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Colour Adjustment - Part 1 - part 3 of 1 2 3 4

by Mike McNamee Published 01/10/2002

Another trick you can employ to assist is to highlight the numerical adjustment values in a Photoshop dialogue box (by double clicking them to make them show blue). Now, when you hold the shift key down and click on the up or down arrows, the values jump by 10 units, producing a rapid shift in colour. If you move without the shift key the values jump 1 digit and you might have adapted to the shift by the time you get 10 clicks in. By jumping in big increments you can do a Goldilock's Porridge adjustment (i.e. - this one is too big; this one is too small; this one is just right!).

A final word before we get into the nuts and bolts of adjusting colour. Do not waste your time chasing perfection in your greys off 6 colour ink jet printers - you will never get there if you are very discriminating. The most recent work by the colour scientists at Derby University has shown that colour discrimination around the neutral is close to 1 delta E. This is also close to the precision of the measuring instruments we use to build profiles. Profiles are intended to harmonise the whole gamut and in practice they tend to sacrifice accuracy around the neutrals in favour of correcting across the whole range. This is particularly so with the 6 ink systems which have the additional complexity of trying to switch from light cyan to deep cyan etc. Experience of showing grey scales to very discriminating photographers has taught the author that some people will be troubled by greys that are incredibly close to neutral. If you are one of these people you should get your enlarger back out of the attic and start mixing chemicals again! In practice your clients are less likely to be bothered by such small errors.

Measuring Colour

This series of articles are planned to be a reasonably complete explanation of the topic and we must therefore set the scene with some care. We have so far explained how colour is perceived now we need to explain how it is measured. We have to understand the relationship between measuring and correcting colour so we know where to put our effort in.

Providing you have either a colourimeter or a spectrophotometer, measuring colour is simple. You simply place the image on a flat surface, plonk the instrument onto the relevant bit of the image and press the measure button. The instrument then sends the colour values to your computer or to its own little screen. It is from here that things get complicated! There are literally dozens of ways of expressing the colour values. This is a bit like currency. A jar of coffee may cost £3.45, $5.18 or 5.48 Euros. The coffee in your trolley does not change but the numbers it costs vary according to how you are going to pay for it. Colour is the same. The colour in the image stays the same but the numbers (currency) you use to describe its value vary according to how you measure it.

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1st Published 01/10/2002
last update 09/12/2022 14:52:34

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