On 2015-10-30 12:38, Whisky-dave wrote:
On Friday, 30 October 2015 16:06:15 UTC, nospam wrote:
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.

but batches of sensors are differnt too, which is whwere teh original
Q came from in that two cameras set set to ISO 100 should expose at
exactly the same level of brightness, but they don't.

If they're the same model camera I'd expect them to be too close for
casual measurement no matter what the batch. (Because I'd expect the
manufacturer to store calibration data on the sensor).

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

doesn't help if you have more than one camera though.

It's not so big a difference that you can't work them both. You have to
know your cameras well. This is the same across my Hasselblad lenses
.... on some the timer is a bit long (1/3 stop) which doesn't sound like
much unless shooting E-6.




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.

So hom come the OP was asking why his ISO one one camera is difernt
from teh other. You'd have thopugh with 10 bit minium resolution they
could get it right wouldn't you.

For a given model they should be too close for casual measure. And in a
precise test I'd assume far less than 1/3 stop.



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.

So why have it ? Are you expecting the IOS to be measured at 5600k or
5400k or 3200k or anything else, or perhaps red, blue, green, or
yellow magenta or cyan light would be best. Moonlight might be OK too
for hi ISO.

The standard sets an objective. Engineers at each co. interpret it as
they see fit.





it's at best, a rough guide.

Yep. I remebr on boxes of film the sped was always given a s a guide
rathe rthan a dead cert, a bit like setting a gues make or temerature
on an oven. Yes I've even doen that and if I set my oven to 180C I
can get temeratures of 165-200.

Oven controllers are not very accurate - I agree with the range you show
there.