Ray, read DXO’s description of how they make those so-called ISO speed measurements and get back to me. You will see that the “lower than stated” values have nothing to do with the sensor being given less exposure (which would involve the camera using a higher shutter speed than selected!). It is instead based on measurements of raw file data after any gain (analog or digital) applied in pruducing those raw files. So you (and perhaps DXO) are ignoring the issue of how much gain is applied when at a high EI setting. Clearly, and for good reasons, most or all camera makers choose a gain level that places metered midtones below the highest level permitted by the ISO standard, for the sake of reducing the risk of amplifying highlights into clipping that were not blown out in the photo sites. Conversely, it would be folly for a camera to give less exposure than indicated by the EI setting, because it make the camera look worse in testing of noise levels.
BJL,
One of us seems very confused here. Perhaps it's me. As I understand, it's the internal processes of the camera that apply the gain at higher ISOs, in accordance with the manufacturer's design and decisions.
We don't have a choice in the matter. Nor does DXO.Pixel saturation, in at least a part of the sensor, is a required starting point to measure DR. At higher than base ISO, pixel saturation is achieved through in-camera amplification, not through the absorption of too many photons. This is why ISO-less, or ISO invariant cameras have an advantage. At fast shutter speeds, when one might normally raise the ISO setting, one can avoid using the camera's in-built amplification system, and do one's own amplification in Photoshop, thus removing the risk of blowing highlights, yet still maintain the same image quality as a correctly exposed shot at the usual higher ISO setting.
Unfortunately, Nikon DSLRs are not exactly ISO-invariant. There's usually a slight advantage in using the camera's in-built amplification, especially between base ISO and ISO 800. For example, if a camera is truly ISO-invariant then the drop in DR should be 3 EV when one uses ISO 800 instead of underexposing 3 stops at ISO 100.
However, with the D850, using the camera's amplification by choosing ISO 800, instead of using the same exposure at ISO 100, results in a drop in DR of only 2 stops. One gains a full stop of DR by using the ISO 800 setting on the D850. I find this type of information one can glean from the DXOMark graphs very useful in a practical sense.
Whilst many manufacturers over state their ISO settings, in relation to the ISO standards, most of them over state them by only 1/3rd to 2/3rds of a stop. Olympus over states the ISO sensitivity of the E-M1 MkII by over one full stop, approximately 1 & 1/4th of a stop. I see an element of marketing salesmanship here. By doing this the manufacturer is basically stating that their camera has an elevated ISO range at both the high end and low end, from ISO 200 to ISO 25,600. The actual range, according to the ISO standards, is from ISO 83 to ISO 10,916, approximately equal to ISO 100 to 12,800.
Do you see the deception? I've used cameras that have a base ISO of 200, such as the Nikon D700 full-frame, although the DXO-measured ISO was 158 which is approximately 1/3rd of a stop down, as is the case with most cameras (except Olympus, apparently).
I've found such cameras to be very advantageous for hand-held shots because at a real ISO of 158 (as opposed to a real ISO of, say 83, (which the E-M1 MkII has at base ISO), there is far less need to mess around changing the ISO settings back and forth. ISO 200 is useful for more conditions.
To advertise a camera as having a base ISO of 200, when in practice it is ISO 83, seems deceptive to me.
Now this leads on to the nub of the issue. At base ISO I presume that the camera's in-built software
does not amplify the signal to compensate for the underexposure of the sensor that takes place as a matter of course at higher ISOs. Right?
If we were to compare a Nikon D700 and an Olympus E-M1 MkII, shooting the same scene, at the same nominated ISO of 200, with equivalent focal length lenses to give the same FoV, using the lenses at the same F/stop, and ensuring that the lenses had the same transmission or T-stop,
would the shutter speeds be approximately the same for both cameras, if we were shooting in RAW mode and attempting to achieve an ETTR shot for maximum DR and the lowest noise?
That's the $64,000 question. Can you answer it JBL and dispel my confusion.
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I am assuming that the E-M1 MkII, in those circumstance, at base ISO, which should involve no analogue amplification of the signal, will require approximately double the exposure of the D700 to get an ETTR shot in RAW mode.
I won't answer the other points you've made until I am clear about this issue. It would be really useful if someone reading this thread happens to own both a D700 and E-M1 Mk II and is prepared to do the comparison under the conditions I've described. However, an awareness of the T-stop factor is also essential for accurate and meaningful results.