Is this CA problem on my 50/1.4 ASPH, it's normal or not?

[jaapv]

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I would think most modern Leica lenses would be APO corrected to a considerable degree. I am also not sure how you would define when a lens becomes defined as "APO" rather than Achromatic, for the degree of perfect convergence of all three prime colours. Maybe within a certain multiple of the median wavelengths along the optical axis.

 

I am not sure it is longitudinal CA, as I would have expected the Noctilux 0.95 to be worse than the 50 ASPH Summilux but in my experience it is better.

 

Wilson

Achromatic is when CA is eliminated at two points of the spectrum, APO is when it is eliminated at three.

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[Peter Branch]

Cards, camera brand, firmware, whatever, has little to nothing to do with this. As soon as a lens is capable of rendering high contrast edges, this problem will appear on any sensor.

Many sources attribute this problem to CA in the lens. However, I do not buy that. For one thing, the better the lens is corrected, the more chance that it is prone to purple fringing.

Secondly, if it were true CA, one would expect a green fringe on the contralateral side of the contrast edge (yes, I know about longitudinal CA). Furthermore one would expect the same effect on film, which not.

 

I am convinced that it is an interpolation artifact in in that the Bayer demosaicing gets disturbed by the contrast difference.

 

However, whatever the cause,as this is an universal problem, so almost all postprocessing programs have a slider to remove the fringe.

Simply use the tools in your Lightroom or Photoshop or C1 to remove this.

 

E. Puts, I can't remember were, has attributed such effects to light leakage between sensor pixels on high contract edges. Nothing to do with the lens except that very well corrected lenses will have higher contrast sharper edges.

 

FWIW the 50mm f/1.4 Summilux-M ASPH is to all intents and purposes an Apo lens. Quite why Leica declined to recognise this in the lens description has been the subject of much speculation. E. Puts has expressed his view on this matter in print and confirmed that by the standards of other manufacturers the lens would undoubtedly have been labelled as Apo.

[pgk]

Nothing to do with the lens except that very well corrected lenses will have higher contrast sharper edges.

If I may comment, I would suggest that its less about the contrast and more about the accuracy of delieation of high contrast edges (ie. they are imaged with higher precision in a well corrected lens) - the difference is subtle but I suspect important.

[Peter Branch]

Achromatic is when CA is eliminated at two points of the spectrum, APO is when it is eliminated at three.

 

It seems that that used to be the simple definition of Apochromatic and indeed some early Leica Apo lenses, e.g.the 180mm f/3.4 Apo-Telyt - R conformed, but in that particular example the "Third" wavelength brought to the same focal plane was in the near IR.

 

Since those times things seem to have become more complicated. It is clear that different manufacturers have adopted different definitions of what constitutes an Apo lens. This has been possible because the simple definition is just that - simple. For example does the elimination of CA extend to both longitudinal and lateral CA? Does it have to apply to all points in the image field or just the centre?

 

In practice, as the graphs published by E. Puts clearly demonstrate, current Leica and Zeiss Apo lenses make no attempt to eliminate CA at three wavelengths. These designs minimise the secondary spectrum across the full range of visible wavelengths. This is not the same thing but in ordinary photography gives a better result than the strict three wavelength definition.

[Luke_Miller]

E. Puts, I can't remember were, has attributed such effects to light leakage between sensor pixels on high contract edges. Nothing to do with the lens except that very well corrected lenses will have higher contrast sharper edges.

 

I can duplicate the issue shown in the original post with virtually any lens and digital camera I've used. Some better and some worse, but all will show it. Just a digital fact of life. As others have said - no fault in the lens.

[jaapv]

It seems that that used to be the simple definition of Apochromatic and indeed some early Leica Apo lenses, e.g.the 180mm f/3.4 Apo-Telyt - R conformed, but in that particular example the "Third" wavelength brought to the same focal plane was in the near IR.

 

Since those times things seem to have become more complicated. It is clear that different manufacturers have adopted different definitions of what constitutes an Apo lens. This has been possible because the simple definition is just that - simple. For example does the elimination of CA extend to both longitudinal and lateral CA? Does it have to apply to all points in the image field or just the centre?

 

In practice, as the graphs published by E. Puts clearly demonstrate, current Leica and Zeiss Apo lenses make no attempt to eliminate CA at three wavelengths. These designs minimise the secondary spectrum across the full range of visible wavelengths. This is not the same thing but in ordinary photography gives a better result than the strict three wavelength definition.

 

From what Erwin told me I distilled that the problem is not so much that manufacturers in general do not adhere to the definition of three primary colours, but that the areas outside those points remains undefined. Leica and Zeiss try indeed to get close throughout the spectrum, some other manufacturers are no as scrupulous as that.

[adan]

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Purple fringing around highlights is NOT chromatic aberration.

 

It is excess light energy spilling into surrounding black and dark areas/pixels, and is due to a contrasty lens in contrasty lighting.

 

It is purple/violet because the violet/ultraviolet end of the visible spectrum is the most energetic (the same reason that it is UV light, and not red/infrared light, that burns your skin in sunburn). And thus spills further from the guilty blown highlight into the dark areas, and is the most emphatic color.

