Don Lashier said:
Unfortunately Canon, Nikon, and Sony don't appear to publish sensor specs so it's hard to compare with DSLRs.
Ah, but then there are pixel peepers (like me ;-), and others) who use alternate methods to empirically derive those specs ... !
Manufacturers cannot hide it, the truth is out ... ;-).
Most of the difficulty in uncovering that information reliably, is in systematically eliminating as many external sources of influence as possible. One obstacle is the Raw converter, which may also do all sorts of noise related stuff with the data. It is therefore helpful if the Raw sensor data after Analog to Digital Conversion (ADC) can be accessed.
The empirical determination is based on a range of exposures, and on measuring the signal and noise levels in the resulting data numbers (DN) of the Raw linear gamma files.
Part 1 would be a determination of the "read-noise" level at the shortest possible exposure time that the camera allows, while blocking any external light from contributing to the 'exposure' (use the lens cap, and cover the viewfinder).
The shortest possible exposure time will reduce the potential buildup of temperature related dark noise. All the noise we then detect in the resulting file is due to the electronics, and thus represents the lowest possible signal level in a given camera at that temperature and ISO (= amplification) setting.
Part 2 would be a determination of the actual saturation level of the sensels. By photographing a uniformly lit surface, and by keeping that surface out of focus to avoid surface structure influence, we can take a range of exposures with small increments in exposure time until the resulting DN no longer increases (the signal saturates the sensel's charge storage capacity).
I simply use a piece of opaline glass held flush with the filter threads of a lens which is pointed at a uniformly lit area or cloudy sky. I also use a medium range aperture and a modest tele lens to reduce the influence of vignetting, and only analyze a small area in the center of the image. That helps to get an as uniformly lit exposure area as possible.
There are several checks and balances that need to be built-in in (especially) the part 2 tests and evaluations (like subtracting 2 exposures for better statistical performance and elimination of sensor dust influence). You can find an example of such a workflow
here. Although it is not a medium/large format camera that is tested there, the principles apply equally to the Phase One's (and similar) of this world.
The easiest part is the Read-noise determination, which gives the noise floor for all other Dynamic Range calculations. That too will benefit from subtracting 2 equal exposures at each ISO setting one would like to test, because it can reduce the influence of e.g. hot pixels.
Try it, it'll be fun to build a better understanding of real world limitations. It may require a Raw converter like DCRAW to extract the linear gamma data before demosaicing and white balancing, but that is a hurdle that can be taken once the double Read-noise shots are taken. If there is broader interest in this fundamental evaluation for various cameras, we could start a new thread "Dynamic Range evaluation" in a more appropriate place, like for the particular camera models.
Bart