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Re: [OM] Hep! Dr. Flash needs... um, flash help

Subject: Re: [OM] Hep! Dr. Flash needs... um, flash help
From: <chucknorcutt@xxxxxxxxxxxxxxxx>
Date: Mon, 29 Dec 2008 01:29:43 +0000
If the camera believes, at the start of exposure, that the exposure time will 
be faster than 1/60 second the flash will not be fired and the second curtain 
will be started on its way before the first curtain is fully open.  If the 
camera believes, at the start of exposure, that the exposure will be 1/60 
second or longer then it will wait for the first curtain to be fully opened and 
the flash will be fired.  It takes the first curtain 1/60 second to get across 
to the other side.  Even if the second curtain is released instantly the 
exposure will be 1/60 second.  It can't be faster.   If the flash doesn't fire 
or is underpowered then the camera (in auto mode) can extend the shutter time.  
But if the flash is overpowered there's nothing that the camera can do.  Even 
if it could move the shutter curtain faster it wouldn't help.  The exposure has 
already been made.

Dr. Flash


>  -------Original Message-------
>  From: usher99@xxxxxxx
>  Subject: Re: [OM] Hep! Dr. Flash needs... um, flash help
>  Sent: Dec 29 '08 00:50
>  
>  Hmmm,
>  
>  Now I don't just seem to get how the estimated 2nd curtain with the OTF
>  metering can't be faster than  1/60 sec and thus exceed the synch speed
>  if the camera doesn't know a flash is attached.  Say the flash is in
>  auto and the sensor sees a darker background and thus would overexpose
>  relative to the OTF metering---as the flash duration is very small
>  relative
>  to the exposure time, it would seem the 2nd curtain could be caused to
>  limit the exposure to something under 1/60th and thus lead to uneven
>  exposure of the frame.
>  Perhaps I just need to re-think about it after coffee in the AM.
>  Perhaps I'll  also regret this post and slap my forehead.
>  
>  A. Still Confused Mike
>  
>  (in review, OM2 in TTL auto, T32 in normal auto)
>  
>  
>  
>  
>  
>  >
>  > Correct for TTL mode where the camera is in control of the
>  > flash exposure.  For auto or manual flash where the only
>  > control available to the camera is the shutter speed you
>  > wouldn't get uneven exposure nor underexposure  but you could
>  > get overexposure.
>  >
>  
>  Well assuming a fully manual calculation of flash exposure or the flash
>  set
>  to "normal auto" with say an aperture of f8.  As long as the ambient
>  light
>  would lead to an exposure of > 1/60 (say (1/8), first curtain opens
>  fully,
>  flash fires, OTF metering sets the second curtain on it's way, the
>  resultant
>  exposure is probably about 1/60 f8.  If the flash is giving less light
>  than
>  expected then the OTF metering just holds back the second curtain until
>  the
>  ambient makes up the difference.  You may get overexposure if your
>  manual
>  flash calculation isn't correct or if the "normal auto" mode gives more
>  light than it should.
>  
>  ...Wayne
>  
>  --
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>  
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