In article ,
Whisky-dave 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.

different cameras have different sensors.

two copies of the same camera will have the same sensor. you'd need lab
instruments to find *any* difference, and if there is one, it would be
much smaller than the manufacturing tolerances of the shutter and
aperture. in other words, it can be ignored.

because of that, many photographers bought film in large quantities so
that they could calibrate their workflow for one batch.

Some of that was to save money and always have film in stock.
I used to buy 50ft reels of HP3 , 100ft reels of HP4.
I still have the bulk film loader somewhere.

I'm not to sure that had such a thing as workflows then.

of course they did.

they had to
freeze most of it so that it didn't expire before they had a chance to
use it. crazy!

No most didn't freeze it they kept it cool in the fridge.

that depends how long they're going to keep it.

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.

didn't need to as each was done individually and looked at.
It was only when machines started bulk processing was that needed.

it absolutely was needed.

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.

that's the whole point.

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.

because there's no standardized rating system.

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 ?

good question.

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.

it should be standardized.

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.

that's because film is not as good as digital.

technology advances. film is obsolete.