Hertz,
without touching on other issues already mentioned by others here
are a few other issues that have some influence on your problem. Any digital
measuring system like the 2S or 4 has "quantization uncertainty" of the
analog to digital conversion process. That is, there are only a fixed number
of different values that the micro-processor can measure due to the digital
nature of the signals convertion process. In addition there are errors in the
mechanical coupling parts so that the same setting when repeated is not quite
the same. There is also display resolution uncertainty (the display on the 2S
only has bars every 1/3 stop, I believe). Thus there could be an apparent
error of about 1/3 stop just from display resolution. For continuously
variable resistors (shutter speed ,ASA and aperture) there is a resolution
limit so that you cannot set all values (sort of like a switch with a lot of
positions,not infinite number) and worse turning the dial in one direction
and stopping in one spot does not necessarily produce the same value as
turning in the opposite direction and stopping on the same spot. (mechanical
hysteresis) Also to stop the display flickering a lot between one value and
the next designers often add a tiny amount of digital hystersis to the
displayed values. All these issues make comparison testing harder than you
might think between different digital display cameras under "the same
conditions".
Ultimately the manufacturer makes an overall accuracy specification for
the camera meter system. For example as far as I can remember the service
manual for the OM2N and OM1 specifies an overall absolute accuracy of about
0.6 Stop and a repeatability of 0.3Stop at any one setting. Typically what a
good manufacturer will try to do is ship the camera new with a much tighter
spec to allow the camera to age gracefully and still meet spec. In a
manufacturing process this translates to having an internal final test spec
much tighter than the published spec. This is often done by implication
through a statistical design analysis and more limited testing. The allowable
field service spec may be much looser than the factory spec to account for
inferior test equipment and the lower average skill level in the field.
In tests I seem to remember that in fact the OM digital cameras rated at
or near the top of all cameras tested in exposure measurement accuracy in Pop
photo(?).
Bottom line: side by side comparisons are harder to do in a repeatable
way than you might think.
Tim Hughes
hi100@xxxxxxx
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