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[OM] The (well almost) definitive word on T-32 coverage

Subject: [OM] The (well almost) definitive word on T-32 coverage
From: Chuck Norcutt <chucknorcutt@xxxxxxxxxxx>
Date: Sun, 09 Jan 2005 17:14:59 -0500
Today I decided to dispense with all theory and approximations and make 
some actual measurements of T-32 output over the field of view of a 24mm 
Zuiko.

First, I should make clear that no actual film was impinged upon by 
photons during this test.  Although a 24mm Zuiko was used it was used 
only as a spotting device to determine the measurement locations for a 
Sekonic L-358 flash meter located approximately 10 feet (3 meters) from 
the flash.  The meter was positioned at the center, the top center, the 
left center and the top left corner of the field of view as seen through 
the camera's viewfinder.

Measurements were taken in a smallish room of my house that was not wide 
enough to accomodate the full field of view of a 24mm lens at a distance 
of 10 feet.  Consequently, the right side of the field was viewing 
things (like my TV set) much closer than 10 feet and measurements were 
only made at the center and left sides.

To assure that the flash was fully and evenly charged for each shot, 
power was supplied by an SP Systems (Quantum Turbo compatible) 330 volt 
power pack.  This battery pack can recharge a T-32 after full dump in a 
bit less than 2 seconds and time between measurements was much longer 
than this.  Measurements at the same spot were always perfectly 
repeatable so there was no difference found in flash power output.

Two separate T-32's were measured becuase I thought I had previously 
noticed 1/3 stop differences between some of my four T-32's.  I think 
the former measurements were biased by less than fully charged batteries 
since, with the SP systems power pack, no discrepancies were seen 
between the two flashes.  Note that, while recharge time is improved 
with NiMH batteries, the maximum flash power output is lower than if 
fresh alkalines are used.  I assume this is due to the fact that four 
NiMH or NiCad batteries are only producing 4.8 volts total instead of 6.

Fall-off from the center as 0 point

Top center      -2/3 stop
Left center     -1 stop
Top left corner -1-1/3 stops

Note that what was measured was the flash output and not the light 
reflected back and actually reaching the film.  Fernando, for example, 
reported disappointment in the amount of vignetting visible in his 
images taken with a T-32 and 28mm lens outside at a distance of 10 
meters.  No doubt the 28mm lens has some degree of vignetting of its 
own.  If, for example, the light falloff of the lens itself at the 
extreme corner was 2/3 stop then, given the 1-1/3 stop loss from the 
flash, there could be a total 2 stop loss overall in the corner. 
Clearly, this amount of light loss would be quite visible.  Fernando's 
results may also be a bit more extreme since, being outside, there were 
no walls or ceiling to recycle any of the stray light back into the image.

So, does the T-32 support a 24mm field of view?  The answer is yes 
beause, in a world with no standards, the manufacturer says it does. 
Does it support 24mm to an appropriate degree of evenness of 
illumination to suit your purposes?  That's up to you.

Me, I try never to be dependent on a single flash if I can help it.  I 
also try to avoid using them straight on which is the only case in which 
coverage angle is meaningful.

Chuck Norcutt



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