Re: [Leica] M6 Shutter accuracy?

[Dennis Painter <dpainter@bigfoot.com>]

Dan Post wrote:
> 
> Wayne-
> Focal plane shutters, like all objects, have a certain inertia, and will
> have to accelerate. The actual exposure is determined by the time the film
> is exposed- a focal plane shutter is essential a moving gap that travels
> from one side of the film to the other. since both will have to accelerate
> and brake equally to give an even exposure, photograph an evenly lit
> surface, and if you have a desitometer, check the density of both ends of
> the 36mm of the negative to see if one of the curtains are hanging.
> Dan

It doesn't work this way. The first curtain does indeed accelerate as it
crosses the film plane. If the travel speed of the curtains was
identical then the slit width would be constant and since the rate of
travel increases, the exposure would be shorter at the far end of the
curtain travel. (constant slit width moving at different speed across
the film)

What the Leica shutter does is control the rate of acceleration of the
2nd curtain such that the slit width widens as the curtains travel
across the film plane. Since the curtains are moving faster at the end
of the travel across the film, the slit must widen to allow an equal
amount of light to strike the film, portionate to the slower curtain
travel and more narrow slit at the beginning of the travel.

Dennis