Today’s Question: In semi-automatic modes like shutter priority and aperture priority, we set two of the three important settings for exposure: ISO and either shutter speed or lens aperture. The camera picks the third and has control of the overall exposure. If we use manual exposure but with auto-ISO, doesn’t this become a third semi-automatic mode with the camera having control of one of the three settings? Then it really isn’t manual at all? Is exposure compensation then an important and effective setting just like in the other semi-automatic modes?
Tim’s Quick Answer: Yes, if you enable auto ISO when using the manual exposure mode, the camera is then operating in a semi-automatic rather than fully manual exposure mode.
More Detail: A proper exposure involves the use of a given lens aperture, shutter speed, and ISO setting. There are a variety of ways you can achieve specific settings, including fully automatic, semi-automatic, and manual modes.
In a fully automatic mode, the camera typically chooses all three exposure settings for you. In a true manual exposure mode, you need to adjust all three settings yourself. And in semi-automatic modes you are typically able to select one or two of the exposure settings, with the camera selecting the other setting for you.
So, if you enable auto ISO when using manual exposure mode, the exposure control is no longer fully manual. As you adjust the lens aperture or shutter speed, the ISO will adjust as well to maintain the metered exposure. That also means that exposure compensation become important when using manual mode in conjunction with auto ISO, just as exposure compensation is important with other semi-automatic exposure modes.
Exposure compensation enables you to shift the exposure brighter or darker relative to the exposure calculated by the camera based on the meter reading. Normally when using manual exposure mode, the exposure compensation setting for the camera wouldn’t apply, as you would simply adjust the exposure settings to the desired meter reading. However, with auto ISO you can apply exposure compensation with manual exposure. This would alter the selection of the ISO setting to achieve the target exposure based on compensation, and in conjunction with the lens aperture and shutter speed settings you have selected.