Re: Slide Colour Balancing

[P3D Dr. George A. Themelis <fj834@xxxxxxxxxxxxxxxxxxxxx>]

Andrew Woods writes:

>SLIDES on the other hand don't need colour balancing because under 
>normal circumstances the eye adjust to the slide's colour balance and 
>you won't notice the error in colour balance.

In theory, yes, the eye adjusts.  However it takes time for this 
adjustment and, in practice, you will notice a shift in color balance as 
you switch between slides in a projector or viewer.  You will not notice 
a color shift due to the light of the viewer or projector, when using, 
say regular vs. halogen bulbs (unless you switch viewers or projectors), 
but you will notice shifts in color between individual slides.

The adjusting power of the eye is amazing!  In work we have this "green 
room" for inspection.  After a while, the green light looks white, until 
you step out of the room where everything looks pink!  Someone mentioned 
the possiblity of viewing anaglyph WITHOUT glasses utilizing this 
effect.  I'll have to try it.  The eyes will also adjust to a slide 
where the two images are of different exposure.  After a while this 
difference will not be noticeable any more.  BUT, it takes time for the
eye to adjust and such changes will be noticed in a quick succession of 
images.

>If the colour balance of a slide was very bad I can imagine a need
>to correct it.  BUT is there anywhere which does this and does
>anybody actually do it?

I do that with slide duplication.  You can use filters to correct or 
alter the original.  My current batch of slide duplication film has a 
magenta/pink shift.  I use green filters to correct, but when I get a 
slide with obvious green shift (due to either FL lights or shooting 
through a thick glass with green cast) then I can duplicate without 
filters and get nice white (neutral?) balance.  I use slide duplication
to improve composition and alter/correct exposure and color shift. I
only do this for PROJECTION slides (PSA and Detroit competitions).

George Themelis


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