Today, in a thread on the GetDPI forum, I posted the following rant. It relates to an image posted by a forum member who, having posted some images from her new Sony A7r, had been told by some people that they could see colour shading in the sky and by others that they either could not see it at all or that it was normal or natural.

"Since we're all having our 2 Cents, here are mine:

Cindy's file does indeed seem to have a tiny, tiny, tiny magenta issue. It is absolutely within 'normal' variance for almost any system.

By 'system' I mean 'soup to nuts' - from lens to sensor thru RAW pp to display thru to eye and brain.

As other people have noted, normal vignetting can create small colour shifts, as can the natural variance of colour in a sky, and the uneven distribution of IR and UV across a sky, especially when taken into account with the amount of filtering on lens (screw-on and coating) and on sensor.

Then there is the fact that even very good monitors can have uneven display colour and brightness. I, like Jono Slack, have rather slacked off the frequency of calibrating my monitors because even when freshly calibrated, my 30" Cinema Display, when viewed next to my Eizo 30", gives the game away: the Apple display is blue and the Eizo is red, relative to each other. Viewed alone, each looks very good.

Next is the issue of the normal variance of human colour vision. Naturally the medical profession has decided to categorise and pathologise this, preferring to see it in terms of Norms and Abnorms rather than variance, though countless studies have shown that 'colour blind' people can determine patterns that 'normal' people cannot, and can often distinguish colour variations that 'normal' people cannot. Variations in abilities are what sustain populations, let's leave it at that.

Next, the fact that we perceive colours culturally and with gender bias. We are all familiar with the complaint that asian manufacturers favour certain skin tone renditions that look odd to western eyes. We are also aware that females, (and experienced male photographers!) are more likely to be able to use accurate language to distinguish between magenta and pink, or purple, mauve and violet. Using accurate language places a requirement on the speaker to perceive more diligently: my colour vision has become a lot more accurate since I started trying to see accurately, rather than seeing like a British boy raised in the 60's when knowing the names of anything other than red green and blue would cause a large pink question mark to appear over your head...

In every step of the imaging chain, in every situation, there is room for variance from what is 'true', 'accurate' and 'normal'. Your file is very comfortably within the sum of those variances IMHO. And when something is clearly wrong it is clearly wrong - such as the colour casts in this white frame from an 18mm Super Elmar M on an A7R - it will be obvious:


Additionally you will not, ever, accurately bottom out (let alone be able to eradicate accurately in post) a colour cast with a shot of a blank grey sky or a white wall. The light falling on a wall will be almost impossible to get perfectly illuminated and, unless lit by very accurately temperature controlled flash, will be subject to the same variances as a sky. To do this properly (and even then not 100% reliably or accurately) you need a Lens Cast Calibration sheet as used by MF shooters. They cost a few dollars and save a lot of time and yet very few people bother to source them. If you use one, you will see that, for example, a file from a Sony RX-1 with in camera colour shading corrections turned on still has some colour shading. And that is a camera with an extremely closely tuned ecosystem of sensor, lens and bespoke processing.

Most work does not require perfect colour accuracy. Photographers who work in fields where it does, do not use zooms with adaptors on what are fundamentally consumer cameras. 

Inaccurate colour renditions can be part of a photographer's style. 

There are highly successful artists, in both photographic and other media, with highly personal colour vision - and yet their work can be enjoyed, like the work of a deaf Beethoven, by a wider audience. 

Rant over. The file is fine."