Compensating for dot gain in film using transfer functions

When using CMYK color profiles, you cannot customize dot gain settings. However, you may be able to compensate for dot gain from a miscalibrated imagesetter by using transfer functions.

Transfer functions enable you to compensate for dot gain between the image and film. For example, the Transfer function makes 50% dots in the image print as 50% dots on film. Similar to dot gain curves, the transfer functions let you specify up to 13 values along the grayscale to create a customized transfer function. Unlike dot gain curves, transfer functions apply only to printing--they don't affect the image color data.

Use the following guidelines to determine the best method of accounting for dot gain:

• If you are using a custom CMYK profile, use the dot gain settings in the custom CMYK dialog box to adjust dot gain so that it matches the printed results.
• If you are using an ICC profile and the dot gain values do not match the printed results, try to obtain a new profile with values that do match.
• Use transfer functions only if neither of the previous methods is an option.

To adjust transfer function values:

1. Use a transmissive densitometer to record the density values at the appropriate steps in your image on film.
2. Choose File > Print with Preview.
3. Select Show More Options, and choose Output from the pop-up menu.
4. Click the Transfer button.
5. Calculate the required adjustment, and enter the values (as percentages) in the Transfer Functions dialog box.

For example, if you specified a 50% dot, and your imagesetter prints it at 58%, an 8% dot gain occurs in the midtones. To compensate for this gain, enter 42% (50% - 8%) in the 50% text box of the Transfer Functions dialog box. The imagesetter then prints the 50% dot you want.

When entering transfer function values, keep in mind the density range of your imagesetter. On a given imagesetter, a very small highlight dot may be too small to hold ink. Beyond a certain density level, the shadow dots may fill as solid black, removing all detail in shadow areas.

Note: To preserve transfer functions in an exported EPS file, select Override Printer's Default Functions in the Transfer Functions dialog box and then export the file with Include Transfer Functions selected in the EPS Format dialog box. (See Saving files in Photoshop EPS format (Photoshop).)

To save the current transfer function settings as the default:

Hold down Alt (Windows) or Option (Mac OS) to change the Save button to --> Default, and click the button.

To load the default transfer function settings:

Hold down Alt (Windows) or Option (Mac OS) to change the Load button to <-- Default, and click the button.

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