 

True chromatic aberration is due to angled glass surfaces splitting white light up into bands of color, just like Newton's prism. Resulting in bi-colored edges to opposite sides of objects (red/cyan or yellow/blue or purple/green) as the different light colors spread in a rainbow (latin: aberrare - to wander from the correct path).

 

An APO lens minimizes the splitting of light into colors, and thus corrects chromatic aberrations.

 

It has no effect on or connection with purely purple fringing, which is caused by a completely different phenomenon of physics.

 

As mentioned, ANY lens, whether APO or not, can produce purple fringes on ANY digital sensor - if a highlight is blown out through massive local overexposure and excess light overflows into the surrounding areas.

[pgk]

As mentioned, ANY lens, whether APO or not, can produce purple fringes on ANY digital sensor - if a highlight is blown out through massive local overexposure and excess light overflows into the surrounding areas.

But not all do, which still puzzles me!

[jaapv]

I would say it is purple because the number of green pixels is higher, thus when there is a sampling/interpolation problem purple will be dominant.

 

Purple fringing around highlights is NOT chromatic aberration.

 

It is excess light energy spilling into surrounding black and dark areas/pixels, and is due to a contrasty lens in contrasty lighting.

 

It is purple/violet because the violet/ultraviolet end of the visible spectrum is the most energetic (the same reason that it is UV light, and not red/infrared light, that burns your skin in sunburn). And thus spills further from the guilty blown highlight into the dark areas, and is the most emphatic color.

 

True chromatic aberration is due to angled glass surfaces splitting white light up into bands of color, just like Newton's prism. Resulting in bi-colored edges to opposite sides of objects (red/cyan or yellow/blue or purple/green) as the different light colors spread in a rainbow (latin: aberrare - to wander from the correct path).

 

An APO lens minimizes the splitting of light into colors, and thus corrects chromatic aberrations.

 

It has no effect on or connection with purely purple fringing, which is caused by a completely different phenomenon of physics.

 

As mentioned, ANY lens, whether APO or not, can produce purple fringes on ANY digital sensor - if a highlight is blown out through massive local overexposure and excess light overflows into the surrounding areas.

[CheshireCat]

Purple fringing around highlights is NOT chromatic aberration.

It is excess light energy spilling into surrounding black and dark areas/pixels, and is due to a contrasty lens in contrasty lighting.

 

This does not make sense. Surrounding sensels are mapped also to red and green.

The fact purple fringing happens also when the sensor is not saturated (e.g. tree branches over a blue sky) is enough to disprove that theory.

 

As a nice experiment, the OP can take a couple other photos of his Toyota driving wheel, one frontfocused with respect to specular highlights, and one backfocused (like the one he posted). The fringing in the frontfocused shot will be green instead of purple, and much more difficult to notice.

 

As mentioned, ANY lens, whether APO or not, can produce purple fringes on ANY digital sensor - if a highlight is blown out through massive local overexposure and excess light overflows into the surrounding areas.

 

Most APO lenses are not corrected for UV, therefore they still suffer from purple fringing in the presence of strong UV light. However, purple fringing caused by the visible light component will be strongly attenuated with respect to normal lenses.

[adan]

Yes, cheshire cat, but I am talking about light spilling, not electrons spilling. So the red (and especially) green pixels do not respond to the spilled light, any more than they respond to other blue/violet light.

 

Same goes for jaap's suggestion - green pixels don't see much blue light - thus no green fringes unless there is green light (due to lens CA).

 

A blue sky is close to saturated in the blue/violet wavelengths, therefore the same spill occurs (but would not with a red sky at sunset).

[CheshireCat]

E. Puts, I can't remember were, has attributed such effects to light leakage between sensor pixels on high contract edges. Nothing to do with the lens except that very well corrected lenses will have higher contrast sharper edges.

 

A citation is required.

In any case, E.Puts was probably talking about CCD bloom, an issue that does not affect CMOS sensors.

If you care to read E.Puts review of the APO Cron 50 (cited in one of my previous posts), you will understand that he is talking about lens aberrations, not sensor issues.

 

FWIW the 50mm f/1.4 Summilux-M ASPH is to all intents and purposes an Apo lens. Quite why Leica declined to recognise this in the lens description has been the subject of much speculation. E. Puts has expressed his view on this matter in print and confirmed that by the standards of other manufacturers the lens would undoubtedly have been labelled as Apo.

 

Luckily, Leica does not use the standards of other manufacturers. Owners of both the Lux and the APO Cron could verify if and how "much better APO" the Cron is at f/2.

[thighslapper]

All this has been argued about at length on this forum before ...... as has the cause of the 'Italian Flag' and why each side of the sensor has different colours ..... and although fairly convincing arguments have been proposed (including info from Leica) I have yet to find anyone who is an expert in sensor technology and optics provide a DEFINITIVE explanation of these phenomena........

 

From what I can see, the manufacturers know these things exist, accept them as a characteristic of the product and try and minimise them as much as possible ..... and I'm sure some R&D boffin out there somewhere could explain the interplay of the different factors ...... but I assume they are too busy out skateboarding to pop into the Leica forum to illuminate us ....

