Digital BW, The Print

> From: Andre <am1000@...>
 
> 
> 
> 
> I've read numerous times that the images from a digital camera is
> somewhat linear, whatever that means. Would you say that applying an
> S-curve in Photoshop helps restore some analog (film) look about the
> image ?
> 


Andre

If you are going to work in digital you really should spend some time
understanding this - it will really help you put some of the jargon in
perspective.  All digital is linear.  The pixels in a plasma or lcd display
act in a linear fashion - they illuminate in direct proportion to the bit
value they are fed.  In order to get around this, we introduce gamma (a
fancy name for curvature rather than linearity).  By applying a gamma
function, when we convert RAW pixel data to RGB whether it be a camera or
display, we align these devices with things like photo paper or film which
are not linear in their luminance vs exposure and the way the eye sees.
These products exhibit curvature when luminance is plotted against exposure
(pixel value) but they ARE linear if you look at the LOG of each side of the
equation.  This is why when you calibrate your monitor you must decide what
gamma you want, and when you do a scan you choose a gamma, and why when
doing a RAW pixel to RGB conversion you choose a gamma.

[All LOG is is the first derivative of an exponential (ie curved) equation.
It gives you rate of change of an exponential function.  You may remember
this from your maths in high school.]

The single best summary of exactly this topic that I have seen is, as I
mentioned before, on Norman Koren's site, specifically this page:

http://www.normankoren.com/digital_tonality.html

It goes through all the issues that have recently been dicussed in this
thread:  gamma, film vs digital, tonal range, how the eye sees, and the
practical application of all of this to using an S curve in RAW pixel value
conversion to RGB.

I would also read the two bits headed up "Gamma and Black Level" and "Why
gamma?"  on this page:

http://www.normankoren.com/makingfineprints1A.html

I am in no way affiliated with Norman! But I really did find the stuff at
these two locations very very informative.  A lot of the myth attached to
the jargon disappears.  Go through it once and if you don't get it put it
down for a while and then go through it again.  I can't emphasise enough the
value of doing this.

Steve