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Old October 30th 15, 05:06 PM posted to rec.photo.digital
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Default How to measure ISO

In article ,
Whiskers 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.


one of the downsides of film is that every batch is different, one of
numerous flaws that digital solves.

because of that, many photographers bought film in large quantities so
that they could calibrate their workflow for one batch. they had to
freeze most of it so that it didn't expire before they had a chance to
use it. crazy!

they often took test shots, usually a macbeth chart or similar, so that
they could adjust the exposure and colour balance to match their
previous batch of film.

fortunately that insanity is no more, as digital is consistent shot to
shot for the life of the camera.

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.


that's what having a standardized rating is supposed to solve.

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.


the fact that there's more than one way to measure it makes the rating
rather useless.

it's at best, a rough guide.