Digital EOS ISO vs Film ISO

[DocFrankenstein]

I'm just wondering. Does anybody know the relationship between them?

[daveh]

=

[alan sh]

Incestuous ?

Seriously - what do you want to know

Alan

[Jesper]

They should be the same.

For a certain light level, you'll get the same exposure setting (combination of shutter speed and aperture) when you're using a film camera with ISO 100 film as when you set your DSLR to ISO 100.

[slin100]

The relationship is not quite that straightford. The problem is that the determination of a film's ISO rating is based upon the amount of light necessary to produce a minimum density (0.1) that is generally agreed to produce resolvable detail in the shadows. Digital sensors, being linear devices, don't exhibit the same kind of "toe" characteristics of film. As a result, manufacturers have had quite a bit of leeway in assigning ISO ratings to their sensors. You may very well find that ISO 100 corresponds to two very different sensitivities between two manufacturers.

There's a proposed standard for determining the ISO rating of a digital sensor, called ISO 12232. But even this document details two different ways to determine a sensor's rating. The first is the maximum exposure level that avoids clipping of the highlights. The second is the minimum exposure level that results in a certain signal to noise ratio. Taken from Usenet, an excerpt of a draft of the standard:

With appropriate electrical or digital gain, a DSC can provide an
appropriate output signal level for a range of sensor exposure levels.
The maximum exposure level is the exposure level where typical picture
highlights will be clipped as a result of saturating the image sensor
signal capacity or reaching the camera signal processing maximum
signal level. The minimum exposure level depends on the amount of
noise that can be tolerated in the image. These situations lead to two
different types of speed values, saturation signal based values, and
noise based values. The ISO speed is preferably determined using a
noise-based method. The saturation-based value is preferably used to
indicate the camera's overexposure speed latitude. A second
noise-based value is preferably used to indicate the camera's
underexposure speed latitude. For some types of DSCs, such as those
employing lossy compression methods that significantly affect measued
image noise values and cannot be bypassed, it is not possible to
correctly determine the noise based ISO speed. In such cases, the ISO
speed of the camera is determined using the saturation-based
measurement, and the ISO speed latitude values are not reported. In
other cases, the noise-based ISO speed may be lower than the
saturation-based speed, in which case the saturation based-speed is
reported.
I suppose the second method could in some way correspond to the method used for film. More from the standard:

The ISO speed ratings described in this standard are intended to
harmonize with film ISO speed ratings. However,there are differences
between electronic and film-based imaging systems that preclude exact
equivalency. DSCs can include variable gain, and can provide digital
processing after the image data has been captured, enabling desired
tone reproduction to be achieved over a range of camera exposures. It
is therefore possible for DSCs to have a range of speed ratings.
DSC stands for Digital Still Camera, btw.

[photofinish]

You might want to check this out for a little more info, too:
http://www.adobe.com/products/photoshop/pdfs/linear_gamma.pdf

It is very enlightening! :wink: