Bill, as we discussed briefly on the Adobe User-to-User forums, the issue here is the baseline exposure differences between various cameras and across vendors. There is a fundamental tradeoff between (a) honoring the original distribution of raw values (between black point and white point) and ( b ) having a given exposure (e.g., f/8, 1/10th sec, ISO 400) be rendered the same way across different cameras. In order to have common controls such as Exposure compensation behave the same way across the different cameras, Camera Raw applies a baseline exposure compensation that varies from model to model in order to get them all to behave similarly when the exposure compensation control is set to its default value of zero.
I think the main problem is that the camera vendors are not using uniform calibration procedures for the ISO of the sensor and the rest of the camera system and thus the results are not consistent between different cameras of the same or different makes.
The main variables are:
1) Calibration of the light meter itself
. According to Phil's tests on DPReview, Nikon has always been spot on for meter calibration and the recent Canon's are also. The calibration is done by comparison with the reading of a calibrated hand held meter. A source of confusion is that the ISO standard for light meters is for the equivalent of around 12% gray, not the commonly assumed value of 18%. See [a href=\"http://www.bythom.com/graycards.htm]Thom Hogan's Post[/url] for details. The upshot is that if you determine exposure by a reading from a Kodak 18% gray card, you have to add 0.5 EV to the exposure if you want the result to come out as middle gray in the photograph.
2) Variations in the shutter speed and aperture control.
Electronic shutters in modern digital cameras are quite accurate and repeatable (precise). The aperture control is mechanical and subject to more variability, but the mechanism usually works well unless something is sticking.
3) Absorption of light by multiple lens elements.
With complex lens designs, this can be a significant factor. It is taken into account by TTL metering, which will then give more exposure than indicted by the hand held meter.
4) Calibration of the ISO sensitivity of the sensor itself.
This is where I think most of the variations arise. Norman Koren
explains the ISO 12232:1998 standard for determination of the ISO of digital cameras. The standard defines two methods: saturation based and noise based. the Saturation based standard is usually used. When exposed according to a standard illumination (determined by a meter reading or derived from luminance values directly), an 18% gray card should result in the sensor being 18/106 saturated resulting in a raw pixel value of 18/106 of full scale since the sensor is linear. The denominator of 106 rather than 100 is used to allow some headroom for highlights. If a gamma curve of 2.2 without any other adjustments is applied, this results in a pixel value of 114 in the resulting 8 bit file. Most cameras and raw converters apply additional tone corrections, so it is best to examine the percent saturation in the raw file directly. In ACR you can try a linear setting with all the sliders on the main panes set to zero (exposure, recovery, brightness, contrast, etc) and the point curve set to linear. If you then take card of the baseline exposure factor and take a picture of an 18% card using the exposure indicated the camera meter, you should get a pixel value of about 114 if everything is working correctly and the camera is calibrated to the 1996 standard.
The new ISO standard is 12232:2006
as explained on Wikipedia and it is slightly different. It retains the saturation based method, and amplifies on noise based methods and adds a standard output sensitivity (SOS), which will lead to a pixel value of 118 in a gamma 2.2 space with 8 bit files.
With the saturation method, an exposure of an 18% gray card based on a standard light meter reading will result in an image with a grey level of 18%/√2 = 12.7% of saturation, which corresponds to a pixel value of 100 in a gamma 2.2 space. The square root of 2 is to allow 0.5 EV of headroom for specular highlights. Since the standard light meter is calibrated on the basis of the equivalence of 12% reflectance, you have to add 0.5 EV of exposure if you want the gray card image to have a pixel value of 118 in the gamma 2.2 file.
The Wikipedia article concludes, "Despite these detailed standard definitions, cameras typically do not clearly indicate whether the user "ISO" setting refers to the noise-based speed, saturation-based speed, or the specified output sensitivity, or even some made-up number for marketing purposes."
Bill Claff, a respected Nikon guru, has published ISO values necessary to attain 18% saturation of the sensor (essentially the SOS standard) on his Web Site
. For example the D3 is rated at ISO 200 but an ISO setting of 138 is needed to attain 18% saturation. By contrast, the D80 is rated at ISO 200 and Bill has rated the native ISO at 93. The high ISO performance of the D3 may not be quite so good as it is cracked up to be.