blue fringing?

[jgwill1]

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Just managed to get my hands on an M9 for a few days with a view to eventually buying one. Now there are some wonderful qualities to this camera of which I'm sure you are all aware, but that said, I must say I'm alarmed at the blue fringing I've found in the top corners of the frame. I was shooting tree branches against an overcast sky with a 35mm F2 Summicron Asph. Is this normal with a digital rangefinder; something I'm going to have to live with? Any thoughts would be welcome.

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[thrice]

It's normal with defocused areas with blown out exposures on almost every camera, poorer lenses will mask the CA with spherical aberration. I should clarify - by "defocused areas" I mean anywhere except the plane of exact focus, ie. the exact distance at which the rangefinder was focused. If the phenomenon is more severe in the corners it might be lateral CA (but then you'd get two colours, not just blue) or may be an indicator of curvature of field.

 

A sample shot with aperture and shooting info might help more.

 

A few choice APO lenses will be immune to this phenomenon, like the Leica 100mm APO-Macro-Elmarit-R, Voigtlander 180mm APO-Lanthar and the Coastal Optics UV-VIS-IR 60mm f/4.0 Macro. I have yet to come across another lens that shows no defocus colour shift in high contrast transitions, it's one of the reasons I haven't purchased a 75mm APO-Summicron, while it satisfies the 'APO' condition of having no lateral CA in the plane of focus, it (and the 50 Summilux ASPH) have blue/green bokeh in some situations which I find slightly unsightly, an indication of excellent correction for spherical aberration though.

[jaapv]

There are a few threads on this. It is normal with a high-resolution lens on a digital sensor. However, it is very easily removed in RAW conversion.

http://www.l-camera-forum.com/leica-forum/digital-post-processing-forum/115965-fringing-technical-problem.html

[jgwill1]

Many thanks for your help. You're right my info was a little sketchy! Here's the offending article cropped from the very top left of the frame. Aperture was around f4 i believe.

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[adan]

Edge fringing along strong light-dark borders is so common in digital imaging that most RAW processing programs now have a specific "defringing" setting - in Adobe products it is under "Lens corrections" - that finds edges and desaturates any narrow color bands along them. In addition to the regular red/cyan or blue/yellow corrections for chromatic aberrations.

 

There can be multiple sources for this fringing - lens CA, sensor microlens CA, spillover of excess energy from bright pixels to dark pixels ("blooming"), and so on. Fortunately it usually occurs in a mathematically predictable way, which makes computed correction fairly easy.

 

In your case, I see just a tad of yellow along the opposite (left) sides of the branches, (less strong then tha purple/blue, and partly hidden by the brightness of the sky) so you are likely getting a mix of lens CA and energy spill.

 

I note that my own preset corrections for the 35 Summicron ASPH in Adobe Camera RAW include a minus-7 setting for blue/yellow chromatic fringing, as well as "defringing" all edges.

[thrice]

Andy, your description of blooming is new to me, I have only seen it when a pixel (quantum well) has become saturated and clipped. You do get noise clipping as the 'well' approaches saturation, but you will not get blooming. I'm not sure whether Kodak used anti-blooming drains or anti-blooming gates but there is definitely some precaution taken to control bloom. I would consider it impossible to get bloom below the saturation point of the pixel, and we get pure white from the charge-transfer condition being exceeded well before the 'well' is actually full.

 

Feel free to correct me as my knowledge of the KAF-18500 is limited and you may have some technical knowledge that I do not.

 

I do agree though, jgwill1's problem looks like lateral CA exacerbated by the slight 'stretching' in the extreme corners with the 35mm summicron asph, which can be seen in the distortion curve on the technical data document leica provide here: http://us.leica-camera.com/assets/file/download.php?filename=file_1743.pdf

[jgwill1]

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Many thanks to all who responded. Though a professional photographer for many years I remain quite a luddite in my grasp of digital technology. I suppose having used Leica Ms with black and white film almost exclusively, I expected the qualities of my lenses to be be replicated identically with digital technology; I have now been thoroughly disabused of this notion! Out of interest I took the same picture with a Canon 1.4 lense on a 5D body and the CA was much less evident, but in every other way, as expected, the Leica M9 was far superior.

Not looking to put my MP and M6 on ebay quite yet but so far very impressed with the M9. It's still an awful lot of money but if it means less time arguing with airport security over hand searches then it will probably be money well spent!