Interesting discussion. For me, manual is a means to control exposure in a different way from either of the semi-auto modes (Tv or Av). In these latter settings the photographer is making a specific decision about one of the values they want to set - aperture in Av mode, for example. The ISO may, or may not be a fixed value. However, the camera's metering system may not consider issues with the specifics of a particular subject's tonal values. For example, shooting in snow, where the sensor will try to make the snow 18% grey: the tone one would normally associate with relatively common images, such as vegetation, but making the snow grey too. In such an event, the use of Exposure Compensation provides a fast means to over-ride the camera's preferred exposure. This, in itself, might result in a change to the shutter speed, and/or ISO - depending upon the algorithms involved in the camera's metering an control system.
Auto ISO is extremely valuable when one is using one of those modes and one wants to set one variable let's say Aperture, which would then usually commit the camera to choosing a specific shutter speed. But what if one is shooting in variable lighting conditions and doesn't want the shutter speed to suddenly drop? Having variable ISO creates a new variable where the algorithm will look at the shutter speed and try to not let it drop too low by providing a variable in ISO value. In many cameras there is a specific menu feature that allows a user to configure this algorithm: I present this image from the Canon 5DIV manual as an example:
In Manual Mode, I would suggest that one is obviously able to control all three settings, ISO, Aperture and S/speed and, by setting two of them, simplify the exposure process, thus have a means to avoid Exposure Compensation by varying the third with its own control. This will still allow the user to introduce changes to exposure that would otherwise be handled by Exposure Compensation. There are times when this is extremely advantageous, I will give an example by Sean Tucker who is a respected street photographer: I suggest going to timestamp 15:05 for his practical demonstration for his specific situation - which is to shoot light rather than people. That is quite different from a wildlife photographer, but that simply expresses that photography is made up of many genres, artistic expressions and techniques.
How to Nail Exposure using Manual Mode - Bing video
So, the question is, does using Auto ISO and exposure compensation with Manual mode contribute to the user's flexibility or conflict with their level of control? I think the answer lies with a foot in each camp. Let me explain...
Case 1: The user wants total control over the exposure and only use the settings they choose for Shutter speed, ISO and Aperture. Obviously, this would preclude any automated features from the camera, so no auto ISO or Exposure Compensation. The photographer thus has to be mindful of what the camera meters as correct exposure, and change all the parameters accordingly to compensate for the image's tonal qualities - however, one would normally expect that they would pre-determine at least one of those settings as non-negotiable.
Case 2: The user wants specific values for both aperture and shutter speed, but because of the potential for variations in light, is prepared to allow the ISO to be varied by the camera. This would provide a good case for using Auto ISO. However, the camera is still varying that setting by its own metering of the image and, as before with the snow scenario this may not be appropriate. Being able to use Exposure Compensation would still allow the user to override the camera's metering to introduce more or less light for specific conditions or effects, and within the bounds of the algorithm will change the ISO accordingly, rather than the other two settings.
I have seen situations where this conflicted with the Auto ISO settings, and then the camera would either flash to indicate that the exposure parameters were invalid, or attempt to change the other two manually-chosen settings, but this is likely a function of the make and model, and is likely to be a relatively rare occurrence.
Whether this is seen as a practical solution is really down to the kinds of conditions under which one shoots, the type of subject and how an individual photographer approaches their control methodology.
