So, why does this mode create problems for so many photographers?
Manual Mode with Auto ISO is a very useful tool that can improve your results greatly in certain situations, but issues arise from a lack of understanding of its functions, its limitations, and how to identify the appropriate times to use it.
As we now know, Auto ISO can account for your subject travelling through variable lighting. Its ability to alter your exposure during moments when we aren’t able to work quickly enough to do so ourselves is its main benefit. This sounds appealing, but we also need to consider the pitfalls of a mode that is continually adapting to perceived changes in the light, be that on your subject or its surroundings…
Inconsistent Exposures
I only infrequently encounter situations where the light on my subject is changing rapidly. So already, my need for Auto ISO is limited to very rare occasions. In fact, I generally try to avoid such situations whenever possible, as the uneven lighting tends to create undesirable effects in my images, regardless of whether or not they’re properly exposed.
Furthermore, what I do encounter during 100% of shoots are changing backgrounds, and variation in the size of my subject in the frame. Both of these variables can have a significant bearing over your exposure if shooting in Manual Mode with Auto ISO. These changes don’t have to be significant either; something as subtle as a duck swimming on the water towards you and turning more broadside, revealing a larger area of its darker plumage, is enough to change how the camera meters the scene. When framing up a scene at 600mm – a very narrow angle of view – a bird can make a subtle shift along a branch, and as you follow it through the viewfinder, suddenly the tone of the background can change dramatically. As your camera acknowledges these shifts, it will respond by changing the ISO, and thus the resulting exposure. Should these changes to your exposures be undesirable (as they often are) adjustments to your exposure compensation have to be made, which can be impossible at times given how quickly these changes in exposure can occur. This can be an irritating task that takes extra time to consider, and more often than not, photographers don’t even address it, leaving them with severely under or overexposed images – sometimes irreparably so.
In short, while touted as an amazingly practical mode for wildlife photographers, the way this mode adapts to perceived changes in light actually becomes your worst enemy in many of the most commonly encountered applications.