Re: [OM] new to the list/ precision of metering-dial 2S, 4 Ti

Subject: Re: [OM] new to the list/ precision of metering-dial 2S, 4 Ti
From: HI100@xxxxxxx
Date: Fri, 6 Aug 1999 02:58:37 EDT
         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 
    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 
    Bottom line: side by side comparisons are harder to do in a repeatable 
way than you might think.

Tim Hughes

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