Re: TECH-3D digest 177

[T3D John Ohrt <johrt@xxxxxxx>]

T3D Eric Goldstein wrote:

> So, acknowledging that yes, there are transfer function and gamma
> differences between photographic medias and computer imaging/display
> software and hardware which are currently being compensated for, does
> anyone know the equivelent contrast ranges in f/stops of computer
> imaging versus traditional photographic materials?

The short answer is there is no answer in terms of fstops because fstops
ignores the recording technology.  Fstops merely indicates the range of
light levels detectable, not the dynamic range.

You have to compare density (dynamic range).

For example, say we shoot a typical film which can record 12 fstops with
a density range of 3.0.  Now we substitute a CCD back with 12 bits per
channel.  It too will record 12 fstops with a dynamic range of log12 =
4.8 density, ie linear.  Obviously, all you need is a density of 3.0 for
the film to display what it recorded, ie 10 bits per channel, but
because the film is nonlinear you may miss subtle detail in certain
level ranges.  And we just keep going in circles.

You have to decide whether the film is to be presented in its current
artistic transfer function, or in a linear transfer function.  Only then
can you determine the dynamic range (density) appropriate for the
output.

Now if you are going to settle for a very limited dynamic range from a
cheap hardcopy device, say density 1.5 and you are not looking for a
full dynamic range duplication of the original film, then you are much
better off.

Ideally, scan with a gamma correction that maximizes the linear dynamic
range of the data recorded, which is much less than that suggested by
the density.  Clip and baseline clip to throwout the data which lies
outside the linear dynamic range.  Using brightness and contrast
adjustments supported by histogram analysis to restore the data to a 0
to max range of representation (ie. 0 to 255 for Photoshop etc). 
Process as desired.  In an ideal world, you have the option of wysiwyg. 
Print.

Is the water less muddy?
--
John Ohrt,  Regina, SK, Canada
johrt@xxxxxxx



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