FotoDave@aol.com
Sat, 17 Apr 1999 02:03:02 -0400 (EDT)
In a message dated 4/16/99, PM 03:25:56, alt-photo-process-l@skyway.usask.ca
writes:
<<Does optical brightness in Phototoshop occur on a logarhythmic scale?
Yes.
So if you want to use the familiar H&D curve, plot density vs log(rgb) rather
than density vs. rgb. There is usually a slight nonlinearity which you can
correct with curve or dot gain feature. For example, you could use gamma to
adjust the contrast and manually adjust the curve for 50% (rgb=128). This is
similar to checking Kodak's AMB points in Kodak gray scale.
Or you could go more details and calibrate for 25%, 50% and 75% tone, but
usually this is not even necessary.
>> If I were to create a step wedge that increased in increments of 5% (for a
total of 21 steps), would the information from that wedge be similar to a
manufactured one? Or could I compute each percent step by using log scale?
>>
No. You will have a step tablet that is equally spaced in transmittance, or
you can say that it is equally spaced in percent ink coverage; but the
manufactured step wedge is equally spaced in density, which is
log(1/transmittance).
It doesn't mean that you cannot use the step wedge that you create in %
increment, but you just need to understand that it is not equally spaced in
density.
Dave S
This archive was generated by hypermail 2.0b3 on Thu Oct 28 1999 - 21:39:31