How much effect does that have on comparing cameras? For example, if camera A is touted as great for low light because it is usable at ISO 12,800 could this be because it is actually equivalent to camera B at 6,400?
Hi,
It's anybody's guess what that means, until one calibrates for actual Raw file data levels.
Really, it's very simple, it can also be done with Fast RAW Viewer (FRV) automatically if one doesn't have RawDigger or similar. With FRV, just set the preferences to "ETTR style Autoexposure (shift histogram to the right)", and set the "Saturate up to" option to 0.0% pixels. Now shoot a uniform surface (well lit gray card, or as I do shoot through a piece of opal glass or white translucent perspex flush with the filter threads of the lens) with the camera on Aperture priority metering, and set that aperture to something like f/8 (or narrower).
FRV will tell you after Auto Exposure (Shift A), how many EVs the Raw exposure levels are removed from clipping the Raw data. To reduce the Photon shot-noise tail from clipping, you can also use an exposure correction (e.g. +2EV) on the Aperture priority, and add that to the remaining over exposure latitude.
I have a series of Flat-frames for several of my lenses on my EOS 1Ds Mark III, and I've exposed them at +2 1/3rd EV through a sheet of opal glass. At ISO 100 FRV gives me the following remaining latitudes; +0.82 EV, +1.08 EV, +1.09 EV, +1.02 EV, +1.02 EV, +0.98 EV, etc.
Depending on the camera used, at higher ISOs you may get other values, just try it for your camera.
The variation is due to random noise, slight variations in exposure time due to daylight changing after metering and before exposing, and irregularities in aperture blades and shutter curtains closing to the same opening. But on average it shows a 1 stop latitude in addition to the 2 1/3rd EV I pushed the exposure over the measured exposure level. So on average a total of 3 1/3rd EV above medium gray measurement. That was already what Rawdigger had told me, but now it's calculated for me.
Cheers,
Bart