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Old 14-06-2015, 16:35   #1 (permalink)
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Difference between relative and absolute colour

My daughter and I are trying to think of a way of recording accurate colour differences between photographs of exposed rock strata. So, for example, we hope to take a series of photos of rock faces which we can then compare colour differences across a rock face.

We can appreciate that each photograph will be taken under different lighting conditions, that there will be shadows, different times of day etc etc.

So, is there a way of calibrating each photo so that we can compare like colour with like colour. Could it be as simple as placing colour card against the rock as a reference?

The experience of others would be very much appreciated.

best regards
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Old 14-06-2015, 18:56   #2 (permalink)
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Re: Difference between relative and absolute colour

Welcome to Pixalo Alec

Colour management is a can of worms. I'm no expert, and am a bit colour blind to boot, but I think the trick is to get consistency throughout your whole process.

Your monitor needs to be calibrated. Including a grey card in your picture, of if that is not practical taking a picture of a grey card in each lighting situation will enable you to get the right white balance for each shot and if you shoot in RAW the white balance can be tweaked in your processing too. You can also set the white balance on the camera to suit different lighting conditions and some cameras allow you to set a custom white balance in the camera too. This should be re-done for every different lighting condition.

One way of ensuring that you get consistent lighting is to use flash. If, for instance you use the camera on a tripod and have two flashguns on light stands so you can replicate the set up with regard to distance and the positioning of the flashguns in relation to the camera and your subject you will get consistent lighting provided you use everything in manual mode.

Your ambient lighting that you view the images in can also affect the way the colours look. Then you have to factor in that different people will perceive colours differently too

You don't say if you are viewing the pictures on a monitor or prints, but printing adds a whole lot more complication.

I don't know precisely how accurate you need to be and it might be that not quite getting everything exactly spot on will not make any difference for your use.

I dare say that other will chip in with their thoughts too. I've probably not covered everything here.
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Old 14-06-2015, 20:42   #3 (permalink)
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Re: Difference between relative and absolute colour

It just depends how serious you are to get accurate colour. Top fashion and product photographers need to achieve very accurate colours so calibrate end to end. I do calibrate end-to-end though not because I have to as it was an academic exercise but, now that I have done it, it seems sensible to continue.

I used an X-rite Colour Checker Passport to take a series of photographs in different lighting conditions. Then I used the relevant software (either that which comes with the X-rite CCP or the free Adobe Labs DNG Profiler) which produces the calibration profiles. You also need to be working in Raw and have your monitor calibrated as well. When taking critical photographs, I also take a shot of the CCP as well as the lighting changes and you can use this frame to accurately determine the White Balance and thus apply to other frames. Please look at my web site for more information. Few photographers (professional or amateur) will go this far but I do get accurate and consistent colours. As well as my main DSLR, I also calibrated my Panasonic Bridge Camera and my Infra red DSLR. The profiles are associated with the camera serial number so Lightroom automatically assigns the correct profile to each image as it is imported.
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Old 14-06-2015, 23:04   #4 (permalink)
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Re: Difference between relative and absolute colour

Thank you, this is helpful. It doesn't matter if the image is reproduced in print or o a monitor. The important output is the data to compare each image with the other. Thank you for the suggestions, we will experiment.
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Old 14-06-2015, 23:05   #5 (permalink)
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Re: Difference between relative and absolute colour

many thanks.I assume standard referece cards can be bought online.I will investigate.
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Old 15-06-2015, 19:20   #6 (permalink)
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Re: Difference between relative and absolute colour

The colour checker passport is a precision colour device. You cannot just buy a set of colour cards from ebay for this purpose. The colours are added in a laboratory and guaranteed to be accurate within a tight tolerance. Naturally because it cannot be automated, it is not cheap. I paid 70 for mine about 3 years ago but this would not be considered to be too costly by a top fashion or product photographer. I know one club member who used calibration with a colour checker passport because he does weddings at the weekend. He uses two Nikon cameras but the colour from each is slightly different unless calibrated.
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