On 2015-10-30, nospam wrote:
In article , Eric Stevens
wrote:

Not a big problem, but the question is which camera has the correct
ISO setting, of if perhaps both cameras deviate from the true ISO
100.

While you may have a point, it would not affect me to know that the
ISO setting in my camera is 100 off or not. I know what I can
expect from a setting, and set the camera accordingly.

Things worked just as well in the days I set my exposure to the
Weston scale. The numbers don't really matter but when you have more
than one camera its nice if they share a common scale.

exactly the point.

Using film, you could use the same batch of the same brand in all your
cameras - if you were worried enough about the actual 'speed' you'd
shoot a test strip for each batch of film to determine the best 'speed'
setting for that batch for you. You'd also be aware of differences
between the cameras and lenses - and adjust shutter and aperture
settings accordingly, or the film speed setting if the camera had its
own 'automatic' exposure meter.

Electronic sensors generally aren't interchanged between cameras, and
their performance is remarkably stable over time, but they do vary one
from another, and of course cameras and lenses still do so too. So
you'll get to know that 'this camera tends to blow the highlights' or
'this lens is a bit dark at short focal lengths' or 'the fast shutter
speeds tend to underexpose' and compensate to get the results you want.

ISO 'speed' ratings are guides rather than absolute measurements. For
electronic sensors there are at least three different methods of deciding
what the 'speed' of a sensor is, and all allow for some subjective
judgment. Wikipedia is a good place to start reading.

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