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This page is aimed at intermediate level. Although beginners should be able to read and understand this, it may help to have read the beginners' items first.
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One of the problems with scanners and digital cameras, is that they tend to give a "colour cast" to a picture - that is, colours in the resulting image may appear to be slightly more blue or red or green than they should do. This problem is further compounded by the photo developing process, which may also give a colour cast to an image if not done correctly. Here, we take a quick look at how to fix typical colour cast problems.
Note that the demonstration pictures here are included as small versions - if you click on the picture, a larger version can be seen.
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Before we begin, here is the picture I'll be working on. This picture is another photograph I took in Spain - this is a clock tower in Zaragoza. Take a good look at it now. (Don't forget you can click on the picture to see a larger version.)
The picture is horrible, isn't it? All red and orange, with no contrast. You'll notice also that the colour cast is not even, and the bottom right hand corner is not so badly affected. This will cause us further problems. Alas, a lot of my photos from Spain looked like this - a combination of a bad developer, heat damage, and airport X-ray machines. Sigh. Well, at least we can try and salvage it a bit. Before we do that though, a word about scanning. When I scanned this originally, the scanner software was very confused by the picture, and chose some enhancements for the picture contrast and brightness. This was all well and good, except it over adjusted, and all the detail in the picture was lost. This was easy to fix by rescanning without any adjustments, but it is worth watching out for. Automated systems aren't always the best. |
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Okay, so you've seen the sample picture. To fix this, I used the Tone Balance control, and I would advise you to as well. If you don't know what the Tone Balance control is, see the contrast and brightness tutorial. If your software doesn't have a tone balance control, look for something called Colour Balance. This should behave similarly to the percentages controls for contrast and brightness. (Again, I refer you to the contrast and brightness tutorial.)
Right. Quick and easy this one. First problem with the picture, too much red! Switch your tone balance (TB)/colour balance (CB) control to adjusting red. Now, if you are using the TB control, pull the shadows marker for red up to the start of the histogram. CB control, just decrease the red brightness percentage a lot. You may want to increase the red midtone value a bit too. This should eliminate most of the red for us, but leave the picture slightly green. So now switch to the green colour. The green cast is not as bad, so adjust it in a similar way to the red, but be sure to be less extreme. When the green is starting to disappear, it is probably a good idea to decrease the blue midtone value slightly (thus making them brighter). It gets fiddly after this. You have to keep riding the different controls until the colours look about right. Often, you can only see when you've got it wrong if you play with the controls and it suddenly looks better. When you think it is looking okay, you might need to add a bit of contrast to the picture, although adjusting the colours will put a lot of contrast back into it. Then just sit back and admire. |
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Unfortunately, the bottom right hand corner is now very green. But why? Ah - keep reading... |
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One of the difficulties of this picture, is that the colour cast covers most of the area, except for a small section at the bottom right corner. As I improved the colour in the bulk of the picture, the colours in the corner, which were almost okay, were ruined. This is a real problem.
To try and fix this, I cheated. A lot. Please forgive me. First, I copied the corner area into the corrected picture, and then adjusted its colours to match the rest of the image. This failed, because what looks like a clean transition between "red" and "not red" actually has a stripe in the middle which is partially affected. So, I copied a smaller area into the picture, now that I knew where the stripe was. Again, I tried to colour match it. Then I masked off the area containing the stripe, and tried to adjust that too. The result was... well...
An important lesson here though. There are rarely shortcuts. If you want a professional job, take your time over it. Don't rush things like I do. If any of you out there fancy fixing this picture properly, I would love to see the result... |
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For my final word on this matter, a brief comment on casts of "odd" colours. By odd I mean the colours are not straight forward red, green, or blue. Yellow is a common one - photos taken under tungsten lightbulbs (i.e. the normal household lightbulb) tend to come out very yellow if the flash isn't used.
These can be fixed in the same way as a red, green or blue cast, but you have to remember how the colours combine to do this. I'm planning a complete tutorial on colour spaces soon, but in the meantime, it might be useful to know how the colours combine. Combinations work like this:
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Well, I hope that helps you. This tutorial is a bit rushed. If there any problems, email me at the address below.
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