What is 'proper' Exposure, when your camera can capture 14 stops of DR, but the typical output medium can only display 6-8?
ADL is a way to squeeze an image with a high dynamic range (i.e. data that would spill off
both ends of the histogram) so that it will better fit within the chosen color space (histogram). In other words it reduces the dynamic range of your image, so that you may retain more visible detail in
both shadows and highlights, while at the same time compressing information and reducing image contrast. The overall result is generally a better looking perceived image.
Because of the compression in information and contrast, it is imho best to use it only when the effect you are trying to achieve calls for it: if you shoot Jpeg or NEF+Jpeg, you set exposure by looking at the in-camera histogram and the data does not indicate clipped shadows, there is no particular reason to leave ADL on permanently. It is simply another tool like those available in Picture Controls to set the camera up properly for the particular scene that you are trying to shoot.
For those who own Exmor-sensored cameras, shoot NEF+Jpeg, like a certain level of automatism, like to fine tune most keepers in PP and also like to keep the +Jpeg as an OOC record (or possibly to share/view/print instantly after the shoot as-is) there is a strong case for setting up the camera with Matrix metering and ADL Auto and forgetting that it is even there.
With this setup the camera decides if the dynamic range of the image calls for ADL or not based on data provided by the Matrix meter and -if necessary- may reduce (or increase, hear hear) exposure by up to 1EV to make the data better fit into the Raw data. You are still in control if you use the in-camera histogram to ETTR, but your shadow end is 'protected'.
I should add that if you use this strategy and you post process the NEF, imho it is easier
not to turn ADL off in CNX2, because that will most likely result in an overexposed, non-shadow-protected image that you will then need to correct yourself, adding to your work without, in my experience, any perceivable additional benefits.
Additional information on ADL in this
informative Leon Goodman link.
Cheers,
Jack
PS Most of this comes from previous posts