Re: slide colour balancing (was re: APS and 3D slides)

[P3D Andrew Woods <andrew@xxxxxxxxxxxxxxxxxxxxx>]

Don Chaps <dchaps@xxxxxxx> wrote:
> Good slides can be made from negatives if you use a film designed for 
> that purpose. The problem that would stop me from doing that on a regular 
> basis is the color balance problem. Unless you pay big bucks for a pro 
> job, the slides will be the positive equivilent of minilab 4x6s. Getting 
> the same exposure and color balance on each of your stereo pair would be 
> the problem. If you make 
> prints you sometimes have to browbeat the printer to get matching prints 
> (Grand Photo is the exception. They make it painless). 
> If you were going to make an edition of slides, it would be worthwhile to 
> go to a professional output bureau, negotiate the color balance, and put 
> the time necessary to get great results. 
> 	I think the reason that color slide film is used widely 
> professionally even with its disadvantages of higher contrast and limited 
> exposure range is because the color balance is fixed and not subject to 
> interpretation by an unknown third party.

The reason you don't need to colour balance your slides is because your
eyes do the job for you - yes, your eyes can perform colour balancing!
Slides are usually viewed under certain distinct circumstances - projected
large in a darkened room or viewed through a viewer.  In both of these 
circumstances, the only thing your eyes can see is the slide image.
The eye's colour balancing works over the eyes entire field of view.
In the above circumstances, the slide is the eye's only stimulus therefore
the eye performs colour balancing of *ONLY the SLIDE*.

4x6 prints on the other hand are viewed quite differently.  you hold them in
your hand and the print only fills a very small proportion of the eye's
field of view.  The eye's colour balance will therefore be weighted heavily
on your surrounds.  prints therefore NEED to be colour balanced by the lab
because you will easily notice when their colour balance is wrong.

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.

Now let me hypothesise a little: 
Consider if slides were viewed in a different manner - say with a hand
held unit where the slide was back projected onto a 4"x6" screen.
The image on the screen would look just like a 4x6 print but the image
comes projected from the slide.  
Now would you notice the colour balance problems in your everyday slides
from your 35mm camera?  Based on the criteria discussed above I would
think yes, but there are other factors involved which further complicate
the issue.  
A print is illuminated by the same ambient light which lights the room.
In the unpatented Woods viewer model A mentioned above, the slide image
is lit with the colour temperature of the projector bulb which is 
different from the room lighting...

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?

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