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Color correction?

May 14, 2012, 02:18
Post: #1
Color correction?
I took some shots last weekend with a camera that had been set to tungsten lighting rather than the actual sun lighting. So, my photos all are blue tinted.

Some time ago, I found a tutorial that told how to create a grey scale chart that went from pure white to pure black in 10 steps. You place the chart in your photo and it allowed you to then correct colors by using "curves" to match the grey scale charts colors to those in the "curves" tool.

Unfortunately, I cannot find the tutorial, or any other mention of this technique on the net.

Can someone point me to the process for correcting colors using a grey scale chart please?

I figure the next sunny day, I'll take a photo of the grey scale chart (with the camera set to tungsten) to establish what the curves should look like, then apply the same curves to my photos from last weekend.

TIA
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May 14, 2012, 06:31
Post: #2
RE: Color correction?
(May 14, 2012 02:18)prickett Wrote:  I took some shots last weekend with a camera that had been set to tungsten lighting rather than the actual sun lighting. So, my photos all are blue tinted.

Some time ago, I found a tutorial that told how to create a grey scale chart that went from pure white to pure black in 10 steps. You place the chart in your photo and it allowed you to then correct colors by using "curves" to match the grey scale charts colors to those in the "curves" tool.

Unfortunately, I cannot find the tutorial, or any other mention of this technique on the net.

Can someone point me to the process for correcting colors using a grey scale chart please?

I figure the next sunny day, I'll take a photo of the grey scale chart (with the camera set to tungsten) to establish what the curves should look like, then apply the same curves to my photos from last weekend.

TIA
That won't work. The correction is specific to the lighting for a picture. You can at best use the same correction for a series of pictures taken together.

For most of my color corrections problems I use:

http://registry.gimp.org/node/72
http://registry.gimp.org/node/73

whitebalance is rather clever to use, if you have a color in your image that should be pure gray or white, sample it and set the foreground color with it. whitebalance will then adjust the curves to make it pure gray.

Having a color chart is a good idea, but to use it properly you shoot it at the beginning of the series of pictures. But if you do think about doing this, you can as well adjust the color temperature of your camera (either one of the fixed settings, or if the camera allows it, a custom setting obtained by shooting something white).

Home-grown grayscale charts (and, even more, color charts) are rarely worth their weight in paper. To be effective the printer should be calibrated and to calibrate it you need another chart.

Now answering your questions on
https://Gimp-Forum.net
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May 16, 2012, 02:39
Post: #3
RE: Color correction?
(May 14, 2012 06:31)ofnuts Wrote:  
(May 14, 2012 02:18)prickett Wrote:  I took some shots last weekend with a camera that had been set to tungsten lighting rather than the actual sun lighting. So, my photos all are blue tinted.

Some time ago, I found a tutorial that told how to create a grey scale chart that went from pure white to pure black in 10 steps. You place the chart in your photo and it allowed you to then correct colors by using "curves" to match the grey scale charts colors to those in the "curves" tool.

Unfortunately, I cannot find the tutorial, or any other mention of this technique on the net.

Can someone point me to the process for correcting colors using a grey scale chart please?

I figure the next sunny day, I'll take a photo of the grey scale chart (with the camera set to tungsten) to establish what the curves should look like, then apply the same curves to my photos from last weekend.

TIA
That won't work. The correction is specific to the lighting for a picture. You can at best use the same correction for a series of pictures taken together.

For most of my color corrections problems I use:

http://registry.gimp.org/node/72
http://registry.gimp.org/node/73

whitebalance is rather clever to use, if you have a color in your image that should be pure gray or white, sample it and set the foreground color with it. whitebalance will then adjust the curves to make it pure gray.

Having a color chart is a good idea, but to use it properly you shoot it at the beginning of the series of pictures. But if you do think about doing this, you can as well adjust the color temperature of your camera (either one of the fixed settings, or if the camera allows it, a custom setting obtained by shooting something white).

Home-grown grayscale charts (and, even more, color charts) are rarely worth their weight in paper. To be effective the printer should be calibrated and to calibrate it you need another chart.

Thanks. I ended up just using levels and eyeballing the results. They seemed to turn out ok.
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May 16, 2012, 02:47
Post: #4
RE: Color correction?
P.S. I'd still be interested in instructions for adjusting using the grey scale sheet though for future photo shoots.
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May 16, 2012, 07:03
Post: #5
RE: Color correction?
The standard way is to take a picture of the grayscale chart at the beginning of the shooting session (and for long session, a couple times during the session), and to obtain the correction on that picture that make the grays true grays, and then replicate this correction on the rest of the pictures.

But if you want true color, you should be shooting in "raw" mode, and doing the color correction while processing the raw images. This way you process the 12-bit/channel image with a 16-bit/channel capable image, instead of being stuck processing 8-bit channel images (Jpeg) with 8-bit channel software (Gimp).

Or, if you stick to Jpeg, use the manual balance on your camera so that the color balance processing is done before the image is committed to 8-bit.

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May 16, 2012, 23:52
Post: #6
RE: Color correction?
(May 16, 2012 07:03)ofnuts Wrote:  The standard way is to take a picture of the grayscale chart at the beginning of the shooting session (and for long session, a couple times during the session), and to obtain the correction on that picture that make the grays true grays, and then replicate this correction on the rest of the pictures.

My question is how to "obtain the correction". I vaguely remember having to convert the picture to grey scale, then using curves, then converting back to color. But I have no idea how to do it now.
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May 17, 2012, 00:16
Post: #7
RE: Color correction?
Good software is just indicated what should be gray/white and does the rest.

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