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04-21-2019, 10:56 AM   #1
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Purple fringing with Pentax M 50mm 1.7

Yesterday evening I took some photos at a local pond where there is a swan nesting. I used the 50mm 1.7 and also a Super Takomar 135mm 3.5, and with the 50mm I notice there was a lot of purple fringing on the shots of the nesting swan. The swan was very bright Inthe photo and the background darker. Is this expected or could there be a problem with the coatings on the lens?

04-21-2019, 11:00 AM - 1 Like   #2
Zav
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I would expect it.
04-21-2019, 11:10 AM - 3 Likes   #3
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QuoteOriginally posted by mccririck Quote
Is this expected or could there be a problem with the coatings on the lens?
In absolute terms, neither is expected per se. Purple fringing is how longitudinal chromatic aberration is expressed with many digital sensors and is exacerbated by strong contrast boundaries and even mild overexposure. Swans and wedding dresses are common subject examples and even with mild LoCA, some fringing may occur in those cases: sparkly sun on water and sunlit chrome are others. I have not done extensive evaluation of my copy of the Pentax-M 50/1.7 for PF, but such would not surprise me at f/1.7 or f/2. Some degree of LoCA is typical of fast lenses wide open or nearly so.* The usual solution is to avoid shooting wide open with that type of subject and perhaps bracket exposures to avoid overexposure in the whites. (A mild underexposure may be pulled up in post-processing.)

Addendum: I just tested my copy on my K-3 at f/1.7 and f/2.0 in bright diffuse light on a chrome fixture, 2-stops overexposed. I was able to produce mild PF at f/1.7 and none at f/2.0 or with one stop less exposure.**

Note: Purple fringing is not something one sees in color film photography; yes, there may be LoCA, but not the contagious spreading fringe seen with PF. I did some extensive testing a few years back with a few lenses with a reputation for PF and was unable to get PF on film...waste of a good roll of Gold 200.


Steve

* About five reviews on this site indicate mild purple fringing at f/1.7 while one other indicated strong fringing wide open and one more reported terrible flare and strong purple fringe when doing night shots (two related?). Others made a point that the lens is CA and PF free. The total number of reviews was 220.

** This is one way to provoke PF. Another is low angle sun on water riplets, again, overexposed. Twigs with bright overcast behind is supposed to be another way, but I have found that method less consistently successful. The acid test is electronic flash on crinkled aluminum foil. I may be difficult to get the combination of flash, wide aperture and slight over exposure, but if a lens is going to fringe, the foil will bring it out. I might add that it is good to make sure the light spectra is fairly rich towards the blue end.

Last edited by stevebrot; 04-21-2019 at 11:46 AM.
04-21-2019, 11:47 AM   #4
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QuoteOriginally posted by stevebrot Quote
In absolute terms, neither is expected per se. Purple fringing is how longitudinal chromatic aberration is expressed with many digital sensors and is exacerbated by strong contrast boundaries and even mild overexposure. Swans and wedding dresses being common examples and even with mild LoCA, some fringing may occur in those cases. Sparkly sun on water and sunlit chrome are others. I have not done extensive evaluation of my copy of the Pentax-M 50/1.7 for PF, but such would not surprise me at f/1.7 or f/2. Some degree of LoCA is typical of fast lenses wide open or nearly so. The usual solution is to avoid shooting wide open with that type of subject and perhaps bracket exposures to avoid overexposure in the whites. (A mild underexposure may be pulled up in post-processing.)


Steve
Thanks. So I should have stopped down to 5.6 maybe?

04-21-2019, 12:23 PM - 1 Like   #5
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QuoteOriginally posted by mccririck Quote
Thanks. So I should have stopped down to 5.6 maybe?
That would probably do it, but might be overkill. The threshold in my test was only 2/3 stop narrower than full open (f/2.0 vs f/1.7). It might be easier to avoid even slight overexposure of whites. You can "chimp" the preview image at full resolution (100%) for fringing and and adjust aperture and/or exposure as seems reasonable.


Steve
04-21-2019, 12:23 PM - 1 Like   #6
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Stepping down helps a lot. There are also ways to remove it in post processing.


BTW, this one of the reasons why Pentax just brought out a new 50mm f/1.4 and why it is so big and expensive. Eliminating PF is not easy.


Pentax M, FA, and D FA* 50mm F1.4 comparison - Review Announcements | PentaxForums.com
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