ABOVE: A useful way to think of the ISO setting is as a volume control on an amplifier. When you increase the ISO setting, you’re turning up the signal.
When HTP is turned on, the camera’s metering system works out the exposure needed based on the currently selected ISO value, but when it reads the signal from the sensor it sets the amplifier gain corresponding to the ISO setting one stop below it. It then tags the image in the metadata to indicate it was shot with HTP enabled.
As an example, if you photographed a scene without HTP at ISO 400, the camera’s metering system might suggest 1/400 second at f8. When the signal is read from the sensor, the amplifier (ISO) will be set to ‘400’ and the resulting signal will be converted to brightness values corresponding to the full range of tones in the original scene (assuming the metering in the camera was correct).
When you enable HTP, the camera will still meter the scene at ISO 400 and still select the same shutter speed and aperture as before. Exactly the same signal will be recorded on the sensor. The difference is that this time the amplifier will be set one stop ‘quieter’ (so ISO 200 instead of ISO 400). This means that the RAW file that’s generated will have numerical values that are half of the corresponding values that would have been produced without HTP enabled. If you could view the image at this stage, you would see it as being one stop underexposed and hopefully without any blown-out highlights.
However, the camera marks the file as being taken using HTP and applies a tone curve that brightens shadows and mid-tones, but which holds back the highlights. It is this modified adjustment that you see on the back of the camera – the RAW file is converted to JPEG and processed in-camera based on the settings used (this is regardless of image quality settings). Since the file is tagged as being taken with HTP enabled, the same adjustment is applied if you open the RAW file in Canon’s Digital Photo Professional.
