[butangmucat]

Hello,

I have been mainly using film-era lenses on my digital body and I noticed that lenses ranging from lower-end ones like Tokina RMC 28mm F2.8 Lens Reviews - Tokina Lenses - Pentax Lens Review Database to some more premium ones like the Super-Takumar 1:2/35 which I acquired recently all exhibited noticeable CA on my Sony digital body. It seems that all film-era lenses will exhibit some extent of CA on digital bodies, is that true or just that I am not getting lenses good enough?

Another thing is that I read somewhere that a filter that cuts blue to purple lights can suppress CA showing up on an image, and I have tried from simple Tiffen UV protectors to their Haze-1 filters, and also their skylight 1A filter. It seems that I am not suffering from purple fringing but cyan fringing, is this related to the usage of these filters?

Sincerely

[GUB]

Hello,

I have been mainly using film-era lenses on my digital body and I noticed that lenses ranging from lower-end ones like Tokina RMC 28mm F2.8 Lens Reviews - Tokina Lenses - Pentax Lens Review Database to some more premium ones like the Super-Takumar 1:2/35 which I acquired recently all exhibited noticeable CA on my Sony digital body. It seems that all film-era lenses will exhibit some extent of CA on digital bodies, is that true or just that I am not getting lenses good enough?

Another thing is that I read somewhere that a filter that cuts blue to purple lights can suppress CA showing up on an image, and I have tried from simple Tiffen UV protectors to their Haze-1 filters, and also their skylight 1A filter. It seems that I am not suffering from purple fringing but cyan fringing, is this related to the usage of these filters?

Sincerely

The colour of the fringing is dependant on whether it is closer than or beyond the focussed point.

[Wild Mark]

The colour of the fringing is dependant on whether it is closer than or beyond the focussed point.

Yes, it comes down to the optical formula. The best way to manage this is to stop the lens down and/or be careful in subject selection.

[Digitalis]

As others have said above CA is dependent on the optical construction of the lens - apochromatic lenses are designed to magnify red/blue/green by the same amount* so they produce as little CA/fringing as possible - however, they are extremely rare at focal lengths wider than 100mm. With an achromatic** lens which are much more common at focal lengths below 100mm the blue and red spectrum are often corrected, but the green/magenta spectra is usually out of kilter***. As focal length gets longer, this deficiency becomes increasingly obvious And close focusing only exacerbates the differential in magnification, which is why it is better to stop down at minimum focus distance as nearly all lenses are optically compromised at MFD.

Filters will have no effect on CA.

*Or as near enough as physically possible.
**A precusor of the apochromatic lens, achromatic lenses are only corrected for two wavelengths of light - typically red/blue. Apochromatics are corrected for three wavelengths. Super-apochromatics are corrected to focus four and sometimes five wavelengths at the same plane.
*** Fringing can appear in different spectra too, green/magenta, yellow/blue, blue/red fringing are rather common.

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Last edited by Digitalis; 07-20-2016 at 07:40 AM.

[butangmucat]

As others have said above CA is dependent on the optical construction of the lens - apochromatic lenses are designed to magnify red/blue/green by the same amount* so they produce as little CA/fringing as possible - however, they are extremely rare in focal lengths wider than 100mm. With an achromatic** lens which are much more common at focal lengths below 100mm the blue and red spectrum are often corrected, but the green/magenta spectra is usually out of kilter***. As focal length gets longer, this deficiency becomes increasingly obvious And close focusing only exacerbates the differential in magnification, which is why it is better to stop down at minimum focus distance as nearly all lenses are optically compromised at MFD.

Filters will have no effect on CA.

*Or as near enough as physically possible.
**A precusor of the apochromatic lens, achromatic lenses are only corrected for two wavelengths of light - typically red/blue. Apochromatics are corrected for three wavelengths. Super-apochromatics are corrected to focus four and sometimes five wavelengths at the same plane.
*** Fringing can appear in different spectra too, green/magenta, yellow/blue, blue/red fringing are rather common.

I think the theory of using a filter which cuts blue/purple light is that by limiting the blue-purple portion of light entering the lens, purple fringing can be suppressed.

