Basic colour printing is not terribly difficult to learn, but it is best to work slowly at first and gain a good understanding of what you are doing and why. It is very easy to use up large amounts of expensive paper and chemicals by guessing or by making test strips which are too small to evaluate properly. Also, never try to judge anything from a wet or damp test or print, since it will be both lighter and have a bluish cast.
The first step in making a colour print is to make test strips to determine the basic printing exposure. (All the test strips in this TechTalk have been simulated in Photoshop.) Remember to use the most important part of the image for making test strips. For example, in a portrait use the face and clothing. Test strips of the sky, grass or black background do not make for easy printing.
The following assumes that the enlarger that you are using has a 'colour head', the type that has three knobs that control variable cyan, magenta & yellow filters inside. If it doesn't have an automatic timer, then use some other reliable method. (See Method 2 which uses a constant exposure time & which you might find easier to use.)
There are two methods that can be used for making this initial exposure test strip.
Method 1 - Constant lens aperture, varying exposure time.
This is the same method used for making a test strip when B/W printing.
1) Place the negative to be printed in the enlarger, compose and focus in the same way as for a B&W print. Close the enlarger lens aperture down two or three stops from fully open. Set the filter controls on the enlarger head to Cyan00, Magenta50, Yellow50. Ensure that the filters are in the beam of light. This can be checked by watching the image on the baseboard as you operate the filter IN/OUT control (if the image is dim, place your hand a few inches below the lens as you operate the control). If the filters are OUT then they will not have any effect on the print, usually resulting in the print having a strong orange colour cast.
2) Set the timer to 5 seconds and switch off the enlarger focus light.
3) Place a test strip in the masking frame and expose the whole strip of paper. Cover up about a fifth of the strip with a piece of card and repeat the first exposure. Cover up the second fifth and expose for twice as long. Repeat this process for the remaining parts of the strip, doubling the exposure each time. This will give a strip with the following exposure times:
Covering up from the Left ->>>>>
4) Write the exposure times, f-stop and filter settings on the back of the test strip using a waterproof pen or pencil. Process the strip.
5) When the strip is dry, examine it in white light. The strip should display a series of exposed strips going from underexposure to overexposure. Use the strip with the best exposure to determine the starting exposure time that you should use. If there is no reasonable exposure on the test strip, then you will have to repeat the test using a longer or shorter exposure time or you can open-up or close-down the lens aperture. Remember that if the whole strip is under or overexposed, then simply adding or subtracting "a few seconds" will have little effect. Effectively, each step is equal to a one stop difference in exposure, so increase or decrease the exposure time by the number of stops (steps) that you exposed the test strip for. For example, 5 steps give an exposure range of 5 stops. If the original time set was 5 seconds then the exposure range would be 5, 10, 20, 40 & 80 seconds, therefore beginning the new test with a set time of 80 seconds (with the same lens aperture) would give an overlapping test of 80, 160, 320, 640 & 1,280 seconds. Note the large jump from 5 seconds to 80 seconds still overlaps the first step of the test strip. Since 1,280 seconds is over 21 minutes, this would not only get boring, but could result in other problems such as reciprocity failure (see TechTalk 4). So since opening-up the lens would reduce the exposure the times, we could open-up one stop to halve them (40,80,160,320,640), two stops to quarter them, (20,40,80,160,320) and so forth. Similarly if the exposure time gets too short, closing-down will have the same effect in the reverse direction.
6) When you have a reasonable combination of exposure time and f-stop, set these on the enlarger.
Method 2 - Constant Exposure Time
This method, which keeps the exposure time constant, uses the aperture control on the enlarging lens to vary the intensity of the light falling on the paper. This has the advantage of minimising any colour shifts that can be caused by reciprocity failure in the emulsion of the printing paper (see TechTalk 4). Although RA4 process papers tend to have very good reciprocity characteristics.
A set of card "fingers" are required for this method. These can quite easily be made from some scrap card using a scalpel and steel rule, or a matte cutter. The piece of card should be large enough to cover a reasonable test strip (e.g. 8"x3"). If the card has light & dark sides, have the darker side facing the paper (so the projected image can be seen on the fingers). Each of the fingers should be cut out from the board and stuck back with a sticky tape hinge. It is a good idea to either cut off one of the bottom corners of each finger or stick some tape on the bottom of each finger to make a "handle" so that the fingers can be lifted easily in the dark.
The fingers look like this:
These fingers are also very useful for doing colour correction test strips.
1) Place the negative to be printed in the enlarger, compose and focus in the same way as for a B&W print. Open the enlarger lens aperture fully. Set the filter controls on the enlarger head to Cyan00, Magenta50, Yellow50. Ensure that the filters are in the beam of light. This can be checked by watching the image on the baseboard as you operate the filter IN/OUT control (if the image is dim, place your hand a few inches below the lens as you operate the control). If the filters are OUT then they will not have any effect on the print, usually resulting in the print having a strong orange colour cast.
2) Set the timer to 20 seconds and switch off the enlarger focus light.
3) Place a test strip in the masking frame and place the fingers on top, checking by touch that the paper is correctly positioned under the fingers. Close all the fingers, open the first finger (it doesn't matter which end you start at) and expose the paper for the time set. Close the first finger. Stop the lens down by one stop (this may have to be done by listening for the click, if the lens has an illuminated aperture scale, then you can turn on the focus light on the enlarger, after making sure that all the fingers are closed). Open the second finger and expose this for the set time. Repeat this process for the remaining fingers. This will give a test strip of equal exposure times, but where the intensity has been reduced by 1 stop each step.