[CheshireCat]

Yes, cheshire cat, but I am talking about light spilling, not electrons spilling.

 

Andy, how can you explain photons spilling out of a sensel and back into another sensel, and back again into several other far pixels ? Photons are not water.

 

Instead, the purple ghost can be explained with longitudinal chromatic aberration (i.e. the purple image is magnified with respect to the "normal" image).

[k-hawinkler]

I just curious and ask anyone who own the Lux50 asph have problem above??? or this is normal?

 

Normal ....... and this fringing is the same with virtually all fast lenses used wide open on digital sensors in situations where there is sharp edges with blown (or near blown) highlights adjacent.

 

From previous discussions I recall the consensus being that it is a mix of CA and sensor issues.... not purely just one or the other ......

 

Given the right conditions it can be truly awful and I am sure no lens is immune ....... and no sensor either .....but it can also produce some interesting images ...... this is the 50/1.4 inadvertently fairly wide open while taking a shot of the sea (cropped)... this is mostly the poor old sensor and the bayer matrix getting all confused by the sharp changes in illumination in adjacent cells ....

 

Terrific image! Thanks.

[thighslapper]

Andy, how can you explain photons spilling out of a sensel and back into another sensel, and back again into several other far pixels ? Photons are not water.

 

Instead, the purple ghost can be explained with longitudinal chromatic aberration (i.e. the purple image is magnified with respect to the "normal" image).

 

Sorry, but I think you are wrong and Adan is fundamentally right .....

 

The actual sensor and the individual elements are in reality extremely crude and rough .... the microlenses on top are pretty crappy as well ....... this light spill-over is a fact of life in sensor design ...... there is enormous amount of signal processing, cleaning and manipulation that goes on to clean up the cruddy output of the raw sensor cells ....

[CheshireCat]

Sorry, but I think you are wrong and Adan is fundamentally right .....

 

Sorry, but what you and Andy think is not going to demonstrate anything.

 

As usual in this forum, there are too many words and too few images.

I just shot (100% crops) a couple Palm trees at about 300m, horribly overexposed on purpose. First shot is at infinity (what most users will do), and second shot has been taken rotating the focus ring just a little to move the focus plane to have the same amount of defocus, but this time slightly in front of the palm trees.

 

It should be now clear that purple fringing is caused by the lens, not by the sensor.

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[BerndReini]

The purple fringing is caused by the interplay between the lens and the sensor.

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Hallo,

Look here: Leica M

[adan]

Instead, the purple ghost can be explained with longitudinal chromatic aberration (i.e. the purple image is magnified with respect to the "normal" image).

 

Actually, longitudinal CA is where one color is focused in the wrong plane compared to the other colors, NOT where it is magnified with respect to the rest of the image.

 

http://media.digitalcameraworld.com/wp-content/uploads/sites/123/2012/10/Chromatic_abberration_lens_distortion.jpg

 

LATERAL CA is the result of one color image being magnified with respect to the other colors- the type where a branch or whatever is imaged as overlapping and misaligned sharp red and cyan or purple and green or blue and yellow images. Resulting in dual complementary color fringes. Which are the easiest to fix in processing, since it only requires scaling of one or more color channels to reverse the "magnification" disparity.

 

As a user of the 75 Summilux, I am painfully familiar with longitudinal CA. And it has several characteristics that make it fundamentally incompatable with the images from the 50 Summilux ASPH shown previously.

 

1. the fact one color is actually out of focus and not just displaced, means the images are low-contrast. It's like running a partial gaussian blur on the image - overall monochrome contrast is reduced.

 

2. The color fringes occur throughout the image, not just in or beside the brightest highlights, with a corresponding staining of the highlights as well as the dark areas as one color is "subtracted" from bright areas to be spilled into the shadows.

 

http://www.l-camera-forum.com/leica-forum/digital-forum/145063-what-chromatic-aberrations-really-look-like.html

 

As to:

 

how can you explain photons spilling out of a sensel and back into another sensel, and back again into several other far pixels ? Photons are not water.

 

Actually, the wave theory of light considers light to be very much like water, in useful ways, and uses that similarity to explain a lot of optical phenomena, such as diffraction, that cannot be explained by a classical "particle-only" theory of light.

 

Ultimately the wave-particle duality of photon behavior requires digging into some pretty dense quantum electrodynamics to explain the observed phenomena. (And let's not even go near "quantum entanglement"! )

[jaapv]

Sorry, but what you and Andy think is not going to demonstrate anything.

 

As usual in this forum, there are too many words and too few images.

I just shot (100% crops) a couple Palm trees at about 300m, horribly overexposed on purpose. First shot is at infinity (what most users will do), and second shot has been taken rotating the focus ring just a little to move the focus plane to have the same amount of defocus, but this time slightly in front of the palm trees.

 

It should be now clear that purple fringing is caused by the lens, not by the sensor.

 

You defocused -i.e. widened the contrast transition, which turns this into a less sharp lens, which will not show purple fringing.

Nor does this experiment explain why the effect does not occur on film.