[rgonzale]

I'm seeing significant purple fringing in some circumstances with the Pentax 50mm f/1.7 FA lens on a K5 body with the lens wide open. The lens has a little fungus around the front edges but is otherwise clear. See the leaf with sun glare above the left lip of the pot:



Yet the same scene using an old Pentax 50mm f/1.4 SMC-M lens wide open (on the same K5 body) does not show purple fringing:



And a Rikenon P 50mm f/2 lens wide open has only a touch of fringing:



If I stop down the Pentax 50mm/1.7 FA lens to f/2.8 or smaller the purple fringing starts to disappear. Is purple fringing a known problem when using this lens wide open?

[UncleVanya]

The first exposure looks brighter than the other two. Also the light from the window looks a bit more flared. Maybe the angle of the light changed? I would try a few more shots in different lighting. My f1.7 af Pentax is the F series and I don’t recall a ridiculous amount of CA but it isn’t immune either.

[GUB]

I'm seeing significant purple fringing in some circumstances with the Pentax 50mm f/1.7 FA lens on a K5 body with the lens wide open. The lens has a little fungus around the front edges but is otherwise clear. See the leaf with sun glare above the left lip of the pot:

Attachment 560908

Yet the same scene using an old Pentax 50mm f/1.4 SMC-M lens wide open (on the same K5 body) does not show purple fringing:

Attachment 560911

And a Rikenon P 50mm f/2 lens wide open has only a touch of fringing:

Attachment 560913

If I stop down the Pentax 50mm/1.7 FA lens to f/2.8 or smaller the purple fringing starts to disappear. Is purple fringing a known problem when using this lens wide open?

Fringing tends to happen in areas outside of the focus area. I suspect your focus differs slightly between the two shots. Set up a test along something like a ruler so that the perfect centre of focus is identifiable. I have done it looking up at power wires against the sky but in a way that the wires recede across the image.

[ChristianRock]

I've had lenses with a little bit of fungus (they've been cleaned by Eric Hendrickson now) and fungus will do that against light. The flaring and fringing is normal.

You might not see much effect from the fungus in light that is more spread out, but in this situation, you likely will.

Is the fungus recent? Leave the lens against sun light in a window for a few days. If the fungus is fresh it dies, then after you store the lens away for a few days most of the fungus becomes dust inside the lens. Still cleaning is recommended, but you can get rid of most of the fungus effect that way (I learned in practice, bought a lens lot a couple years ago that had fungus, some I donated, some I sent to be cleaned).

[pschlute]

good job you were not using the 31mm LTD. That is a real CA monster at wide open (when taking pictures with very harsh back-light)

[ChristianRock]

good job you were not using the 31mm LTD. That is a real CA monster at wide open (when taking pictures with very harsh back-light)

Bad copy? I have heard this from the 77mm in some cases, but most users of the 31mm seem to be happy with it, and the copy that OpticalLimits.com tested with the K-5 had basically no problems with CA. Perhaps on full frame edges the story is different....

[BarryE]

Fringing tends to happen in areas outside of the focus area. I suspect your focus differs slightly between the two shots. Set up a test along something like a ruler so that the perfect centre of focus is identifiable. I have done it looking up at power wires against the sky but in a way that the wires recede across the image.

... or lay a metal ladder on a table outside shooting down the length of the ladder. The light reflecting off the metal will give fringing in out of focus areas. Best done on an overcast day so light direction is not relevant and comparisons are consistent. I had purple fringing revealed in all its glory doing this with a 77 Limited while testing the AF fine adjustment. It was stark in the oof areas, easy to replicate and therefore will give good comparisons of different lenses.

[BigMackCam]

Yet the same scene using an old Pentax 50mm f/1.4 SMC-M lens wide open (on the same K5 body) does not show purple fringing

I see green fringing there, on the highlights of the plant pot.

I'm with @GUB - I think this is longitudinal CA you're seeing due to difference in precise focus accuracy... correct AF fine adjustment will eradicate it at the intended focus distance, but you're going to see it in front of, and behind, the focal point in certain circumstances...

[pschlute]

Bad copy?

Nope. The original FA 31mm LTD can suffer from CA at wide apertures when photographing high contrast scenes where the subjects are not all in the same plane of focus. Just like all fast lenses of its era

[UncleVanya]

Nope. The original FA 31mm LTD can suffer from CA at wide apertures when photographing high contrast scenes. Just like all fast lenses of its era

While it can, I found the fa 43 and 77 to be more prone to it than the 31. I have very few shots with pronounced ca from my fa 31. Not zero of course.

[BarryE]

While it can, I found the fa 43 and 77 to be more prone to it than the 31. I have very few shots with pronounced ca from my fa 31. Not zero of course.

Agree, empirically. The 31 seems less problematic. It does, however, flare a lot more than the 77 & 43. Any high contrast, back lit shot is a CA challenge for most lenses. Slight refocussing and direction can change the severity, significantly and render tests like this hard to compare.

[bschriver11]

Is there any fast lens that doesn't produce some fringing when shot wide open in extreme contrast?

[rgonzale]

If there is good sun again tomorrow I'll try to replicate with the same three lenses again. I actually did numerous shots today and only saw the fringing with the FA lens. But it is true that the camera chose a brighter exposure with the FA lens than it did with the manual lenses.

[UncleVanya]

But it is true that the camera chose a brighter exposure with the FA lens than it did with the manual lenses.

Maybe set to manual mode but adjust shutter to keep the equivalent exposure? That’s not to say it isn’t helpful to know it meters differently.

[stevebrot]

I see LoCA (both near and far) in the first image that blooms to PF in the grossly overexposed leaf surface of the first image. This behavior is not unexpected for digital capture and is less a characteristic of the lens as it is of the sensor and low-level image processing. Repeat with actual exposure closer to that of the images taken with the Pentax-M 50/1.4 and see if the results are not more similar.


Steve