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

[Erik Gunst Lund]

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You can't let the firmware in the camera or the software in PP correct for this 'automatically' that's also why you have the sliders in PP.

Similar to moire patterns, not always fixable with a slider.

The issues are far too complex and interacting to make a mathematical fix all solution.

 

With my current lens and camera lineup I rarely see Purple Fringing even though I often shoot into the sun or flashlights... An old habit of mine is to test any new equipment for it's strengths and weaknesses and make a note of them and if I don't have a need for that particular tool I sell it on...

 

Also it's easy to see when someone has 'overdone' or 'cooked' an image in PP with these sliders, too much color will be removed from areas not needing correction.

 

I see a lot of fine images where 'cooking' the image in PP the Bokeh gets ruined or shadow/highlight areas and then they blame the lens. This threw me off when reading reviews in the beginning, later i learned to investigate things better and only trust two or three reviewers and look right trough the rest.

 

Thanks to all for the interesting views so far! Very interesting reading

[CheshireCat]

At high intensity light levels, the information becomes non-linear and depending on its intensity, blooming, whatever, will be increasingly spurious. , etc.".

 

You still don't explain why the "blooming" and the "whatever" phenomena prefer purple.

[pgk]

You still don't explain why the "blooming" and the "whatever" phenomena prefer purple.

Not all fringing IS purple. I'm not sure that I can explain any colour let alone all of them:eek:.

[algrove]

You are far more an expert than i in this area anyway. How could you explain cyan/blue fringing like this one, if any? Just curious.

 

Simple stuff-they are blue spruce.ha-ha.

[lct]

Ha ha! yes and with black spruce we would get black fringing i guess. We could call that "Spruce CA" or even "Cameleon CA" if it works with lizards as well.

Edit: Spruce CA... Epicea... Even Etymologists would not mind.

[Rick]

Rick

 

Whilst your explanations make good sense in some ways, you cannot ignore the sensor/software issues which convert the image thrown onto the sensor into the image file we have to deal with.

 

Yes, the sensor is part of the image chain and we can't ignore it but, it isn't helpful to think of it as the unexplainable dumpster for what we can't explain. LoCA can be almost completely understood and explained with lens optics.

 

At high intensity light levels, the information becomes non-linear and depending on its intensity,

 

The sensor doesn't become non-linear just because the light intensity is high. The sensor is designed to be non-linear regardless of the intensity of light.

 

 

blooming, whatever, will be increasingly spurious. Even when there is underlying CA there will be additional distortions and sometimes even when there is no CA there will still be distortions in the data. So, whilst you may be dealing with CA (or sometimes not) there are other elements causing problems in images.

 

True, there are sensor phenomena that may cause artifacts, just not the ones in the OP's example.

 

You say yourself "nobody has correctly explained the purple fringing that is brighter against a bright background like sky and then is adjacently gone when a branch has a dark background behind it. So, some decide that it must therefore be something else, like the sensor. But, it is still OOF-CA. It just needs to be explained by the optics of the OOF-CA and it can be easily done so.", and go on to say "Yes, I agree that there exist many occasions where there are sensor effects like blooming, and there is flare, etc.".

 

The light that produces the magenta fringe is primarily reflected light from the branch. Most of us have seen this when we have taken a picture of a meter stick to look for focus accuracy of our lenses. We observe that the black markings and numbers on the meter stick are magenta in front of the plane of focus and green behind. There is no other light except what is reflected off of the black markings and some from the meter stick itself. Yet, the markings can be strikingly magenta and green off the plane of focus (POF). And the area of focus is black and contrasty. I think we all agree this is LoCA. What is interesting to me is how far off the plane of focus this effect can be observed.

 

So, imagine that the tree branch is laying on the optic bench and we observe a similar phenomena. But, also observe that the branch is blurred and therefore larger where it is out of focus. It is also less contrasty. So, we could imagine that the sensor has a less contrasty, larger image as well projected on it.

 

Now we get to the branch. The branch is in the way of the light from the sky so, it is casting a distinct shadow on the sensor. But, in the areas off the POF the branch is casting a sort of penumbra. An area that is part wider branch (because it is out of focus) and it is also letting the light from the sky fall on the same area. And, her comes the kicker; it is also projecting a magenta hue both inside the branch area and in the wider out of focus area just like the numbers on the meter stick example.

 

In the case when the lens is back focused we of course get magenta. The magenta plus the white light from the sky add together and you get a bright magenta. Once the branch has brush behind it, most of the effect is not detectable because in these areas there exists no bright white light. It is still there, but there is no white to highlight it.

 

Where we still disagree is that I see a more complex picture than that formed purely by optics (no pun intended;)), although I have to say that often the components of the image that I find problematic are subtle and would be both difficult and time consuming to adequately post on a forum such as this (I'd have to show before and after post processing and explain all the adjustments made and so on - 'though I might try as I come across images which display such problems). I am happy to accept that some fringing is CA, but I'm still far from convinced that all fringing is as a result of CA or, in some cases, even has any component of CA. CA as far as I am concerned, is a culprit but not the only one and sometimes it neither occurs nor cause problems where I would expect it to.

 

[And just in case you think that this is all being rather pedantic, I do have at least one (scientific) application where high contrast edging can cause problems when looking at some 'irridescent' colour markings on small fish which reflects so strongly that it is difficult to maintain highlight detail in them....]

