"Base ISO" for Sigma DP2M (and others)

robertophoto wrote:

I have read that the base ISO is 200, meaning that setting then ISO at 100 is equivalent to using a neutral density 1-stop filter, which one would only do if a slower shutter speed were desired.

First of all, is this true, and how do we know?

Second, is this what owners use as their basic good-light setting, 200?

Third, can someone tell me what the base ISO for my DP1s is, given that it allows a setting of 50 for the lowest ISO?

And finally, has anyone succeeded in seeing for themselves that the noise content of images taken at ISO 200 on the DP2M is as low as it gets?

ISO (the Organization) says:

"6.2 Saturation-based speed
In photographic applications where the scene illumination level can be controlled, for example in studio photography, the photographer normally prefers to use a camera exposure index which provides the best possible image quality. In this situation, a saturation signal-based rating is appropriate. This rating allows the user to set the camera exposure so that typical image highlights are just below the maximum possible (saturation) camera signal value.

6.2.1 Focal plane measurement
The saturation based speed, Ssat, of an electronic still picture camera is defined as:
Ssat = 78/Hsat (5),
where Hsat is the minimum focal plane exposure, expressed in lux-seconds (lx⋅s), that produces the maximum valid (not clipped or bloomed) camera output signal.

NOTE Equation (5) provides 1/2 “stop" of headroom (41 % additional headroom) for specular highlights above the signal level that would be obtained from a theoretical 100 % reflectance object in the scene, so that a theoretical 141 % reflectance object in the scene would produce a focal plane exposure of Hsat."

Although the Standard offers several methods of determination of  the 'ISO', this one is the most applicable for determining the so-called 'base ISO'.

I happen to know that Hsat for one of my Sigma DSLRs is 0.8 lux-sec. That makes it's 'base' ISO equal to 78/0.8 = 97.5 as near to 100 ISO as is reasonable - bearing in mind that the Standard offers considerable latitude between actual and published ISO values for digital cameras.

With it's smaller pixels, a saturation-based 'base ISO' of 200 is not unreasonable for a Merrill. If an ISO of 100 is recommended for a Merrill, it is likely that Sigma has used one of the other available methods.