Fujifilm X-Trans sensors hate red

Red hot chili peppers and black and white photography with a red filter for high contrast of scenes with blue sky have one thing in common: are not well rendered by Fujifilm current cameras employing an X-Trans sensor.

All things in life — and in technology especially — have pros and cons; Fujifilm’s specially designed mostly has advantages over a more traditional Bayer-type digital sensor, but it has weaknesses too. A quick glance at Fuji’s design shows that X-Trans sensors have a lesser amount of red receptors. This translates into what I observe when I photograph RED or when I make a scene red by photographing with a glass red filter in front of the lens.

I am showing here the example of a bunch of red hot chili peppers. This is one frame of a focus-stacked series that I shot at ISO 200:

The yellow rectangle indicates the area detailed below
The yellow rectangle indicates the area detailed below
The area, that should be smooth uniform red, is horribly patched
The area, that should be smooth uniform red, is instead horribly patched (click to enlarge). Demosaic sharp=100

It is clear that the naturally smooth texture of the vegetable’s skin is rendered by Fujifilm’s own RAW converter with plenty of nasty artefacts. I have observed similar behaviour with 3rd-party converters. I could not get rid of this by simple noise reduction.

There appears to be a solution and that is the “demosaic sharp” slider that, although normally has a minimally visible effect on the image, with such extreme red cases can make or break the image. By lowering the “demosaic sharp” to 0, the red image changes drastically:

With demosaic sharp=0, the red is almost nice and smooth
With demosaic sharp=0, the red is almost nice and smooth

Besides affecting strongly the smoothness of otherwise smooth red areas, the “demosaic sharp” also has an effect on the overall brightness and redness of red images, as the following side-by-side comparison shows:

With demosaic sharp=0, the red is overall brighter and more vivid
With demosaic sharp=0, the red is overall brighter and more vivid

The final image, after focus stacking and further editing (of course, starting from a series developed with demosaic sharp=0), is the following:

The final result, after focus stacking (Helicon Focus) anf further light  editing
The final result, after focus stacking (Helicon Focus) anf further light editing
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