Re: filmscanners: Pixels per inch vs DPI

[Arthur Entlich <artistic@ampsc.com>]

Austin Franklin wrote:

  >>As many people probably realize, in a typical rear curtain/focal plane
  >>film cameras (as most 35mm SLRs are), any shutter speed beyond the
  >>maximum flash synch shutter speed exposes the film via a moving slit
  >>opening between the shutter curtains.
  >>
  >
  > I know what you say CAN be certainly true for the highest speeds of some
  > cameras, but I did not know it was specifically related to the synch
  > speed...I believe it's more related to shutter design than
specifically tied
  > to sync speed.  Would you mind citing a source for that information?
  >
  > That is certainly not the case with vertical shutters, which all but
one of
  > my 35mm cameras have (Contaxes and Nikons), the exception being my
Leica M.
  >


As a result of the continuing and escalating acrimony between Austin and
myself, and his incessant nitpicking of my postings, I do not intend to
respond directly either publicly or privately to his postings in the
future.  I bring this to the attention of the other members so that you
understand that my silence to Austin's challenges is not necessarily
because I am unable to defend my position on either technical or other
merits, but because I simply have decided his challenges are not worth
my time to pursue.

Further, the issue he has brought up to question above was an aside and
tangential to the main point I was making in my post, that of the
possibility of a design using one or more moving tri-line CCD arrays,
across an aperture, if CCDs sampling response time was improved.

However, since the question of why focal plane shutters have limited
flash synch speeds might be of further interest to others, I provide the
following expanded information for their edification.

Austin, have a ball, nitpick at it as much as you like.

Most camera mounted electronic flashes typically operate at between
1/1000th and 1/10-50,000 of a second, in fact most flashes operate in 
that range, or above, which is well above flash synch speeds on focal 
plane shutter cameras. The sequence of events is the shutter curtain 
opens fully, then sometime during the fully open shutter period, usually 
very nearly after the first shutter curtain is fully open, the flash 
goes off.  With some flash systems you can adjust the flash to go off 
just before the second curtain starts closing, which can be useful for 
some effects involving movement.

The limitation in flash synch speed is that the shutter opening has to
be complete when the flash goes off, since the flash lighting only lasts
a small fraction of the total exposure time.  This is also why ghosting
occurs when there is ambient light.  In most cameras, including vertical
shutters, once you get above the flash synch speed, there is either not
enough time, or no time that the shutter curtain is fully open before
the second shutter is beginning to follow.  That is also why faster
shutters can have faster flash synch speeds because they can have more
"open" time before the shutter has to begin travel to close.

As the shutter speed is increased, the opening between the first shutter
curtain and the second decreases.  So if a camera has a flash synch of
1/250 sec. and you try to use 1/500 sec, you will find that the flash
will have gone off as the first curtain has fully opened, but by that
time the second curtain will have already begun its travel, and you will
get part of the frame missing flash lighting.

There is probably one speed, or perhaps even two, above the flash synch
speed where the shutter might actually be open fully, but it is too
short a time to allow for the electronic flash to go off and finish its
flash duration before the second curtain starts its movement.  So,
"factually" it might be possible that the shutter remains open fully on
one or more further speeds beyond maximum flash synch, but not long
enough to accomplish the necessary steps to complete a flash lighting
before the second curtain begins its travel.

I can think of no advantage for a camera to have a slower maximum flash
synch speed offered than the shutter is capable of providing, so I can't
see why any manufacturer would do so unless they manufactured a camera
which  had unreliable shutter travel.

Art