[Wild Mark]

I think the theory of using a filter which cuts blue/purple light is that by limiting the blue-purple portion of light entering the lens, purple fringing can be suppressed.

What you say makes intuitive sense. The physics suggests otherwise. Perhaps the idea 'lessens' the effect? Can you test this idea?

[Digitalis]

the theory of using a filter which cuts blue/purple light is that by limiting the blue-purple portion of light entering the lens, purple fringing can be suppressed.

A filter that strong would also negatively impact colour rendition in the blue/purple spectrum*, such an imbalance would be plainly obvious especially in scenes where clear blue skies and purple flowers dominate. Theory Debunked.

*Filters with such a narrow band-pass do exist and are used for astro-photography, however I have never seen them used for casual photography.

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Last edited by Digitalis; 07-20-2016 at 07:41 AM.

[Wild Mark]

A filter that strong would also negatively impact colour rendition in the blue/purple spectrum*, such an imbalance would be plainly obvious especially in scenes where clear blue skies and purple flowers dominate. Theory Debunked.

maybe and maybe not. It depends on what you are shooting. If you are shooting scenes with limited blue spectra then the benefit would be apparent. PPing could be use to manage loss of blue spectra. But as you suggest, such effort would really narrow your subject material (e.g. astrophotography) and the effort exerted seems counter intuitive to the photography experience (i.e. lots of effort to get a photo)

[Digitalis]

I work with the SMCP FA77mm f/1.8 - a lens which does have issues with purple fringing. However for 90% of my work it is simply not an issue, I rarely see it. I have the lens stopped down far enough to avoid it. And even if it does show itself, Adobe light room and appropriate lens corrections can me more effective ( and less destructive to IQ) than any filter.

[butangmucat]

What you say makes intuitive sense. The physics suggests otherwise. Perhaps the idea 'lessens' the effect? Can you test this idea?

OK I thing I found what I have read: Using An Ultraviolet Filter

The only practical optical role for Ultraviolet filters in digital photography seems to relate not to scatter, but to an artifact known as Chromatic Aberration or “purple fringing“).

and

Shoot with a Haze-2A or other strong UV-cut filter.

from https://en.wikipedia.org/wiki/Purple_fringing

I haven't touched the physics subject for about three years and I have forgot a lot of things.

---------- Post added 07-20-16 at 08:24 AM ----------

A filter that strong would also negatively impact colour rendition in the blue/purple spectrum*, such an imbalance would be plainly obvious especially in scenes where clear blue skies and purple flowers dominate. Theory Debunked.

*Filters with such a narrow band-pass do exist and are used for astro-photography, however I have never seen them used for casual photography.

Well I found this from here: https://en.wikipedia.org/wiki/Purple_fringing#cite_note-2

I don't know if this is limited to certain circumstances.

[Apet-Sure]

Hmmm....that would be an interesting experiment - shooting leaves against a bright cloudy sky with wide apertures, with and without a UV filter. My gut tells me that the UV filter wouldn't make any difference in suppressing PF. But I was wrong once before.

[Digitalis]

My gut tells me that the UV filter wouldn't make any difference in suppressing PF.

Correct. The biggest problem with eliminating PF is the face that no-one really knows precisely what causes it.

There are plenty of theories, but none of them are really testable, In a scientific sense....sometimes PF happens.

[Adam]

Digital tools for fixing fringing are pretty good these days. Have you tried removing fringing in camera raw?

[GUB]

I experimented with an ir uv cut filter on the Q at one time. At the end decided there wasn't much gain but the first test was quite dramatic. I was using a TV remote to produce ir and the older M series clearly transmitted it where as the Q 01 lens pretty well blocked it. And the ir uv filter blocked it. I was wondering how much infrared was the source of purple fringing.
https://www.pentaxforums.com/forums/136-pentax-q/248682-fringe-killing-q.html

[Digitalis]

I was wondering how much infrared was the source of purple fringing.

There is a theory that the source of PF is intense ultraviolet, which is at the opposite end of the visible spectrum from IR. My issue with this theory is that nearly all digital cameras already have strong UV/IR filters built in and PF doesn't show up on film - even with the same lenses involved.