 

I believe that the OP's example and the one of the window is just LoCA and can be explained with optics. By the way, the Magenta is formed by the lens from Blue and Red that are closer together in the secondary spectrum of an achromatic or apochromatc spectrum. These two colors are at opposite ends of the primary spectrum and can be observed this way in examples of Lateral CA or simple spherical aberration. This is just like the primary spectrum observed in all single lens design and is simply the prismatic effect of the lens where the order is ROY-G-BIV. Contrary to lens doublet designs where a secondary spectrum is created that is an effect of the achromatic lens design.

 

Back to the window example. If you look carefully at the window example, and you really should, you can see magenta with a thin blue line and a thin red line on the border of the wide magenta line. This is proof the effect is a result of the achromatic lens design because it is formed by Red and Blue (from the lens!), LoCA. Also, imagine that depending on the example, the color of the LoCA could be anything from blue to purple to other colors that are hard to explain otherwise.

 

Have at it guys,

 

Rick

[pgk]

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Rick

 

I'll sort out some images and then start a new thread based on what I can illustrate. May take a bit of time. Will post the link idc.

[mjh]

The sensor doesn't become non-linear just because the light intensity is high. The sensor is designed to be non-linear regardless of the intensity of light.

A sensor doesn’t need to be designed to be linear; that is just a by-product of the way it works, namely to use the energy from incoming photons to create electron-hole pairs and to collect those electrons. It is linear up to the point when the pixel’s full-well capacity is reached and clipping (i.e. non-linearity) occurs.

[Rick]

A sensor doesn’t need to be designed to be linear; that is just a by-product of the way it works, namely to use the energy from incoming photons to create electron-hole pairs and to collect those electrons. It is linear up to the point when the pixel’s full-well capacity is reached and clipping (i.e. non-linearity) occurs.

 

Of course the sensor itself is a linear device. If, you double the amount of light the sensor will double the charge up until the pixel is full.

 

But, the human eye is not. The camera or RAW converter applies a tone curve or gamma which ends up looking like an S-curve to the data. This is what is designed to be non-linear.

 

I assumed this is what pgk was speaking of.

 

Rick

[pgk]

I assumed this is what pgk was speaking of.

I was referring to the point close to or near where clipping occurs, which is when the data become spurious and less reliable. As can be seen when a near blow highlight is 'pulled back' in software with both posterisation and colour imbalances resulting (in film days this was known as high intensity reciprocity law failure and we got used to it, accepted it and frowned down upon the digital failures of high intensity problems, which I suspect we are now getting used to).

[CheshireCat]

I was referring to the point close to or near where clipping occurs, which is when the data become spurious and less reliable.

 

A sensel does not become "spurious and less reliable", it just saturates (clipping) in a very predictable way.

 

With strong light, all nearby sensels (R,G, will saturate, and demosaic algorithms will interpret this as white, not purple.

 

Posterization color artifacts appear when clipping occurs.

These are examples of posterization:

 

http://www.l-camera-forum.com/leica-forum/leica-m9-forum/107297-rainbow-skies.html

 

You can see the sky looks normal when no color or only B is clipping, then looks cyan where also G is clipping, and finally looks white where also R is clipping.

Notice no purple artifacts at all.

[pgk]

A sensel does not become "spurious and less reliable", it just saturates (clipping) in a very predictable way.

Predictable problems can be dealt with in software ......

[CheshireCat]

Predictable problems can be dealt with in software ......

 

I wish you were right. I really wish.

[Rick]

I was referring to the point close to or near where clipping occurs, which is when the data become spurious and less reliable. As can be seen when a near blow highlight is 'pulled back' in software with both posterisation and colour imbalances resulting (in film days this was known as high intensity reciprocity law failure and we got used to it, accepted it and frowned down upon the digital failures of high intensity problems, which I suspect we are now getting used to).

 

I don't know what you mean by "data becomes spurious and less reliable." Posterization is not caused by spurious data?

[pgk]

Rick

 

We are getting to the point where it will be increasingly time consuming to provide detailed and adequate posts to explain and illustrate my thoughts and concerns - a limitation of forum posts.

 

I suspect that if we sat down for an hour in front of a computer screen with a selection of images showing fringing and otherwise, that we could have a very interesting and civilized discussion on the whys and wherefores of what is going on;). But for the record, my position remains the same, that whilst I can see your arguments, they do not explain everything and there are anomalous factors which won't explain easily. Given enough time and effort such discussions could go on here and may even reach some conclusions. To do so though, I think that everyone needs an open-minded and inquiring approach and to accept that there are other experiences and understanding of issues beyond there own, and to be honest I think this might be a stumbling block;).

 

So when I get chance I will try to shoot some 'test' images (they will only ever be of 'real world' subject matter as I won't shoot test charts) and post these on the forum to request explanations of the anomalies which hope to illustrate. I hope this is satisfactory as I see little point in simply providing more and more statements of disagreement in this thread.

[Exodies]

I'd love to have a catalogue of image defects and how to spot them both singly and in combination. Has no one published such a thing? It should cover operation within spec as well as broken equipment.

[CheshireCat]

I'd love to have a catalogue of image defects and how to spot them both singly and in combination. Has no one published such a thing?

 

There's a lot of nice information on the Internet, as there's a lot of nice items in the city dump

[CheshireCat]

Another example of LoCA with the APO Summicron 75/2.

Like I said, green behind focus, purple in front... ahem...

 

[sorry, couldn't resist ]

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[Exodies]

Purple fringe is indeed a horticultural problem. Your gardener will sort it out for you.

 

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Hallo,

Look here: Leica M

[lct]

Like I said, green behind focus, purple in front... [...]

Purple fringe is indeed a horticultural problem [...]

Indeed

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