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Re: [OM] OM-4 w/ T Series Flash Confusion....

Subject: Re: [OM] OM-4 w/ T Series Flash Confusion....
From: Kenneth Sloan <sloan@xxxxxxxxxxx>
Date: Tue, 30 Mar 1999 12:06:43 -0600
> I'm confused.  From previous notes, I'm under the impression that my OM-4
> (with a T-32 or T-20 on it) will also include a slaved flash unit in it's
> Off the Film Plane exposure control (see Ken's note below).  In other words,
> the OM-4 measures the exposure needed - weighing all light coming through to
> the film plane; TTL sources and others - and terminates the shutter upon
> proper exposure.
> 
> If this is the case, what information is going between to the OM-4 camera
> and T series flash TTL capable flashes ?  Isn't the camera really
> controlling the amount of flash that's used ?  What is the TTL flash
> circuitry adding to the picture ?
> 
> My original goal - and original question to the group - was based on finding
> a way to have a second/third flash at angles to subjects to handle harsh
> shadows across faces and backgrounds.  Could I skip the expense of adding
> two more T-32s that are "TTL" and simply by two slave units and two cheaper
> flashes that would be positioned to the sides of the subject and that would
> simply go off when the camera mounted T-20 goes off ?

Take this advice with caution.  Correction welcome!

Think in terms of time.  Strobes strobe.  Shutters shut.  They act in
different time frames.

OTF shutter control works with continuous, or long lasting, light.  For
example, the sun, a flood light, or an F280 in Super FP mode.  Open the
shutter, measure the light, and close the shutter when you've seen
enough.  The limit is the speed with which you can close the shutter
(for an OM-4, this is the time between when the first curtain starts to
expose the film and the second curtain starts to hide the film again.)
Let's call this limit 1/2000 sec.  But note that it still takes 1/60sec
(or so) for the curtains to sweep across the film - so the light must be
constant during that 1/60sec (or so).

TTL flash control works with very fast, but controlled light (the
flash).  Open the shutter (all the way - to expose the entire frame at
once), turn on the flash, measure the light, turn off the light when
you've seen enough, close the shutter.  The shutter must be completely
open during the flash, but the flash duration can be as short as
1/50000sec or as "long" as 1/1000second.  The ambient, continuous light
should be insignificant compared to that provided by the flash (because
the shutter is open for 1/60sec).

A simple slaved unit is a completely dumb manual flash.  It sees the
main flash go off, and dumps it's entire store of energy into light
illuminating the scene.  One shot control.  No opportunity for control -
so it's best used for fill and secondary lighting.  Cheap simple slaves
work just like flood lights - their contribution to the exposure is
constant. Expensive radio controlled slaves (or multiple TTL controlled
flashes - not really slaves) behave just like ordinary TTL flash- there
are simply more of them.  Notice that TTL can't control the effect of a
simple slave - because it can't control the slave.  Notice that OTF
shutter control can't control the efect of a simple slave - because it
can't close the shutter fast enough - as far as OTF shutter control is
concerned, strobe output is "all or nothing".

If you want to do multiple flash, you can use either simple uncontrolled
slaves or TTL controlled flash - but you must understand how and when to
control them.  If the multiple flashes are all "primary" - then you need
to use TTL control on all of them.  The camera turns them all on, and
off, at the same time - based on the light reaching the camera.  On the
other hand, it is common to use some of the multiple flashes as
secondary, fill light sources.  These can all be dumb slaves.  Your
camera can't control them - but the photographer can control them by
placement and power.  As long as the primary flash is TTL controlled,
you are OK.  Again - all of this happens with the shutter wide open.

So...treat secondary simple slaves like flood lights (except for the
requirement that the shutter be completely open when they fire).  Use
TTL control for the primary flash - and make sure that the primary flash
is the PRIMARY source of light in the scene.

Donning my flame-proof suit, and awaiting corrections...

-- 
Kenneth Sloan                                            sloan@xxxxxxx
Computer and Information Sciences                       (205) 934-2213
University of Alabama at Birmingham                 FAX (205) 934-5473
Birmingham, AL 35294-1170   http://www.cis.uab.edu/info/faculty/sloan/

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