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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