Re: filmscanners: Pixels per inch vs DPI

["Dave King" <kingphoto@mindspring.com>]

Harvey,

Sorry for the stupid question, but have you done this test in an
"effectively" dark room?  Perhaps you're seeing ambient light begin to
contribute to exposure?  For ambient light not to have any effect on
exposure it should be at least 5 stops below the working setting.

I thought the longest flash durations were in the neighborhood of
1/500th sec.  I don't recall seeing exposure differences at shutter
speeds 1/250 or slower where ambient light isn't a factor.

Dave

----- Original Message -----
From: SKID Photography <skid@bway.net>
To: <filmscanners@halftone.co.uk>
Sent: Wednesday, October 31, 2001 3:26 AM
Subject: Re: filmscanners: Pixels per inch vs DPI


> "B.Rumary" wrote:
>
> > 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.
> > >
> > It _is_ related to the synch speed, because electronic flash is so
fast that
> > it needs the entire image area exposed when the flash goes off. If
the camera
> > speed is set above the synch speed, then the "moving slit" effect
means that
> > only that portion of the film exposed by the "slit" at the moment
of flash
> > will get the benefits of the flash. The "flash-lighted" area will
then be
> > correctly exposed, while the non-lit area will be heavily
under-exposed.
> >
> > Note this only applies to electronic flash guns, which give very
short
> > duration flashes - typically 1/30,000 sec. The old fashioned flash
bulbs
> > "burn" much more slowly and give light for long enough for the
"slit" to do
> > it's full run across the film.
>
> I think you will find that very few, if any, flashes are of such a
short duration.  It has been my experience
> that the difference between, a 250th, 125th and 60th of a second
exposure and almost any brand electronic
> flash will yield very different film exposures, no matter what type
of shutter you are using.
>
>
> Harvey Ferdschneiderpartne
> partner, SKID photography, NYC
>
>
>
>