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Re: [OM] F280 Super FP revisited

Subject: Re: [OM] F280 Super FP revisited
From: "Tim Chakravorty" <suchismit@xxxxxxxxx>
Date: Mon, 9 Sep 2002 01:31:55 -0700
>Therefore, for an "FP" flash to work, it must always have 
> the same, long burn duration, regardless of shutter speed. 


I think you are assuming that the flash remains ON for the entire time it takes 
the slit to traverse the focal plane. That
may not be true. I remember reading somewhere that in super sync mode the flash 
output is not continuous but
a series of pulses. How many pulses would that be..  If we divide the length of 
the rectangular focal plane by the
width of the slit - that gives us the number of slits that can be fitted all at 
once on the plane. And since we want 
illumination in 'all of the slits' there will have to be so many flash pulses. 
In other words, the flash fires every time
the window moves a distance equal to its width. 


Now lets say the length of the focal plane is about 36mm. The max syn speed 
with a regular flash is 1/60 sec.
The window therefore moves at a constant speed of 36x60   ~ 216 cm/s . If the 
shutter speed is any higher than
1/60s the slit only narrows but still moves with the same speed. IMO a flash 
duration of 1/60 s is too long, they 
typically go from 1/500s and up, and is why its pulsed.

-Tim



----- Original Message ----- 
From: "John A. Lind" <jlind@xxxxxxxxxxxx>
To: <olympus@xxxxxxxxxxxxxxx>
Sent: Sunday, September 08, 2002 9:49 PM
Subject: Re: [OM] F280 Super FP revisited


> At 22:48 9/8/02, Pete wrote:
> 
> >I'm confused again.  If Super FP mode allows full synchronization at all 
> >speeds, then the flash duration should be dependent upon the shutter speed 
> >and not be a constant of "about 1/25 sec", no? I seem to remember a post 
> >(by John L.?) explaining that Super FP mode operates as follows: flash 
> >starts, shutter opens, shutter closes, flash stops.
> 
> Yes, that's the sequence.
> 
> >   If true, and I believe it is, then the flash duration will be the 
> > shutter speed plus the time before and after the shutter operates.  And, 
> > the longer the shutter opening, the less flash power that will be 
> > available, according to the charts.
> 
> No.  With focal plane shutters, total time it takes for a curtain to travel 
> across the film gate remains constant, regardless of shutter speed.  You 
> have two curtains, an opening one (leading) and a closing one 
> (trailing).  What varies with shutter speed is the delay between releasing 
> the opening one and releasing the closing one.  In other words, it's the 
> *gap* between them that changes.  Shutter speeds faster than the X-sync 
> speed have a gap that is narrower than the film gate.  At those speeds, the 
> closing curtain is released before the opening one has completely traveled 
> and the result is a slit moving across the film gate.  The faster the 
> shutter speed, the narrower the slit, but the slit itself *always* moves at 
> the same speed.  Therefore, for an "FP" flash to work, it must always have 
> the same, long burn duration, regardless of shutter speed.  And . . . the 
> narrower the slit traveling across the film gate, the less light from the 
> flash that's allowed to reach the film.
> 
> >Because of the radically reduced flash power as shutter speed
> >decreases, it seems that Super FP mode is *only* useful as a fill
> >flash, and nothing more.  No?
> 
> Super FP imposes noticeable limitations on working distance compared to 
> using the same film speed at X-sync with TTL Auto.  The faster the shutter 
> speed, the greater the limitations.
> 
> -- John
> 
> 
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