I have a Sekonic L-358 flash meter which has the ability to tell you
what percentage of an exposure is due to the flash and what is due to
the ambient.
Today, when I was testing the previously mentioned Vivitar 285's I
noticed that one of the flashes was 1 stop lower (f/5.6) at max power
than what I had previously measured (f/8). I checked the batteries,
determined that they were a bit low and changed them for a new set with
expiration 2008. No change. I got an even newer set with expiration
2009. No change. Then I put the newest batteries into the other flash.
Same result. Both flashes were reporting 1 stop lower than what I had
previously measured them when in manual mode at full power.
Obviously, something else was going on. Unless I was willing to
hypothesize that both flashes had simultaneouly failed in the same
manner it had to be the meter. Then I noticed that the shutter speed on
the meter was set at 1/500th. I tried dropping the shutter speed to
1/250 and suddenly the missing stop returned. I was back to f/8 from
f/5.6. I dropped it successively down to 1/30th and reading held steady
at f/8 except that the effect of weak ambient light began to creep in as
the shutter speed grew longer and longer.
So, I learned two lessons today.
1) When at full power a Vivitar 285HV takes longer than 1/500th second
to do a full dump but not more than 1/250th. I'd have guessed the flash
was much faster than that but it is not. While I haven't tested them
yet I assume that this will also be found true for many other portable
flash units.
2) The flash meter is just like the camera with respect to shutter
speed. When set at 1/500th it means the flash is only measuring for
1/500th of a second. If the flash duration is longer than that it won't
see it. I had never stopped to think about it before and assumed all
electronic flash durations were in the range of, say, 1/10,000th of a
second or faster. I didn't thing it mattered what the meter's shutter
speed setting was except in so far as to compute the flash/ambient ratio.
I has only taken me decades to learn this simple little tidbit so I
thought I'd share it and maybe save you 10 or 20 years to learn the same
thing. Treat the camera and meter the same with respect to sync speeds.
Chuck Norcutt
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