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[OM] OM 40 & SCA 300 Flash

Subject: [OM] OM 40 & SCA 300 Flash
From: Hughes <hi100@xxxxxxxxxxxxx>
Date: Mon, 17 Mar 2003 02:54:05 -0800 (PST)
Cc: angel.lobo@xxxxxxxxxx
Angel and others discussed the question of third party flashes and how well 
they emulate Tseries OM flashes:

>>I don't have an OM-40, but I've seen differences between my Metz 40 MZ-3i
(with SCA 321) and the Olympus T-20: with both OM-4 and OM-2S, the little
arrows beside the '60' on the viewfinder's LCD show up just after
connecting the T-20 (even while charging); but the Metz doesn't show the
arrows until the flash is fully charged (and the LED is turned on).

With the OM-2S the LED flashes to confirm correct exposure, with both the
T-20 and the SCA (haven't tried the OM-4 -- there's film in it).

<<

I don't know about the OM40/PC and third party flashes but these comments apply 
to 3rd party flash interfaces:

Unfortunately the third party flashes often do not seem to completely emulate 
the Tseries interface which is surprisingly complex. In some cases the later OM 
bodies can make up for this and in some cases some functionality is lost. 

I have just finished modifying an OM SCA adapter for a Nissin flash so it is 
fresh in my mind.

The difficult issue for third party vendors is the LED flash ready, display 
line from the flash. This must produce a complex series of different currents 
at  different times to fully implement the Tseries interface :

1) Whenever the flash is switched on it must put a current of at least 15uA 
through the flash ready LED line. This is to identify that a flash is connected 
and is ON, so the camera can switch to 1/60 sec.  (If not implemented by 3rd 
party flash adapter then the camera will not switch to 1/60 automatically until 
the flash is fully charged and puts a still larger current through the LED 
line. This is strictly true only for  OM's after OM2. since OM2 does not 
implement this,Also implementation in OM2N is a bit different from say OM2S )

2) When the flash is fully charged it should raise this current to between 
350uA-2.5mA thereby lighting the flash ready LED in the viewfinder. 3rd party 
flashes do this as a minimum.

3) After the flash is triggered it must momentarily reduce the current to the 
LED line to zero to tell the camera it was triggered. In the OM2N this is used 
to limit the flash exposure time so it does not continue timing in the case of 
inadequate light being recieved from the film plane. It does this by switching 
the camera to Off mode! where the timing is automatically limited. If light 
from the film plane was adequate then this is unecessary as the shutter will 
close normally because of adequate integrated light.  In OM2 if course, this 
drop to 0 current has no effect and it continues integrating light if flash was 
inadequate.

4) In case of adequate flash exposure where the flash ready LED must be 
flickered on for a short period the current must first drop to zero as 
described above and then switch between  the ~40uA and  ~2.5mA levels 
repeatedly. It then must stay low ~40uA) until the flash is recharged, when it 
goes back up to 350uA-2.5mA to switch on the ready LED. Even if the flash does 
not implement the flicker mode some OM cameras will still flicker the LED in 
the display when the camera got sufficient light. This is true for my older 
OM2S for example which does flicker the LED despite the no flicker 
implementation in the Nissin SCA adapter. 

An arcane question: what is the difference if any between early OM2S and late 
OM2S where LED is powered by flash rather than camera when used with these 3rd 
party flash interfaces? 

A nice feature which can make up for the lack of full implementation is the 
beep mode which can be enabled on some Nissin and Metz flashes. This beeps 
discreetly when adequate light or a quench signal was recieved by the flash. 
This means you don't need to watch for the flashing LED. I actually prefer this 
mode of operation for most purposes as I sometimes miss the flashing indicator.

Regards,

Tim Hughes

 
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