Dr. Flash didn't have his head screwed on correctly for FP mode. The
sync speed rule only pertains to electronic flash due to the extremely
short burn time. For FP, the burn duration need only be as long as the
shutter speed plus some slop to get the bulb lit and the light level
stable.
In rethinking it I do think auto mode should work provided the light
level is stable during and just ahead of the exposure. For shutter
speeds faster than the sync speed the auto exposure needs to be read
from the shutter curtains ahead of the opening of the shutter (thus the
need for stable light level ahead of shutter release). The reason it's
required is that the shutter curtains are not completely open at speeds
faster than the sync speed and only form a moving slit across the film.
If the camera was to try to vary the speed during the exposure parts
of the frame would be exposed differently. For speeds at the sync speed
the shutter curtains are completely open. The camera then has time to
integrate the OTF reading and decide when to close the second curtain.
I suspect this might not work exactly at the sync speed. It might need
something a bit slower to be able to exercise some control.
So, as Jeff suggests, I think it really should operate like a high power
FP mode electronic flash... especially since that's exactly what the FP
mode electronic flash is trying to emulate. The only thing that bothers
me about this is that the OM-2 manual is clearly saying to use the
camera in manual mode for FP type bulbs. Doesn't yet make complete
sense to me. Maybe they're suggesting manual mode based on the diverse
characteristics of different classes of FP bulbs???
Dr. Flash
On 4/16/2011 10:25 PM, usher99@xxxxxxx wrote:
> Hmm,
> I am not quite following why the burn duration has to exceed the sync
> speed. With the big guys I will be throwing away most of the light and
> the duration is plenty long. It will be a bit tighter for the #6's
> but still is plenty long even after the 10-15msec delay in FP mode.
> The link to the pdf version of the graphs that is much clearer but is
> not working.
>
> I dug up John's post and he thought there may be some moderate
> variability among cams with that delay.
> So if the exposure is proper at say 1/125th, why won't the TTL metering
> just close the shutter? I have no specific knowledge if the cam
> electronics are such that it detects
> something hooked up to the pc socket, but I bet it doesn't know. I
> shoot one in manual just in case especially as Dr. Flash has
> reservations.
>
> Student of Dr. Flash, Mike
>
>
>
>
>
> The numbers on the chart are too tiny for me to read them easily but I
> do note that the time scale is in milliseconds and stretches over many
> of them whatever the actual scale. A small portable electronic flash
> may have a flash duration as little as a 1/50 of a millisecond or even
> shorter. A big studio electronic flash might have a duration as long as
> 1 millisecond. All much, much too fast for the camera to handle in its
> normal auto mode.
>
> In order for TTL FP mode flash to work in auto exposure mode the flash
> duration must be at least as long as the flash sync speed. For an OM
> body that's at least 1/60 second or 17 milliseconds. If you look at the
> ramp up/ramp down curves on those FP graphs you can see that getting
> some stable light will take a while which is why the FP sync starts
> ignition of the bulb before the shutter opens. Some additional timing
> slop would probably also be desirable so I think I'd like to see a burn
> time of 35 milliseconds or more. Even so I'm still not sure it will
> work.
>
> Dr. Flash
>
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