[Leica] shutterspeeds

[Erwin Puts <imxputs@knoware.nl>]

To understand the correct measurement of the shutter speed and its 
interpretation, two different aspects must be considered. The 
following is for speeds higher than 1/50! First is the fact that any 
horizontally running focal plane shutter starts its travel with a 
slower speed than the nominal one(the shutter tapes must accelerate, 
when the springs are released to activate the shutter tapes) and end 
the travel with a much higher speed. The Leica shutter compensates 
for this natural behavior to have drums with a different diameter, so 
that in the beginning of the travel the slower nominal speed is 
accounted for with a narrower slit of the shutter tapes and at the 
end, the slit is opened up as the speed increases. The correct 
working and adjustment of this mechanism is of overriding importance 
and anyone looking at the shutter must ascertain that the variation 
in blind velocity on acceleration is compensated for correctly. Leica 
official repairshops have a device to do this. Then when this is 
accounted for the nominal shutter speed can be tested. As you know by 
now the speed of the slit is not a constant, so it is very important 
where you do your measurement,that is at which point of the slit 
traveling alongside the negative area. Leica chooses the position in 
the middle, but this is only correct IF the compensation mechanism is 
adjusted correctly.
The Leica factory uses a device with three simultaneous measurements 
at the beginning, middle and end of the shutter travel. And then uses 
an interpretation chart to adjust the settings. It is not so very 
easy.
A measurement with a digital device might be dangerous as it is not 
certain where the measurement takes place during the shutter travel 
and if this is known, is it the correct position?
As we are talking here about very slight tolerances, any deviation 
however small, will have its effect on the results and the 
interpretation thereof.
So the results of the digital test should be reviewed with care and 
without the knowledge that this particular shutter has the correct 
working of the adjustment mechanism and without knowing at what 
position the measurement spot does measure the shutter speed, it is 
difficult to interpret the results.
A variation of 1/6 to 1/3 of a stop for the Leica shutter is in fact 
very good.  I have reported about shutter speeds on my site where the 
tolerance of the shutter is discussed and the adjustments that are 
possible are noted, including the technique of adjusting for a lower 
actual speed of 1/1000 to be on the best side for noise, accuracy and 
consistency of travel.

On a more practical level, it may be noted that many Leica shutters 
after a year or more of heavy duty usage will show a top speed of 
about 1/700 to 1/750, which is still being within Leica tolerance of 
40%.
1/700 = 14 milliseconds
1/1000 = 10 milliseconds
More important is the constancy of travel, which can be tested very 
easily by taking a few pictures from an evenly lit  white or grey 
area, or a blue sky. If the pictures show some darker or lighter 
stripes at either side, the shutter is in for adjustment.

Slower speeds are only governed by the correct tensioning and should 
be less prone to variation. An interesting additional fact of focal 
plane shutters is the relation between slit width and exposure 
efficiency. That is the real slit width necessary for a certain 
exposure time. The lens diameter partly determines this efficiency 
and when the aperture is large the efficiency is lower: at an 
aperture of 1.4 the efficiency is about 70% and 90% when stopped down 
to 8.0. So underexposure when using wider apertures might be partly 
caused by the intrinsic characteristics of the shutter. A 1/3 stop 
under exposure at full aperture, where most people will underexposure 
anyway, will add to the total underexposure. So the lesson here is 
not to underexpose when using wide apertures. The focal plane shutter 
is indeed an intriguing mechanism.

Erwin