Colours at high ISO

Girl with large eyes wearing a Christmas hat and white toga-style top. You can tell it is very dark because the image is extremely grainy, there is considerable colour noise and the colours are not true.

An image I took some years ago that I use to illustrate noise with my students. Shot at high ISO, the image is extremely grainy, there is considerable colour noise, some fringing and the colours themselves are not true.

 

A student at one of my beginners’ photography workshops asked why the camera produces different colours at high ISO.  This is the first of two blog posts I have written in response.

Two human eyes, combined with the brain, provide an awesome level of accurately integrated functionality that no camera can duplicate. This is because the photographs we take are created by one lens rather than two, and because the camera processor is a poor substitute for our brains. It takes vision, judgement and imagination to use the camera to create impressions with this limited functionality, and at least partially explains why photography is as much art as science.

Our eyes are in constant motion; even when our attention is directed towards just one thing. The brain builds up composites of impressions, mostly from the brief periods (of 200-300 milliseconds) when our eyes are still (http://theconversation.com/how-do-our-brains-reconstruct-the-visual-world-49276).

Foreground shows a small piece of white table, and chair striped with shadows from the wooden shade above. Mid ground includes wooden fence, sand and small band of blue sea. Background is an azure blue sky. The scene, which is dotted with kite surfers, has a yellow feeling. This is a reflection of the property of light at Soma Bay just before the sun begins to set.

View of kit surfers, table and shadows at Soma Bay just before sunset

Eyes and brains together help us understand that the table-cloth in front of us is both white and bathed in the soft golden light of a summer sunset. It’s automatic. We know it by looking and without having to think. The camera is more limited, so as photographers we have to choose whether to show the table-cloth as white, very light gold (or if we’re playing with white balance a totally different colour) depending on what we want to convey.

Most modern camera sensors work on light intensity rather than colour. Colours are interpreted through the use of a micro-lens matrix in front of the sensor. Most modern digital cameras also compare this to a bank of stored images to interpret colour in the best possible way. Different colours used in the matrix are perceived differently depending on their intensity. The higher the ISO the less light falls on the matrix and the more calculations (and approximations) the camera processor must make.

Different colours used in the matrix are perceived differently depending on their intensity. The direction of shift of colour error depends on the spectral sensitivities of sensor, micro-lens matrix and the intensity of light that has come through the lens. Reds and blues tend to deteriorate more quickly than greens because those filter colours have higher additivity failure (http://www.yorku.ca/eye/additive.htm) and the processor is limited because the camera has only the one static image –  your photograph – from which to work. That is why colours in high ISO digital images tend to be much less subtle in terms of colour, and why, depending on the white balance you have selected, you can get a preponderance of colour that might not even be in the original scene.

Whether we use this phenomenon as a tool in our artistic arsenal or as a limitation within which we work is of course our choice as photographers and artists.

ISO basics for beginners: https://dancetog.com/2014/04/21/iso-on-wordymonday-photography/

Colours at high ISO part 2: https://dancetog.com/2017/10/04/signal-to-noise/

I would love to hear your opinions or to see some of the images you’ve taken that have pushed the boundaries of your cameras’ ISO colour technology in the comments  below.