Re: Twin SLR questions

[bercov@xxxxxxxxxxx (John Bercovitz)]

Stephen Kearney writes:

> I'm thinking about trying some twin SLR work. Before I do, I'd 
> like to make sure I have the viewing options fully understood.
>
> Is it correct that I can view the stereo pair by:
>
> 1.  Mounting the the chips in a European format mount (7-perf) 
> and viewing in my Dr. T - modified Red Button. This would 
> eliminate some of the higher resolution full-frame 35mm would 
> offer.

Yes, you can view 7p in the modified red button if you don't wear 
glasses and your eyes aren't deep-set.  I don't understand how 
you would lose resolution, though.  Maybe you mean you would be 
viewing the transparencies from 40-plus mm away instead of from 
50 mm away as you would want to be if the slides were taken with 
50 mm lenses?  I think the loss of resolution due to this cause 
would be minor.  I'd be more concerned about squash and would head 
for a Star D with 50 mm lenses to match the camera(s)' lenses.  
Either that or I'd get an old Minolta camera with 45 mm lens.  On 
the other hand, you may mean loss of view due to the left and 
right sides of the slide being lopped off.  Whether or not this is 
a problem depends on the shot.  No aspect ratio is right for every 
shot.

> 2. Viewing in a 2x2x2 viewer, and losing the precise alignment a 
> stereo mount provides.

I wouldn't worry about losing _precise_ alignment; the eyes are 
wonderfully compliant in some ways.  Your 2x2 viewers will need to 
hold the slides reasonably well - I wouldn't want to see one slide 
rotated with respect to the other very much, for instance.

> In the balance I'm not sure twin-SLR stereo has much over using 
> a Realist, except for hyper-stereo ability.

On balance, I don't know if you can say much about one vs the 
other which method of taking stereos is superior.  It's too 
situation-dependent.  

If you use a slide bar to take your SLR shots, you can keep the 
interocular or stereobasis normal and not get a change of scale.  
If you want to put two SLR cameras on a single bar, then you will 
suffer from scale change (everything will look smaller than it 
is).  However, you don't have to have stretch or squash (relative 
3rd dimension distortion) if you just match the camera's lens to 
the lenses of the viewer you're going to use.  Most importantly 
with SLRs you will be able to at least equal and maybe even 
surpass the resolution of the better (and more expensive) stereo 
cameras.  An SLR on a slide bar is only really good for stationary 
subjects, like Hayden's subjects.  8-)  You can also install 
longer lenses on an SLR so as to make pairs which are more 
appropriate for projection.

Of course a real 3D camera is much more convenient to use, and it 
will synch better than twinned SLRs which has implications not 
only for moving subjects but also for flash.

Whichever way you shoot, it is more convenient to have the two 
transparencies in a single mount.

John B


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