Photography - The Colour Process

The Colour Process

Here is a detailed description of how the colour process works, which may help you to understand why certain things are important in the process, while others can be safely left to deal with themselves.

  The Film.

  I always used to wonder why, if you developed colour paper, or film, in black and while developer, you still got an image, a B+W image, albeit lacking in contrast, but still, it worked.

  In fact the above statement shows us two things. Firstly, that colour and black and white, processes must be intrinsically linked, and secondly, it shows that I could produce colour prints even without understanding the process! (I hope this serves to *encourage* you!)

  The Process.

  So, why *does* it work? Well, colour film is actually made from a sandwich of black and white photographs... There are three layers, and with the use of filters in the sandwich, and differing emulsions, the three Black and White images are made to be sensitive to the primary colours of light, one Red, one Green and one Blue layer.

  So, when you actually take a picture, a colour picture, you are in fact taking three separate photographs at once. One recording the amount of Red light, one recording Green and the other Blue. In fact, it would be possible to actually take three separate photos, each through a separate coloured filter, to produce the same result, this *would* have been done when colour film was created, but of course they saw the benefits of combining the layers.

  Anyhow, you have your three latent B+W images, what happens next? How could you project this image? How are they processed? Well, the process is very similar to the black and white process of Dev, Stop, Fix baths, however the chemistry in each bath is a little more clever, and again, the baths have two functions built in.

  In the Developer, the first process, is similar to the ordinary B+W process, a negative image is formed due to light exposed silver being turned black. However, once this is done, chemical dyes in the developer start to latch onto the black silver. This is no more than we are used to in B+W when we 'tone' a photograph a certain colour. However, the chemistry of the process does ensure that the different colour dyes latch on to the different layers in the film.

  The dyes used are the complementary colours of the primary ones within the layers of the film... this is a negative remember... Thus Cyan dye latched onto the black image in the Red layer, Yellow dye 'tones' the image in the Blue sensitive layer, and Magenta dye colours the Green layer.

  Ok, so far, well the next bath is the same as with black and white the stop bath, this merely arrests development at a known time, by radically dropping the pH of the solution. (Turning it from alkali to acid). The film is then fixed as a normal film would be. ie. any unexposed silver halides in the emulsions are removed so that they will not decay and spoil the image.

  The film so far, is interesting, but would still be impossible to project in an enlarger, so what's the final process? Well, the final bath contains not just the fix, but bleach as well. This bleach completely removes the black silver images from all the layers in the film, leaving just the dyes... dyes which, since they only filter one part of the spectrum, can be combined to form a projectable image.

The Process Summary.

  So, there you have it. Three B+W images are formed, each storing the amount of Red/Green/Blue in the original scene.

  The images are developed, while still in the developer, they are toned cyan,magenta, and yellow respectively.

  They are then fixed, and the B+W images just bleached away, to leave complementary filters which contain all the colour information of the original scene.

So, what about my original problem? How does the colour emulsion develop in B+W chemicals? Well, you should see by now that, the B+W developer develops the images, but they are never dyed. The fixer fixes the images, but they are never bleached. Thus you are left with the three B+W images all as one, each representing the light for its colour as a shade of grey.

  Cutting Corners.

  So, now that we understand the process, what factors are important, and what can we leave to chance?

  The most important bath is the development bath, do not cut many corners here... This is the bath where you will lose your pictures if you get it wrong. Time in the bath is very important, Temperature is crucial. Higher temperatures increase the speed of development, and so if you do not keep a good eye on the temperature, you could over develop or at the very least come up with erratic colour balances between rolls.

  It should be noted however, that if colour balance standardisation is not an issue, eg if you only develop and print a few films, you could set the temperature right at the start, and then allow it to drift down. As long as this is what you *always* do, then you might be able to take less care, however, colour balances going astray will just mean more time at the printing & filtration stage of the process.

  The Stop bath, again, if you want to attempt to produce a 'standard' colour balance for your process, you might want to keep this as is, however, if you are not too concerned about this, change the stop over to a water bath, short, but with vigorous agitation. The acid nature of the BleachFix Bath will then be the cause of *all* development ceasing. The time in the Stop bath is in no way crucial.

  The BleachFix bath is not to be feared. You'd have to leave a film in it for hours and hours for it to have any effect on the actual dyes...and the B+W images are bleached away quickly anyway. Thus time and temperature are not really very important here. I leave it stewing in cooler bleach fix for a while and take very little care over this bath. If you want to get it over with quickly you can aerate the bleachfix by first putting it into a half full bottle, and shaking, but please be careful you do not want this stuff all over your face. Normal B+W 'hypo' fixer is bad enough to get in your eyes... BleachFix is serious.

  The BleachFix bath and light. This is an interesting point. The film/paper should be kept in the dark till the fixer elements in the bath have had a chance to remover all of the unexposed silver, however, it really is *not* a disaster if light does get in at this stage. I have had a film which due to a machine at the chemist failing, had a gap of not 6 minutes but 6 days between the developer and BleachFix baths, and all that time it was exposed to the light. Similarly, I have had pieces of photographic paper which have *never* been fixed, which have not deteriorated in years...

  All that I am trying to get across is, be careful with the development baths, but the other two are far less critical in there outcome on the negatives.

  Colour balance.

  I have mentioned this a few times since I have started this page. The overall colour balance of the resulting print is *never* accurate. This means that your pictures, their pictures, anyone's pictures, are not an exact copy of the light levels at the original scene. There are just too many variables. The age of the film. The temperature/concentration of the developer when the neg was processed, the temp/conc of the developer when the *print* was developed, the actual chemistry of the film and papers used... All of these conspire to ruin your colour balance, and they succeed!

  They succeed to the extent that your negative is not just 'a little off' is is unprintable without correction. It will have an over all colour cast. (unless you *really* naffed up you development, and then you'll have an uneven colour cast and be really up a certain creak without any form of propulsion).

  Colour Balance is restored, (or more accurately, made to look something like it should do) with filtration at the exposing stage of printing.

  Happily, that's what the 'Exposing' section is all about.

A page from James David Chapman's website. Located at: http://www.users.globalnet.co.uk/~jchap/ Site mirrored here at: http://www.j.chap.btinternet.co.uk
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