>For raw files, you can get the same effect in any camera by dialing in -1 stop of exposure compensation and adjusting to compensation in the raw conveter.

True enough, but the camera preview will be dark and the histogram shifted to the left. Nick, how does the 1DMIII handle this situation? I would presume that the preview would be brightened by 1 f/stop, but what about the histogram? Personally, I would prefer to have the histogram show the true status of the exposure and not some shifted value.

Bill

>Personally, I would prefer to have the histogram show the true status of the exposure and not some shifted value

>What is "true status"? The in-camera histogram is based on the camera settings, which have no impact on the raw data. I would prefer raw histogram shown in-camera, at least as an option, but I don't think many customers share this preference.

This issue has been covered many times. Since the histogram is derived from the JPEG preview with most cameras, you should set the contrast to low. Contrast affects the quarter tones much more than the extremes of the histogram. If you want to see the channels without white balance, at least with Nikon cameras, you can upload a special white balance to the camera where the red and blue multiplers are set to unity (UniWB). Also you can upload a TRC to the camera to undo the gamma encoding, but I've never tried this.

If you do a bit of experimenting, you can correlate the camera histogram with the raw file and have a pretty good idea of what is going on with the raw file, and you can gain quite a bit of control over the histogram. Some cameras indicate clipping in the histogram when there is none in the raw file, and this can be corrected with the proper TRC.

IMHO, a raw linear histogram is not a good idea, since the data would be scrunched up on the left. A log base 2 histogram would correspond to f/stops and that would be my preference and also corresponds with human luminance perception, which is log.

>The exposure with HTP On is the same as with HTP Off, but the ISO will be halved with HTP On (that's the reason for HTP working only from ISO 200 up). The loss in the raw data is the least significant bit. As the 1DMkIII creates 14-bit raw data, this loss is practically meaningless (assuming that the extra bits or at least the first of them is not random).

If you halve the exposure, shot noise will be increased by a factor of 1.4 across the tonal range. Usually dynamic range is limited by the noise floor, rather than by quantization. The 14 bit ADC wouldn't address the increased shot noise, but it might reduce the read noise.

> If you want to see the channels without white balance, at least with Nikon cameras, you can upload a special white balance to the camera where the red and blue multiplers are set to unity (UniWB). Also you can upload a TRC to the camera to undo the gamma encoding, but I've never tried this

These are great options; unfortunately, Canon does not offer such. With my next camera I will spend some time to figure out, based on test shots, which temperature and bias setting comes the closest to a "leave it" action.

> If you halve the exposure

but you don't, at least not by turning on HTP.

> I think that Thomas Knoll's explanation is not quite correct

From the point of ACR it's the same: in order to compensate for it, you do the same as if the exposure had been lower.

> Noise should not increase

That's right, in the sense of the sensor. However, when the exposure will be increased by one stop, the raw values get doubled in effect. Somewhat simplified, the last bit of the raw values becomes generally zero. The effect can appear as noise.

On the other hand, if the 13th bit of the Canon 1DMkIII is not random, that is a substitute for the "lost" bit (this question has not been analyzed in detail yet).

On the third hand, if you look at the meaning of the 12th bit, you find that it is virtually meaningless, unless you are doing extensive adjustments to the raw data. So, the effect may or may not be noticable.

> to duplicate the effect in any camera, you would have to decrease the ISO by half and dial in -1 f/stop exposure compensation

Right, that's exactly the effect of HTP. What I meant above was, that when you are processing the image in ACR, you can't select between compensating for lower ISO and compensating for lower exposure.

So, even though not the exposure gets reduced by HTP, the action in ACR is increasing the exposure (which too is technically incorrect...).

> The preview would be dark and the histogram would show the increased headroom

and the JPEG output would be quite useless. While HTP on the raw level means only convenience, the real advantage is for JPEG shooters.