Ooh — controversial, especially if you’ve spent a lot on camera gear. Surely it’s possible to photograph all visible colours?

Well, it’s technically possible, just not with any readily-available equipment. Let me try to convince you.

First, the photo that started me on this quest of trying to comprehend what was going on. I think this is a snapdragon, but I’m open to corrections.

Yeah, it’s blue. It was violet in real life, but blue on the camera screen and blue when I transferred it to the computer. The camera was properly set up: a Minolta DiMAGE A1, shot RAW, converted with the correct profiles. I also tried straight JPEG and a variety of other cameras, cameraphones and camcorders. Blue every time — yet the flower appeared violet to me, and others described it as “deep purple”.

Violet is not purple

First of all, let’s get that out of the way. The colour we call purple is made by mixing blue and red. Violet is a different colour altogether, although because of the way the human eye works, many people see them a being very similar. Some people, particularly those with very slight colour vision deficiencies like myself, see them as distinct hues.

Here is the full range of “pure” purples as far as your computer and monitor are concerned, which is a combination of a vertical gradient from black to pure blue and a horizontal gradient from black to pure red, showing all 4096 of the 16.7m colours your computer can display that have no green component:

There are a lot of purples in there, and some colours that may appear close to what you consider to be violet. Another graph is needed, this time of the sRGB gamut. As you may know, the picture on your screen is made up of red, green and blue dots. Mixing differing amounts of those colours gives rise to the wide variety of colours you can see. For example, if you turn on all the red and green dots and none of the blue, your eyes see yellow.

This graph shows the CIE1931 colourspace (the sail-shaped thingy) which is a representation of all the colours the human eye can see. This was prepared with a lot of experimentation. The line around the outside represents the spectral colours: those with a single pure wavelength, like you get in a rainbow. Colours within the sail are non-spectral: a mixture of wavelengths. The triangle shows the range of colours your monitor can represent. The points of the triangle are the colours of the red, green and blue dots on your screen, and the colours inside the triangle represent the complete range (gamut) of colours your screen can make by mixing those colours. Obviously, the colours in the diagram outside the triangle are only indicative, as your screen, by definition, cannot show them accurately.

You’ll notice that a large proportion of colours are outside the triangle. This means that your screen cannot display many cyans, greens and oranges that your eye can see. For a demonstration, see the Eclipse of Mars optical illusion.

Another colour that isn’t within the triangle is violet. Spectral violet is the far left (most anti-clockwise) part of the perimeter of the sail by the 300 marker, with some non-spectral violets just inside it. The red-blue mix purples above are all to be found on the bottom edge of the triangle at varying brightnesses.

Your digital camera sensor works in a similar way. Each of its millions of pixels is actually a monochrome sensor, but there’s a colour filter array (CFA) in front of it, that puts a red, green or blue filter in front of each pixel. At some point between pressing the shutter and viewing the picture, the individual red, green and blue pixels are combined into a colour image.

The camera’s gamut is again represented by a triangle as above. Typically, the vertices of the triangle are wider than sRGB, so the camera can record more colours. If you then get the photo out as an sRGB file (which you do, unless you know that you don’t), this wider gamut is compressed, losing data.

However, the “blue” corner of the camera’s triangle is still not violet. Any violet light gets through the blue filter and stimulates the monochrome pixels, but the information about the wavelength of the light is lost. During image processing, any stimulation of the pixels under the blue filter is assumed to be blue — so no violet in your photo.

Some cameras may record it as purple rather than blue if the red filter is designed to partially let through violet light, but my camera records violet as pure blue. No consumer camera has the ability to record violet properly, though.

Please don’t send me photos claiming that they contain violet. They don’t. Some people perceive violet and deep purple as similar or the same, but others (me included) do not.