Obtaining film Gammas for ColorPerfect from a data sheet's plot of characteristic curves


If you have or can obtain manufacturer's data sheets for films beyond those built into ColorPerfect's ColorNeg mode we're always grateful if you send us a scan or PDF of such so that we can include a characterization for those films in a future release of the plug-in. If you don't want to wait for that but would rather start converting negative scans right away you can easily find the Gammas for the film and produce a *.negpos file for use with ColorPerfect yourself. Look for the characteristic curves of the film, which should be a plot of density versus log10 exposure. We'll be using the new Kodak Portra 400 film's characteristic curves depicted to the right as an example here.

Preparation: Matching the axes' units

Before getting started we need to make sure that the unit spacing is the same on both axes, that is, that the distance between 1.0 and 2.0 is the same for the horizontal and vertical axis. For an image of a graph like this one, that can be done quite neatly by changing the image size in Photoshop using a different amount for width than for height. In this example the ratio of height divided by width of any rectangle made of the same units on either axis is 1.25. Therefore we must scale the image's width to 125% while having "constrain proportions" unchecked.


Calculating the film Gammas

The three Gammas can be determined by calculating the slope of the straight portion of the characteristic curve and taking its inverse. Choose two widely separated points on the straight line portion of a characteristic curve for either color as illustrated by the red, green and blue dots. The coordinates for these points can be read by carefully placing the cursor on the line and reading X and Y from Photoshop's info panel. The X and Y readings have been printed on the graph above and the gamma calculation was made as shown. It's the difference between the X readings divided by the difference between the Y readings, taken as a positive number. This is done separately for the three colors R, G, and B. Alternatively, the plot can be printed and measured with a ruler. In any case, the slope required is the (positive) measured distance between the two points on the Log Exposure axis divided by the (positive) measured distance between the two points on the density axis.

Creating *.negpos characterization files in a text editor

To create a *.negpos file ColorPerfect and ColorNeg will read and allow you to use from the User entry in the maker menu simply put one line for each film you want in the file and have the lines contain the film's red Gamma, its green Gamma, its blue Gamma and its name separated by tabs.

Loading and saving characterization files with ColorPerfect 2.15 and higher

With ColorPerfect 2.15 and later, characterization files can be stored in any location as plain text files with a freely selectable (short) file name. The only requirement is that you use the specific file extensions instead of *.txt.

Use the file extension .negpos for ColorNeg mode, .colorint for ColorPos mode, and .pfraw for PerfectRAW. After you have selected the entry "User" from the manufacturer menu instead of e.g. Kodak, you can comfortably work with the files by means of file open or save dialogs.

Here is an example *.negpos file with the characterization we made for download.

Saving & loading characterization files with the legacy Mac 32 bit ColorNeg and ColorPos plug-ins

For our earlier 32 bit plug-ins for Mac OS X things were a bit less straight forward. We really recommend you to get the free upgrade to ColorPerfect 2.25 which resolves these long standing issues and offers so much more.

Using Photoshop 7 and Photoshop CS with the legacy plug-ins you had to copy all auxiliary files you wanted to use into a specific folder. Using more recent Photoshop versions you could organize your files anywhere and specify the folder and filename to use on the Plug-in's user interface using a workaround.

For those not familiar with this workaround and still wanting to use an outdated plug-in for whatever reason the easiest way to get a *.negpos or *.colorint file to work is storing it under a default file name. If you have never edited the file name read-out on the original ColorNeg plug-in the default is ColorNeg.negpos, for the original ColorPos plug-in it is ColorPos.colorint.

Here is the example *.negpos file with the Kodak Portra 400 characterization we made for download as: ColorNeg.negpos. Put that file into the folder detailed for your version of Photoshop below and it will get loaded into the user section of the film maker list the next time you use the plug-in.

Keeping multiple files with different file names was also possible of course. After selecting the User entry from the Maker list and pressing the Load button the set up screen for the auxiliary file path showed. While it was active what's usually the read out for the name of the current characterization file the plug-in became an editable field where you could specify which file you intended to load (without the file's extension).

Auxiliary file locations for the legacy 32 bit plug-ins on the Mac

The following information is a shortened outtake from an old ReadMe.txt file. If you are familiar with that ReadMe.txt already you might want to stop reading here. Of necessity our 32 bit plug-ins for Mac OS X used an odd convention for the folder location of the auxiliary files. The location varies with the version of Photoshop. We would have preferred a better way of doing this, but at least this oddity does not come up that often in normal usage once it has been resolved.

Adobe Photoshop 7 and Adobe Photoshop CS

The location of auxiliary files for Adobe Photoshop 7 and Photoshop CS is very odd and very nearly the same. To get there, double-click Applications in Finder, then double-click Adobe Photoshop x, where x is either 7 or CS. Now locate the Adobe Photoshop x icon you might normally double-click to start Photoshop, but do not double-click. This icon may or may not have .App after the Adobe Photoshop x. Although it does not appear so, this icon is really a folder. Single click with the right mouse button (or control-click on a single button mouse) to bring up a menu of options. Click on the option Show Package Contents. This will open a new Finder window with a folder Contents (and possibly other items). Double-click Contents. That will disclose several other folders. For Photoshop 7 The folder you want is MacOS, while for Photoshop CS, the folder you want is MacOsClassic. That is the folder where the auxiliary files go, so double-click it and transfer in your *.negpos or *.colorint file(s). ColorPerfect and ColorNeg will always read and write their files to and from this folder, so this is where you will always find them. You can create aliases and place them in other folders for easier access if you often work with these files.

Adobe Photoshop CS2 through CS5

These versions of Adobe Photoshop do not write to a peculiar folder, but neither do they make it easy to control where files go. They pick the root folder instead of the hidden folders described for Photoshop 7 and CS. You can just let ColorPerfect put its files in the root folder if you wish, but as the root folder is not really a good place to put files, we provide a clumsy workaround. The root folder is the one with a path that is a single slash, "/" often shown in Finder as Macintosh HD. Put the file ColorPerfectPath.txt in the root folder, then edit that file to contain the path to where you really want the files. As delivered, the ColorPerfectPath.txt file contains the path /Users/MyUserName/Documents, so if you substitute your actual user name for MyUserName , ColorPerfect will put its files in your Documents folder. The path must be a valid path - that means that if you want to use a new folder, you must first create the new folder yourself (probably using Finder) before it will work. Now go to the target folder in your chosen path and transfer in your *.negpos or *.colorint file(s). ColorPerfect will always read and write its files to and from this folder. It is possible to change this path from within ColorPerfect whenever you load or save a file.