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Re: [OM] TTL Fooling circuit

Subject: Re: [OM] TTL Fooling circuit
From: HI100@xxxxxxx
Date: Thu, 8 Jun 2000 05:01:26 EDT
abutler@xxxxxxxxx writes:

<< 
While scanning the rec.35mm.whatever newsgroup somebody alluded to a
 TTL Fooling circuit.  This would be a means of changing the flash ratio
 but still use the TTL function.  Has anyone (Tim maybe) ever heard of or
 have an idea what it would take to make one for the Olympus TTL?  My
 thought was to insert it into a T-cord.  What do you think? 
>>
Mike,
     I have not seen commercial units like this although we had some 
discussions along similar lines to this, for making second curtain sync etc 
before on this list.There is a commercial timing unit for fudging second 
curtain sync.

     Without modifying the internals of your OM you could mess with the 
timing of the "quench" (stop) pulse sent to the flash. This would increase 
the duration and hence the amount of light contributed by the flash. This is 
not very flexible:
1 ) To avoid overexposure you would then need to set a faster film speed. 
2 ) choose an aperture that gave an indicated shutter speed of say for 
example 1/30 (corresponding to a fill factor of 1:1)
3) Your timing circuit would measure the time from the flash trigger to 
camera flash quench signal and then delay the time some 0r multiple of the 
measured time before it passed the quench trigger to the actual flash . 
(Alternately it could measure the actual light flash duration with a 
photocell)
This lengthened timing could be done either with an analog or digital timing 
circuit.

     This has anumber of drawbacks: The fill factor is not very well 
controlled since it depends on the difference between displayed viewfinder 
reading and actual OTF measurement. The new ASA setting is painful. It is 
hard to make the fill amount a small 0art of the overall exposure.

     The difficulty is that there is no way of your circuit knowing the 
relative contribution of flash to ambient at the time the quench signal 
arrives. You either have to do a calculation in your head based on the 
shutter speed indicated in the display for that aperture or you would need to 
have a dial to transfer that information to your timer so that it could 
factor it into a fill ratio.

      If you made youir circuit more elaborate you might be able to get a 
better result but it would not be TTL although it might be closer to what you 
want: 
You would need to delay the flash trigger pulse from the camera by say a 
millisecond before you triggered the flash. During that time you would 
measure the normal reflected light from the scene. Then the circuit would 
trigger the flash and again measure the reflected flash light. When it 
reached some preset 0r multiple of the light measured previously without 
flash it would send out the quench signal. For a 1/60 shutter speed this 
would guarantee a certain light fill ratio even though the measurement was 
for a shorter duration (1ms).You would still need to choose an aperture to 
get a displayed shutter speed of about the right value for a particular fill. 
You could put a shutter speed target for each fill ratio on the devices fill 
ratio dial. Example 1:1 would require about 1/30sec as before.  Greater fill 
would require longer displayed shutter times and less fill shorter times. 
However, now these displayed shutter speeds do not affect the fill ratio but 
only the overall exposure. Note in this scheme the shutter speed is always 
1/60 but you use the viewfinder display to check the overall exposure is 
going to be about right for the fill ratio chosen. This should work for the 
higher end OM's which fix the shutter to 1/60 when T series current passing 
on the quench pin is detected. For OM2 (not 2N) the shutter might continue 
timing after the flash finished hence changing the fill ratio a bit. This 
does make it more like TTL again, although with less controlled fill ratio.

           Just my thoughts,

               Regards,
               Tim Hughes
           >>Hi100@xxxxxxx<<

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