[Leica] Shutter speed accuracy

[Erwin Puts <imxputs@knoware.nl>]

Any electronically controlled shutter will beat any mechanically controlled
one by a large margin. The electronic shutter also keeps its accuracy over
a longer period of time. It is well known that the famous Nikon F shutter
after a year's hard work has a nominal 1/000 that is actually a 1/700 or
even less.
And leica mechanical shutters generally are not that much better after a
long working life.
When I checked (with the leica shutter speed control instrument) my new M6
HM the 1/1000 was =B125% off. The official leica norm for deviation at the
highest speed is more than 30%. The leica repairperson could easily tweak
any individual speed so accurately as to have zero deviation (even better
than Dominique's record). But I asked to set the 1/1000 at a conservative
1/900, so 10% off. Why?
Shutter bounce is less so the camera is more quiet and does not tremble.
The consistency of travel of the blinds is higher and when repeating shots
with the same shutter speed the consistency of the value is also higher. It
is not uncommon when doing speed checks to see the speed values change:
when you are firing the 1/1000 ten times rapidly one after the other the
values could be 1/950, 1/700. 1/800, 1/900 etc.
If you set the speed a little below maximum the consistency of the real
speed is higher.
Now is a deviation of 10% imporant, even when doing critical Black and
white work and keeping in mind that Leica optical quality needs correctly
exposed film.
A full stop exposure difference will result in a density change on the
negative of about (log) 0.21, assuming a contrast index of 0.7. A 10%
change will result in a difference of 0.02 density. That is even too small
to measure, let alone perceive. Now a 30% change results in a density
diffenence of =B10.07. That is visible, but just. It is not dramatical and I
really doubt if anyone, when looking at a 30x40 print can identify  two
small regions in the print with a differnence of D=3D0.07.
Now transparancy has a higher contrast and therefore less exposure
latitude. So 15% difference could be just noticeable. But even here a 30%
difference will give you only a slightly higher density.
When working on colour neg material even a full stop difference is of no
practical consequence.
So an accurate shutter is nice to have, but some tolerance (within the
confins of a mechanically governed shutter) are irrelevant for even the
most demanding work. Let us enjoy the sound, low noise and low vibration
and high consistency of the travel of the leica shutter.

Erwin