Photoshop - Some professionals say dot gain 20%, others gamma 2.2 ??????

The relation between a power function (Gamma) and the
dot gain according to a simplified (!) dot gain model
is shown here on p.17:
http://www.fho-emden.de/~hoffmann/a3gencolortest.pdf
(preview)
http://www.fho-emden.de/~hoffmann/a3gencolorhigh.pdf
(quality)

As already stated by Lou, the Gamma curve for G=2.2
is appropriate if the image is treated like an RGB
image in a working space with G=2.2 (sRGB, AdobeRGB).
For instance: a color RGB image is converted to Gray-
scale and then back to RGB. It's then a gray RGB image,
which leads for many printers to a CMYK print, rich black
instead of K-only. A Grayscale (coded by Indexed Color)
is mostly printed by K-only.

In either case this is correct for desktop printers, but
not for the press, where an estimated dot gain is used or,
much better, a profile which is derived from a standard
CMYK profile like 'Black Ink ISO Coated'.

Best regards --Gernot Hoffmann

Sorry Marco, unless you are referring to photoshop channels, I do not undrstand the question.

Chrid,

Hi Peter. thanks that is an interesting point. I often use and load and manipulate individual channels, especially for sharpening. But maybe the difference are not so apparant and maybe miniscule. But thanks for that tip though.

No idea Aandi. But a video podcast and a web site suggested that if you are not sending work to a printer then set this to gamma2.2 because that is the RGB space of your monitor and printer. If youare using PROPHOTO then set this setting to 1.8? Since I was used to keeping it at the default I was somewhat confused as to what to do here.

Chris.

Whatever you have loaded as your grayscale working space will affect how your individual channels of a color document are displayed, so that may have an impact if you do any amount of channel work.

Hi Aandi, thanks for your reply, Are you Dutch? Double 'a' is a dutch dipthong, tot zo!. Lou, I appreciate your time in this, and no, it was not too technical. I need details in order to fully grasp a concept, so Thankyou.

Chris.

Chris,

What you do depends on a number of factors. Dot Gain is usually used when outputting to a press, since it is more likely to reflect press behavior than a gamma curve. If you had the 'average' actual dot gain for a given press, it would probably give you a closer match to a properly calibrated monitor.

If most of your work is either displayed on web, emailed or printed on an inkjet, gamma 2.2 is probably a better overall choice. The web and email are based on sRGB, which also has a gamma of 2.2, so your grascales will display more accurately online if you use gamma 2.2. As long as you use the correct printer profiles for printing (for your paper/ink/printer), your files will be converted so they look correct on your inkjet. The more accurate your profiles, the better the result.

Photoshop will take either your gamma 2.2 or 20% dot gain encoded file and convert the numbers to your printer profile.

Dot gain curves and gamma curves are not quite the same. You can see the effect if you wish as follows: convert a color original to gamma 2.2. Then, ASSIGN a 20% dot gain curve. Your overall image will look lighter than the gamma 2.2 original. Try assigning 25% and 30% dot gain curves, and as the dot gain increases, your file will appear darker. Although not all parts of a dot gain curve mirror a gamma curve precisely, it appears to me that a gamma 2.2 curve is fairly close to a dot gain of approximately 27% or so.

Probably more theory than you wanted. If you aren't going to a press, gamma 2.2 is a good bet and fairly flexible. But you can get good prints both ways.

Lou

Do you ever convert your photos to greyscale? If not, I don't think
the grey profile is ever relevant (but I'm ready to be surprised).

Aandi Inston