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[OM] T10 flash and Ring Cross Polarizers

Subject: [OM] T10 flash and Ring Cross Polarizers
From: Joe Gwinn <joegwinn@xxxxxxxxxxxx>
Date: Thu, 27 Dec 2001 23:00:50 -0500
At 1:37 AM +0000 12/28/01, olympus-digest wrote:
>Date: Thu, 27 Dec 2001 16:31:46 -0500
>From: "Walt Wayman" <hiwayman@xxxxxxxxx>
>Subject: Re: [OM] T8 or T10 ring flash?
>
>Having been impressed by TOPE 8 entry 19, I am experiencing the 
>urge to get a ring flash, but haven=92t decided yet whether it=92ll be 
>the T8 or the T10.  I'm leaning toward the T8 because, unlike the 
>direct flash of the T10, as the esif describes the T8, "The unique 
>concept of this ring flash unit is to provide soft, indirect 
>lighting for close-up subjects.  The flash is bounced off 
>Reflector 1 or Reflector 2 to reach the subject indirectly. This 
>makes possible completely shadowless lighting for any subject 
>smaller than the reflector. It completely solves the problem of 
>ring-shaped reflections."
>
>Well, it goes without saying that we all must constantly be on 
>guard against the dreaded "ring-shaped reflections!"  
>Nevertheless, I like the idea of the indirect lighting, even 
>though the T8=92s output is less than the T10.  If anyone has 
>experience with both the T8 and T10, or even just with either, I=92d 
>welcome any input.  In fact, I=92ll even entertain opinions of those 
>who have never even seen either one but just have a thought on the 
>subject and feel like expressing it.  

I have a T10 (but also covet a T8).  I almost always use the T10 with a ring 
cross polarizer, which pretty much eliminates surface reflections.  It works 
pretty well.

The scientific principle behind the ring cross polarizer is simple.  The ring 
cross polarizer consists of a ring and a disc, each made of the same linear 
polarizing material, but set perpendicular to one another.  The flash light is 
polarized by the ring, bounces off the subject, and tries to come back through 
the disc polarizer (and thus to the lens), which being perpendicular blocks 
direct transmission.  Light reflected from a surface has the same polarization 
as the incoming light, and thus is blocked.  Light that instead penetrates into 
the subject, bounces around a bit, and re-emerges, is rendered unpolarized in 
the process, and so about one half will get through the disc polarizer.  Thus, 
surface reflections are strongly surpressed, and the color of the underlying 
bulk material is seen undiluted.

The price is that one loses two to four stops of light in the polarizers, with 
two stops being the theoretical minimum.

I first learned of this trick in a book on the Hasselblad system, but Olympus 
packaged it in a convenient form.  I first used the Hasselblad approach to take 
a picture of an oil painting, eliminating a very distracting surface 
reflection.  Because the oil paint was gobbed on, there was no angle that 
eliminated the reflection.  Only the crossed polarizers worked.

The ring cross polarizer is not useful for photographing metallic subjects, 
which photograph as black, and without the polarizer one is likely to see rings 
from a T10 flash reflected in the metalllic surface.  A T8 will instead show as 
white (the etched aluminum reflector) in reflections.  This is the issue.

So, the choice between T10 and T8 depends on what you will photograph. Is the 
subject all surface (like a metal), or do you want to see the color of the 
material versus the reflection?

Joe Gwinn


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