You may recall my recent post where I measured these light falloff
values for a T-32 at coverage angles for a 24mm lens.
Fall-off from the center as 0 point: T-32 @ 24mm
Top center -2/3 stop
Left center -1 stop
Top left corner -1-1/3 stops
(This one is new. See text below)
Fall-off from the center as 0 point: T-32 @ 28mm
Top center -1/3 stop
Left center -1 stop
Top left corner -1 stop
Wayne wondered what the equivalents might be for a T-20 and a Vivitar
550FD at their stated lens coverages. I said I'd test the 550FD and get
back so here it is. I also decided to test my Sunpak 522.
Vivitar 550FD
-------------
Wayne stated that the 550FD supports a 50mm lens. The 550FD
specification is actually given as 34 degrees vertical and 46 degrees
horizontal which translates most closely to a 42mm lens. A perfect fit
for my old, original Petri 7s rangefinder. I therefore decided to test
around the specification by checking both 50mm and 35mm coverages.
Fall-off from the center as 0 point: Vivitar 550FD @ 50mm
Top center -1/3 stop
Left center -2/3 stop
Top left corner -2/3 stop
Fall-off from the center as 0 point: Vivitar 550FD @ 35mm
Top center -2/3 stop
Left center -1 stop
Top left corner -1-1/3 stops
Note that, while the Vivitar spec does not say the flash supports 35mm
lens coverage, in terms of evenness of illumination, it actually
performs as well at 35 mm as the T-32 does at 24 mm. The T-32, of
course, is a full 1-2/3 stops brighter at all points in addition to
covering the much wider angle. Put another way, if you were using ISO
160 film with a T-32 you'd need ISO 400 with a 550FD to work at the same
distance. You still wouldn't be able to go wider than 35mm however.
Sunpak 522
----------
The specification for the 522 is also stated in terms of angle of
coverage as well as focal length for various format cameras. It's given
as 45 degrees vertical and 60 degrees horizontal. The first thing to
note is that the specification is for 3:4 aspect ratio and not a 2:3
aspect ratio. The first indication that this flash really is designed
to be used by medium format photographers as well as 35mm.
It's also interesting to note that, when it comes to specifying the lens
to be used for 35mm format the focal length is stated as 35mm. The
specification of 60 degrees horizontal coverage angle, however, equates
to a 31mm lens. So I decided to bracket this one too by testing at both
35mm and 28mm.
Fall-off from the center as 0 point: Sunpak 522 @ 35mm
Top center -1/3 stop
Left center -2/3 stop
Top left corner -1 stop
Fall-off from the center as 0 point: Sunpak 522 @ 28mm
Top center -1/3 stop
Left center -1 stop
Top left corner -1-1/3 stop
Once again, although the specifications don't say that the flash
supports a 28mm lens, in terms of evenness of illumination, the Sunpak
522 actually performs a bit more evenly at 28mm than the T-32 does at
24mm. Note that the Sunpak is distributing more light vertically in an
attempt to provide better coverage for the squarer medium format films.
T-32 revisited
--------------
All of this led me to go back and test the T-32 for 28mm coverage.
Those results are reported above. I did this a couple of times since I
had previously reported 1/2 stop fallof at the edge of a 28mm field and
these results show a full stop. Part of the error was measuring a 28mm
field with the Minolta A1 whose horizontal covereage is only a 3:4
aspect ratio and not 2:3 as is 35mm. The rest of it I can only
attribute to additional, unidentified brain malfunction.
I did notice that, even though the SP Systems high voltage power pack
has only delivered about 100 flashes since it was charged, the maximum
power output from the T-32 has already dropped about 1/3 stop from my
very first measurements. Apparently still very sensitive to maximum
voltage.
My conclusion. The angle of coverage spec for the T-32 is a bit
optimistic relative to its competitors... or otherwise par for the
course in a world without standards. When using a 28mm lens, the Sunpak
522 actually provides the same level of brightness (absolute power
output) and a bit more even illumination than does a T-32.
Comments anyone?
Chuck Norcutt
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