Zuiko 24mm 2.8 purple fringing at infinity

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Archive 2009 · Zuiko 24mm 2.8 purple fringing at infinity

melcat
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p.2 #1 · Zuiko 24mm 2.8 purple fringing at infinity

All the OM Zuikos focus past infinity unless someone has been in there fiddling. I've had this lens for more than a decade and used it on half a dozen OM bodies, and, like all my other Zuikos except one, the focus stop is about 0.5mm past infinity measured on the barrel. There's a particular tech in this town who likes to "fix" this and I suspect he's had his hands on that one lens sometime in the past.

I'm looking at a shot from my archive taken with my copy of this lens on a Canon 5D. There's lateral chromatic aberration towards the corner of the frame: magenta to the left and green to the right (centre) of some rigging-like spaceframe architecture. It was a very bright day and I remember discarding several other shots taken together with this with another lens that couldn't handle the flare. There's spherical aberration and some coma. All of this is visible in Bridge's loupe tool but it appears I decided not to bother correcting it as it would not be visible at any normal print size. I've also seen lateral chromatic aberration with this lens on slide film - under a microscope.

The Zuiko 24mm f/2.8 MC wasn't APO and made no claim to be.

But there's no green opposite from the magenta fringe on the gable of the OP's shot, so it isn't *lateral* chromatic aberration. And, as I said, this lens's lateral chromatic aberration is much less severe than what we see here.

Oct 09, 2009 at 12:50 PM

Jonas B
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p.2 #2 · Zuiko 24mm 2.8 purple fringing at infinity

olyacme wrote:
Besides the growing difficulty...
(...)
So, the upshot is that a 50/1.4 which was marketed as being as well corrected wide open as at f/2.8, and Apochromatic at f/2.8 on a 135 sensor, would be a great beast of a lens, suitable for capturing 200MP images.

I need some time to digest all this. I think I can get used to call what usually has been, and is, called longitudinal CA for defocus CA instead. I understand the differences between the old plain CA, or lateral CA, and the "new" longitudinal CA.

For an APO and relatively fast 50mm lens, well, maybe a great beast of lens isn't on any manufacturer's agenda... Oh well. I'm tired of defocus CA which in my opinion made us all used to see ugly green stuff in the background OOF parts of images, or strange tone transitions in B&W images. (Maybe the Leica 90Cron AA is the fastest and shortest APO lens I've learned to know. It wasn't perfect though and I sold it for a mix of reasons.)

I'll try to relax.

Thank you for all comments and answers!

Oct 09, 2009 at 02:00 PM

Toothwalker
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p.2 #3 · Zuiko 24mm 2.8 purple fringing at infinity

cogitech wrote:
I've always loved Paul Van Walree's articles on lens aberrations.
Very good reading (even if he does use the annoying term "purple fringing")
http://toothwalker.org/optics.html

A purple fringe is a purple fringe, isn't it? Is there a good reason not to use the term "purple fringing"?

Oct 09, 2009 at 03:11 PM

cogitech
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p.2 #4 · Zuiko 24mm 2.8 purple fringing at infinity

Toothwalker wrote:
cogitech wrote:
I've always loved Paul Van Walree's articles on lens aberrations.
Very good reading (even if he does use the annoying term "purple fringing")
http://toothwalker.org/optics.html

A purple fringe is a purple fringe, isn't it? Is there a good reason not to use the term "purple fringing"?

I think the term is too broad and misleading. It seems to me that "purple fringing" can be caused not only by a combination of red and blue chromatic aberration, but also by many other possible factors, including:

* Chromatic aberration in each CCD cell (microlenses)
* Image processing and interpolation artifacts (almost all CCDs require considerable processing)
* Stray ultraviolet light
* Stray infrared light
* Image bloom from overexposure
* Leaks between cells of the CCD

So, CA can be purple, but "purple fringing" is not necessarily always attributable to lens CA. The terms are not synonymous. For this reason, I prefer that the use of the term "purple fringing" not be used when referring to lens CA which just happens to be purple due to a combination of red and blue CA. If the fringing is purple due to red/blue CA in the lens, then it should be referred to as CA. Calling it purple fringing opens up a can of worms that leads to misunderstanding and confusion on the part of those who are not aware of the many other likely causes of "purple fringing". I've seen too many people conclude that a lens has CA because they see "purple fringing" in their photos. Often enough, the "purple fringing" they see is not an attribute of their lens at all.

This is just my relatively uninformed opinion, of course. I hope you understand that I meant no offense to you for using the term. The documentation you provide on aberrations is the best I have read.

