted" parts that lead to blurry images. You may want to try that out...

http://www.baader-planetarium.de/sektion/s41/s41.htm scroll down a bit... (there is an English list also)


PostPosted: Thu Sep 08, 2011 10:30 pm    Post subject: Reply with quote
visualopsins wrote:
peterqd wrote:
martinsmith99 wrote:
I tried a UV on the Tamron SP 300mm and it made no difference at all.

I can't see why it would. UV light is invisible!

[...]


But not all UV wavelengths are invisible to sensor/film...I'm thinking these wavelengths which are not well corrected are reduced by UV filter, much like a "Haze/UV" filter reduces haze which cannot be seen with eyes, but is seen on film and digital sensor.


Even some humans can see UV....


PostPosted: Thu Sep 08, 2011 10:50 pm    Post subject: Reply with quote
kds315* wrote:
visualopsins wrote:
peterqd wrote:
martinsmith99 wrote:
I tried a UV on the Tamron SP 300mm and it made no difference at all.

I can't see why it would. UV light is invisible!

[...]


But not all UV wavelengths are invisible to sensor/film...I'm thinking these wavelengths which are not well corrected are reduced by UV filter, much like a "Haze/UV" filter reduces haze which cannot be seen with eyes, but is seen on film and digital sensor.


Even some humans can see UV....

Yes, I used to think I could see it myself sometimes, but I never found any way of putting it to practical use. But those days were long ago and I certainly can't see it now. I imagine the visible spectrum diminishes as we age, similar to the audible spectrum.

Getting back to the CA, even if a UV filter does affect blue light I think it's right to say that you would still be left with the unfocussed red light, so the fringing would become warmer in colour but not disappear completely.


PostPosted: Thu Sep 08, 2011 11:29 pm    Post subject: Reply with quote
peterqd wrote:
kds315* wrote:
Even some humans can see UV....

Yes, I used to think I could see it myself sometimes, but I never found any way of putting it to practical use. But those days were long ago and I certainly can't see it now. I imagine the visible spectrum diminishes as we age, similar to the audible spectrum.

Getting back to the CA, even if a UV filter does affect blue light I think it's right to say that you would still be left with the unfocussed red light, so the fringing would become warmer in colour but not disappear completely.
Klaus was referring to aphakic indiviuals (no lens), as the lens absorbs most of the UV. Your spectral sensitivity probably isn't going to be affected with age, as it is highly unlikely that one predominantly loses only one of the cone types. Visual acuity is a different chapter, though focusing problems are going to be the main issue.


PostPosted: Thu Sep 08, 2011 11:30 pm    Post subject: Reply with quote
kds315* wrote:
Even some humans can see UV....


Very Happy Most (all?) human eyes produce the SWS1 opsin, sensitive to UV. ChR2 opsin (blue) sensitivity to UV also seems possible to me. The brain is sensitive to UV as well.


PostPosted: Fri Sep 09, 2011 6:56 am    Post subject: Reply with quote
peterqd wrote:
Not only that, but CA occurs because the lens is focussing different wavelengths at different distances, with green in the centre. Purple fringing occurs because the film/sensor plane is at an average distance from the lens, (ie where the green light focusses), which leaves the red and blue out of sync. UV light (if we could see it) focusses even beyond the blue and IR light beyond the red, which is why some lenses have an IR mark on the focus dial.

courtesy of Wikipaedia:

This only accounts for longitudinal CA and for a single lens element, as shown in the illustration. Actual lenses may be well corrected for green/red LoCA but only display some blue/yellow CA.
Lateral CA is caused by different wavelengths of light having different magnification, not different focus depth.

http://toothwalker.org/optics/chromatic.html


PostPosted: Fri Sep 09, 2011 3:24 pm    Post subject: Reply with quote
My little experience wiith the UV filters to avoid teh CA, told me:

1- In the real word, and for general use, the differences weren't very high. I use only normal lenses. At the time the 1,7/50 hexanon AR.

2- The filters UV (one leica and one B&W) did that the pl/mm were lower. From the 90 to 70.

3- I don't use filters anymore.

Rino


PostPosted: Fri Sep 09, 2011 4:18 pm    Post subject: Reply with quote
AhamB wrote:
This only accounts for longitudinal CA and for a single lens element, as shown in the illustration. Actual lenses may be well corrected for green/red LoCA but only display some blue/yellow CA.
Lateral CA is caused by different wavelengths of light having different magnification, not different focus depth.

http://toothwalker.org/optics

Yes, you're right of course, and thankyou for the link, but I was trying to keep it simple. I couldn't find a suitable diagram for lateral CA and for this purpose the single element diagram on Wiki makes it much clearer then the one with 2 elements. Longitudinal or lateral, the point I made about filters not affecting CA remains the same though, doesn't it?
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