This is hoary old subject related to not only the test conditions used for the
GN rating, (they assume a smallish room with reflective walls) but also if you
are foolishly(?) wanting to cover a wide angle uniformly onto *a flat surface*.
(This is what people are doing when they try to test their flash by
photographing a blank wall say) There is then an inherent power law that varies
as something like cos^3 that is very,very punitive as you go wider. In some
ways it does not make a lot of sense to try to cover a flat surface uniformly
either, when using a direct on flash small reflector, as this would require
very nonuniform high power/steradian at edges, blowing out near objects at
edges of frame. You are better off doing room or ceiling bounce or using a
large area flash diffuser like a beauty dish, light box, umbrella etc. to
reduce cos^3 law effects.
> like the others they also fudge on the recycle time claiming 10 seconds
> from a full dump. At 10 seconds the flash is still 1/3
> stop away from max power and it's still 1/10 of a stop away after 20
> seconds.
Actually this varies a lot by vendor. In the Case of T32 that is true off the
internal batteries , however T32b is much more consistent if used off of the
bounce grip 2. This is not just battery size, the reason is the bounce grip
uses voltage feedback control of the output capacitor voltage, making flash
power much,much more consistent, even when the batteries wear down. Also the BG
starts chirping when it reaches correct voltage, so even when batteries get
older you still know how long to wait to reach full charge. Using a flashmeter
with the T32 on its own, still puts you somewhat at vagaries of how long you
leave T32 to charge again, after the test flash. Even old handle mount flashes
like Metz 45/60 etc or Sunpak 522,544 555 etc regulate the voltage to the
capacitor using feedback control, so these again charge faster (esp 6 cell
units) and work more consistently. Most of them also change their chirp to an
intermittent pulsing (like BG2)once they
reach full charge, giving nice audible feedback as to when they are fully
ready/reached full voltage.
Most modern flashes even smaller units, use voltage feedback control, not only
for flash consistency but for power saving, since the inverter is cut off at
full voltage and only cycles on periodically, for top up (so chirping). I have
a Nissen flash unit that eventually goes into a power save mode that stops
cycling the inverter after it has sat idle for a very long time. You then touch
the on/off switch to put it back into the cycling mode to top up. The unit's
self discharge rate is very low so it stays well charged for a very long time
even after it goes into the sleep mode. A rather nice feature.
Tim Hughes
--- On Sat, 11/8/08, usher99@xxxxxxx <usher99@xxxxxxx> wrote:
> From: usher99@xxxxxxx <usher99@xxxxxxx>
> Subject: [OM] Re: News "Flash"
> To: olympus@xxxxxxxxxx
> Date: Saturday, November 8, 2008, 8:20 PM
> More sound advice from Dr. Flash.
> Thanks,
> Mike
>
>
> Just a different way to fudge the numbers. Some won't
> notice that the
> spec is at ISO 200 rather than 100 and most won't know
> how to do the
> numbers to convert 160 at 200 to 113 at 100. But
> that's an easier
> conversion than trying to compare the T32's guide
> number of 104 at 24mm
> coverage angle to someone else's guide number given at
> 105mm coverage
> angle. It's especially difficult because Olympus also
> fudges the 24mm
> coverage number... it's really more like 28mm as there
> is already 1/2
> stop falloff at the edge of the field of a 28mm lens. They
> also just
> plain lie about the guide number. I've measured 4 T32s
> (all within 1/3
> stop of each other) and they average a guide number of 84.
> And, just
> like the others they also fudge on the recycle time
> claiming 10 seconds
> from a full dump. At 10 seconds the flash is still 1/3
> stop away from
> max power and it's still 1/10 of a stop away after 20
> seconds. Never
> believe a flash manufacturer's specs and always use a
> meter.
>
> Dr. Flash
>
>
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