Re: Printing of photography
Re: Printing of photography
- Subject: Re: Printing of photography
- From: Robin Myers <email@hidden>
- Date: Mon, 20 Dec 2010 10:01:59 -0800
There are several issues raised by your question about including color bars in the images. Usually this means adding something like a ColorChecker or the Kodak Color Separation Guide and Gray Scale to the image. These references are being used for several purposes, some of which are not appropriate and that may be the cause of problems reported by others in this discussion.
There are some pigments, and many dyes, which cause observer metamerism failure when imaged with digital cameras (and film). The causes are related to the pigments or dyes in use, the spectral response of the camera and the spectral power distribution of the light source. These issues can all be solved to achieve an accurate image. I wrote a paper which details the reasons for the metamerism failure and how to solve it, http://rmimaging.com/information/color_accurate_photography.pdf.
Often the ColorChecker, or Separation Guide, is used to check the image capture for accuracy, the assumption being that if the target is correct then the artwork image should be correct. Unfortunately the pigments used in these targets are not the same ones which cause the color shifting problems. So the targets appear correctly in the image, but the artwork does not. This is not a correct use for these targets.
The ColorChecker can be used to create a profile for the image and its gray scale, or the Koday Gray Scale, can be used to check the tonal reproduction curve of the capture. Either of these targets can be used to guide subsequent image tone adjustments. When used with the technique in the aforementioned paper, the charts can provide guides for accurate printing of the artwork image.
Including a color chart in the capture then becomes an issue of providing a capture reference, a check for subsequent printing and a guide for image manipulation, such as tone curve adjustments for printing.
One important note should be made about the chart included in the image. Use of the Color Separation Guide is not recommended for many reasons: there is no published reference for the colors of the Guide, it consists of only four colorants, the substrate fluoresces, it is comprised of halftones, ... The Gray Scale is suitable for a neutral and tonal scale reference.
The main reason many museums and libraries use the Kodak Color Separation Guide is because it is a linear chart which does not take up many pixels in the image. Until recently it was the only linear target available.
There is a newer strip chart from Image Science Associates (imagescienceassociates.com), the Object Level Target, which can now replace the Kodak Color Separation Guide. These are linear targets with all the colors from the ColorChecker but with an extended 12 step gray scale. These would be suitable targets to include with the artwork in the image capture.
Regards,
Robin Myers
On Dec 20, 2010, at 6:18 AM, David Ramsey wrote:
>
> 2 questions;
>
> 1) I am getting ready to photograph some works on paper for a museum and I was asked about including "color bars" in the photos. Because of the nature of the materials used in these works I have to adjust specific colors as we shoot (Color managed workflow and calibrated display). Colors are adjusted individually, for example, purples may go towards blue reds towards magenta. So if I place color bars in the frame and will this not create havoc down the line since I am moving individual colors around? Also which color bars should I use if they are to be included?
>
> 2) My camera produces an native file size of 9"x14"@300dpi(ppi). I have been asked about producing a larger file for a book cover that will wrap the book (11"x18"). There are a plethora of ways this can be accomplished either thru photoshop or other applications. What are the thoughts of the you printers and prepress people out there? I know I can do this but should I? Or let the folks down the line optimize the file for their particular setup.
>
> Thanks in advance for your thoughts and input.
> David Ramsey
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