Starting from the right - <<<<<.
4) Write the exposure time, starting f-stop and filter settings on the back of the test strip using a waterproof pen or pencil. Process the strip.
5) When the strip is dry, examine it in white light. The strip should display a series of exposed strips going from over-exposure to under-exposure. Use the strip with the best exposure to determine the starting f-stop that you should use. If there is no reasonable exposure on the test strip, then you will have to repeat the test using a longer or shorter exposure time. Remember that if the whole strip is under-exposed, then simply adding "a few seconds" or even doubling the exposure time will have little effect. Increase or decrease the exposure time by the number of stops that you exposed the test strip for. For example, 5 fingers give an exposure range of 5 stops. If the original time set was 20 seconds then the exposure range would be equivalent to 20, 10, 5, 2.5 & 1.75 seconds, therefore beginning the new test with a set time of 320 seconds and the lens aperture fully open would give an overlapping test of 320, 160, 80, 40 & 20 seconds. Note the large jump from 20 seconds to 320 seconds still overlaps the last step of the test strip.
6 When you have a reasonable combination of exposure time and f-stop, set these on the enlarger.
Correcting Colour Casts.
The test strips produced by either of the methods above will probably have a colour cast. This is where, although the general density of the print is correct, the colours are shifted, for example too yellow or red. This will be particularly evident in pastel colours, skin tones and greys. Saturated colours, dense black and plain white will show colour casts much less, if at all. It is quite easy to confuse the colour of the cast, particularly a light cast. The task of correcting colour casts seems to cause the most problems for inexperienced colour printers, but it's something that gets a lot easier with practice. Although there are sets of viewing filters that can be bought, the following procedure allows casts to be identified and corrected with the minimum of effort and paper. This method requires a set of card fingers (See Method 2 above). You will find that changing the filter values will lighten or darken the print slightly. The larger the change to the filtration, the larger the density change in the print. This is due to the fact that the filters block some of the printing light. The more filtration in the light path, the more light that is blocked. Do not try to correct this until you have corrected the colour casts in the print.
1) Examine the exposure test strip or test print to determine the colour and depth of the cast. Use the back of the printing paper as a reference white. If the cast is not a pure primary (red, green or blue) or complimentary (cyan, magenta or yellow) colour, decide which is the strongest colour and work to remove that first. Do not try to remove two colours at the same time since this will probably lead to confusion and wasted effort and paper.
2) The following is the connection between the colour of the cast and the printing filter that affects it. The complimentary colours are the most obvious, since they affect themselves:
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One way of remembering which filter affects which primary colour is to remember that the colour 'missing' from white is the one that is affected :
Cyan = green + blue (red missing)
Magenta = red + blue (green missing)
Yellow = red + green (blue missing)
It also has to be remembered that you are printing negatives so it is necessary to add the colour of the cast to reduce it in the print. When correcting casts one printing filter (usually Cyan) should always be kept at zero, otherwise neutral density (grey) is introduced. This leads to longer printing exposures. When it is necessary to use the filter being kept at zero, the other two filters are changed instead. The following list will help to explain.
With the Cyan filter kept at zero, to eliminate a :
Cyan cast subtract from magenta and yellow.
Magenta cast add to magenta.
Yellow cast add to yellow
Red cast add to magenta and yellow
Green cast subtract from magenta.
Blue cast subtract from yellow
The amount you will need to adjust the filters will depend on the depth of the cast. The value of the filters are known as units and you need to decide on how strong the cast is in units. This will get easier with experience, but if in doubt, err on the higher side.
3) Divide the estimated depth of the cast by the number of card fingers you have. (e.g. if you think that the cast is 50 units and you have 5 card fingers, add or subtract 10 units per step.) Make a test strip using the exposure that you have previously determined. Use the card fingers to cover the strip and expose each step individually in turn either adding or subtracting density a step at a time from the chosen filter. Note that this will place or remove quite large amounts of filtration in the light path. This will result in the print image growing noticeably lighter or darker in each step. Do not try to correct this at this stage.
4) If all is well, this will show the colour cast reducing as the filtration is changed, finally reversing to the opposite cast as the neutral point is passed (e.g. blue turning to yellow). If it doesn't, but shows a new cast appearing, then the wrong colour was chosen. This is a very good way of differentiating between casts which are hard to tell apart, such as cyan and blue, and a quick way of finding the correct filtration.
5) If a second cast exists in the print (e.g. if the original cast was orange and you chose to eliminate the red with your first test strip), then repeat the procedure to eliminate the second cast.
6) You should now have the filtration set to give a reasonably accurate colour rendition. Make a test print using a whole test strip, placed over the most important part of the image.
7) Examine this test print carefully. Check that the print density and colour rendition look accurate. If the print is too light then you will need to increase the exposure time, or decrease it if too dark. Think in f/stops the same as for B/W printing when estimating the exposure change. Remember that filtration and exposure changes are linked, changes to one may affect the other. If any changes are needed, make another test print to check their effects. It is quite easy to make a mistake at this point and make a change in the wrong direction. (It's a good idea to keep the final test print & write on the back the exposure & filtration values. This gives a good starting point should you want to do any reprints, but do remember that many things can change the exposure and filtration required. Different enlargers will have different colour light sources & the age of the bulb in the enlarger will affect the colour of the light it gives out. Different lenses can also affect the image colour slightly. It must also be remembered that different makes of colour paper will need different exposures and filtrations and possibly even different batches (packets) within the same make and type of paper.)
8) When you have an acceptable test print, check the enlarger focus then make and process a full print.
© Barry Leighton FRPS