Re: DOF in photomicrography

[P3D Sam Smith <sam@xxxxxxxxxxxx>]

>A simpler method was written up several (6-8?) years ago in a magazine that
>our staff photographer got -- Photomethods?  Since then I've tried without
>success to find that article again, although I have seen the same topic
>elsewhere.  I don't remember its proper name.  It should work for micro or
>macro methods.  It illuminates the subject from the sides (in a plane
>perpendicular to the optical axis) using a slit aperture on the light
>source.  This is set to illuminate only the depth of the subject that is in
>focus; the rest of the subject is not illuminated and does not expose that
>area of the film.   The exposure is made, the focus is moved and the
>illuminating aperture moves with it, another exposure (or maybe a continuous
>exposure) is made on the same film, and so on, building up a complete
>in-focus image on film.  They had some good examples in the article.  Of
>course the subject has to be motionless while all this happens.  

Cool. I expect you could use this technique with a good macro lens at
extreme magnification, say 4:1 to 10:1. I like the theory behind this, but
my area of interest is insects and arachnids directly from nature. The
"motionless" bit makes this technique sound impossible, or does it?

Let's examine what we have to work with. Designing a stereo rig to shoot two
simultaneous stereo images at high magnification and with a greatly reduced
base is both possible and feasible. Just like magicians, it can all be done
with mirrors. Moving the light source/slit/focusing rail in complete
synchronization is also mechanically feasible. The problem seems to be being
able to do all this in a very short space of time to minimize movement of
both subject and camera. For the sake of argument, lets say 1/125th of a
second as the minimum exposure time for the entire subject. The main problem
seems to be the light source. You would require a great intensity of light
while panning the slit to achieve correct exposure.The only light source
that's practical is electronic flash. The problem is that the minimum flash
duration of most flash units is 1/1000th of a second, leaving no time to pan.
The only solution I can see to this dilemma would be to forget about panning
the flash. You could have several flash units at fixed distances with fixed
slits all aiming at the appropriate part of the subject. The focusing rail
would be connected to a spring mechanism. This would move the camera lens a
pre-determined distance, panning the focus from the furthest details of the
object to it's closest. When the shutter is pressed, the lens starts moving
outward. As it slides forward, it triggers the flashes in consecutive order
to illuminate the parts in focus. If you need numbers, lets say the required
depth of field would be a distance of 10mm. The lens can be stopped down to
cover 3mm. That would mean if you used three flashes and slits, they would
have to cover 3.3mm each.

Make sense? I don't expect this all to fit in your pocket, but again I'm
just interested to see if anyone has any comments whether it would work or
not. New territory is always appealing. 

Sam 


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