Oct 09, 2009 at 03:56 PM

olyacme
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p.2 #5 · Zuiko 24mm 2.8 purple fringing at infinity

cogitech wrote:
I think the term is too broad and misleading. It seems to me that "purple fringing" can be caused not only by a combination of red and blue chromatic aberration, but also by many other possible factors, including:

I'm ambivalent on the term, as it pretty much boils down to being a subset of LoCA.

cogitech wrote:
* Chromatic aberration in each CCD cell (microlenses)

Microlenses are one per sensel so CA in them shouldn't have much effect. The very nature of a microlens works against this, as they are designed to capture light that lands anywhere on their surface and concentrate it upon the photosite that lies under their (approximate) centre. Even if it did, there is no clear reason why they'd scatter light only into neighbouring red and blue sensels, and not green sensels, to create a purple fringe.

cogitech wrote:
* Image processing and interpolation artifacts (almost all CCDs require considerable processing)

Again, there's no clear reason why interpolation artifacts should be purple and not some other colour, so I'd discount this as a cause. Processing can make an existing purple fringe worse, but unless it's broken it won't create it.

cogitech wrote:
* Stray ultraviolet light
* Stray infrared light

These are much reduced by typical cameras and lenses, but even if they get through, and happen to be interpreted by the sensor as purple, I'm OK with calling them purple fringes.

cogitech wrote:
* Image bloom from overexposure
* Leaks between cells of the CCD

Blooming is charge leaking across cells, but, yet again, there's no obvious reason why charge would prefer to leak to photosites capped only with red or blue, but not green, filters. I'd very strongly discount blooming as related to purple fringing, which negates the concern about term ambiguity.

cogitech wrote:
So, CA can be purple, but "purple fringing" is not necessarily always attributable to lens CA.

It's not a perfect term, but the vast majority of the time it's going to be LoCA at fault. It'd be a good thing if, when people used "purple fringing", they also understood that LoCA was the underlying cause though.

cogitech wrote:
The terms are not synonymous. For this reason, I prefer that the use of the term "purple fringing" not be used when referring to lens CA which just happens to be purple due to a combination of red and blue CA. If the fringing is purple due to red/blue CA in the lens, then it should be referred to as CA.

Red/Blue CA would show up far more strongly, as these two together are a much bigger component of white light than violet. It's not impossible to find a lens that leaked these, but it would be a very poor performer. Even still, I'd be OK with calling the result "purple fringing".

cogitech wrote:
Often enough, the "purple fringing" they see is not an attribute of their lens at all.

I've yet to find convincing examples where this was the case. It can probably happen in exotic circumstances, but it's certainly not even the cause of a significant minority of "purple fringing" cases with the cameras and lenses we're using.

Oct 09, 2009 at 04:39 PM

cogitech
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p.2 #6 · Zuiko 24mm 2.8 purple fringing at infinity

I'll take your word for it. I have no doubt that you and Paul know better than I do.

I'm trying still to understand why I see distinct blue and red components in the CA samples posted here, but people seem fine to call it "purple".

Oct 09, 2009 at 04:52 PM

olyacme
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p.2 #7 · Zuiko 24mm 2.8 purple fringing at infinity

cogitech wrote:
I'll take your word for it. I have no doubt that you and Paul know better than I do.

I'm trying still to understand why I see distinct blue and red components in the CA samples posted here, but people seem fine to call it "purple".

For what it's worth, I'm not at all sure that simple CA is causing the colour bleed that started this thread. If it's the leaves that are at issue, then it looks to me like they're mostly just out of focus, mixing their dark shades with the blue of the sky. Both sorts of CA are likely also there, judging from the colour spreads around the window panes. Coma could be in there too, to give the foggy look and further blend the blue sky into the leaves. But it's really hard to say exactly what's going on from an uncontrolled test like this.

This image is copyrighted by the owner

Oct 09, 2009 at 05:52 PM

PhotoMaximum
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p.2 #8 · Zuiko 24mm 2.8 purple fringing at infinity

Agreed...

I would redo the test with the image in focus this time...

Oct 09, 2009 at 06:02 PM

Toothwalker
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p.2 #9 · Zuiko 24mm 2.8 purple fringing at infinity

cogitech wrote:
This is just my relatively uninformed opinion, of course. I hope you understand that I meant no offense to you for using the term. The documentation you provide on aberrations is the best I have read.

Thanks - and there is no way anyone could offend me with such harmless remarks. When people say they observe purple fringing, we know they see purplish fringes but we don't know the cause. When people say they observe chromatic aberration, we know the cause but not the color scheme of whatever artifact they see. Both observations are valid statements, just of a different order. When people say they observe purple fringing due to chromatic aberration, we have a more complete description, but I think this is too much too expect. Further I agree with olyacme's analysis.

Oct 09, 2009 at 06:50 PM

min lee
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p.2 #10 · Zuiko 24mm 2.8 purple fringing at infinity

I posted two images, the first was in focus. Anyway, I made the adjustment to the focus ring. It works fine for me now.

Oct 09, 2009 at 06:53 PM

Jonas B
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p.2 #11 · Zuiko 24mm 2.8 purple fringing at infinity

min lee wrote:
I posted two images, the first was in focus. Anyway, I made the adjustment to the focus ring. It works fine for me now.

The first image shows less details though than the second one. Look at the roof to the left of the windows, or the wall just to the right of the windows. To me it seems as the ugly image showing all the CA is a bit more in focus. Or, where did you focus?

Oct 09, 2009 at 07:34 PM

AhamB
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p.2 #12 · Zuiko 24mm 2.8 purple fringing at infinity

I agree with Jonas; the image with the CA is definitely not focused beyond infinity.

Dec 08, 2009 at 10:33 PM

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