Magenta cast in reflections

[hydeca]

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I've noticed a magenta cast in localised reflections (e.g. shimmering water) that are over-exposed. I don't see this in larger over-exposed areas (e.g. sky).

 

It would be interesting to hear how others deal with this issue in post.

[adan]

A simple fix - assuming there is no other magenta in the picture (swimsuits, etc.) - is to desaturate magenta only and shift the hue towards blue.

 

Another approach is to run Photoshop's Median filter (Filters>Noise>Median) at about 3 pixels radius, and then Fade the filter for Color only - that will blur the magenta (or any "hot" localized color artifact) into the surrounding colors (and vice-versa) without affecting the actual sharpness of the highlights or other details (which are a function of Luminance).

 

That last technique also works to reduce color noise, or those occasional glaring yellow or red highlights in colored car signal lights (brakes, turn signals, etc)

[lars_bergquist]

Why does this happen with M cameras but not with other digital camera I know of? Is it the absence of an anti-aliasing filter? And why magenta, specifically, if the light is white?

 

The increasingly spectral old man from the Pre-Digital Age

[adan]

1) Something of a mistaken premise - it DOES happen with a variety of digital cameras. Depends on a host of factors, from lens used (focal length, sharpness, CA), to distance to subject (size of the reflections).

 

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2) Yes, the lack of an AA filter will also increase the visibility of the fringes (just as it increases the visibility of any small detail).

 

3) Because:

 

a) it's actually a violet fringe - but violet diluted by the original white highlight looks "magenta" to most people - see attached.

 

violet is the most energetic end of the spectrum (ultraviolet light will give you sunburn and skin cancer, red and infrared light won't). The extra energy means violet light will "spill" further before running out of steam.

 

c) There may be a lens CA component to the fringes (in addition to energy overflow) and since lenses tend to be corrected for green first (middle of the spectrum, most important for human vision), CA tends show up towards the ends of the spectrum - and violet + red = magenta. Additionally - the reflective surface itself may have chromatic aberrations - I've never heard of APO-corrected water.

 

A side note: there really is no such thing as a "magenta" wavelength on the spectrum - magenta is white light, with the green part in the middle of the spectrum removed.

[scrubs]

I also have had terrible problems with this and the only way is to desaturate

the magenta and purple completely and pray nothing else of this color

range is in your image - if that is the case then use lightroom for

localised desaturation with a brush. In any respec -t it's rather annoying

and could be fixed (covered up more) with a better lens profile made

by Leica - when they get round too it.

 

What lens are you using? is it coded or manual detection set?

 

This is taken with a summicron 35mm ASPH

 

https://rapidshare.com/files/347668300/MagentaFringing.DNG

 

look not only in the water, but top left of image and also large area in buildings above centre.

in fact it is everywhere and would be quicker to tell you where it isn't.

 

I have shot with many cameras and never experienced purple fringing as bad

as I do with the M9.

[jaapv]

How could it be fixed with a lens profile:confused:

[jaapv]

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I have shot with many cameras and never experienced purple fringing as bad

as I do with the M9.

That is because you never focussed so well nor had proper edge contrast before...

[lars_bergquist]

I have shot with many cameras and never experienced purple fringing as bad as I do with the M9.

 

They probably have had AA filters over the sensor.

 

I have experienced this with the M8 and the M9 and any damn lens I have used for shooting across backlighted water – and there's lots of water in Sweden and the sun does shine occasionally. My X100 (arrgh!) seems to be immune. So the solution seems to be: use substandard camera ...

 

Thanks Andy for the enlightenment. So it is essentially a 'blooming' thing, i.e. electron overspill on the sensor. Damn – all those lovely sailing boats in backlight – have to do them in black and white just as in the old days!

 

But – would an apochromatically corrected lens really show less of this?

 

The old man from the Age of Contrejour

[sandymc]

Lars is correct on the AA filter. (Although not on the blooming - modern sensors don't bloom, at least not in the technical sense of the term, and an AA filter has no effect on blooming anyway). It's really an aliasing artifact, often exaggerated by the effects of the demosaicing that the raw converter does. You might want to try a different converter - some are better than others in this regard.

 

Sandy

[scrubs]

How could it be fixed with a lens profile:confused:

 

No idea, Just going off what Leica told me when i sent the image to them.

 

I got a response from our quality control department. Considering your information about the purple fringing issue they suppose an aberration of the lens. I have forwarded your image to our product management to check this issue and to decide on a reworking of the 6-Bit correction profile.