If you buy a new camera today, the Auto ISO is usually already activated. This also applies to the exposure modes “Aperture priority”, “Shutter priority” and “Manual”. Many photographers like to use the Auto ISO to always have a sufficiently short exposure time for handheld shots, even in different lighting conditions. However, you should be aware of how Auto ISO works and what its limitations are in the various exposure modes.

Note: The following descriptions refer to Canon cameras. On cameras from other manufacturers, the various exposure modes may behave differently in combination with Auto ISO.

Auto ISO and Aperture Priority

In aperture priority mode, the exposure time is selected depending on the focal length of the lens for a specific aperture so that the risk of camera shake is minimized. For a focal length of 100 mm, for example, an exposure time of 1/100 s or 1/125 s is set. The camera then selects the ISO number so that the exposure is ( usually) correct.

Exposure compensation is possible in this mode. Unlike with a fixed ISO number, however, the compensation is not achieved by changing the exposure time, but via the ISO number.

This mode can be quite useful. If you are a cautious person, you should limit the maximum ISO number so that the image quality remains acceptable.

Auto ISO and Shutter Priority

In Shutter Priority mode, an open aperture is selected for a specific exposure time and the ISO number is set so that the exposure fits. Only when the lighting conditions are such that a correctly exposed image is no longer possible with the lowest ISO number is the aperture closed further.

Exposure compensation is also done via the ISO number in this mode.

Auto ISO and Manual Mode

The combination of the Auto ISO with the manual exposure mode allows you to always work very comfortably with the same combination of exposure time and aperture, even in different lighting conditions. This way you can make sure that the photos are not blurred and at the same time the depth of field is always in the desired range. So basically, this is a very interesting option.

However, this method has a serious functional limitation with most camera models, which makes the combination of Auto ISO and manual exposure mode unusable, at least for me: Exposure compensation is no longer possible in this mode! The ISO number is always set exactly so that the exposure is “as metered”.

It would be so easy to simply use the ISO number for exposure compensation. Why don’t the programmers of the camera software think ahead?

Note: If there is a way to do exposure compensation at M + Auto ISO on Canon cameras, I’m dying to know how that works! If you have an idea, then please write a corresponding comment.

Auto ISO and Tripod

In many situations, such as architectural shots at dusk or at night, and landscape shots that are often taken just before or after sunrise or sunset, the camera is usually on a tripod. In these cases, the Auto ISO is usually counterproductive.

Since the camera does not recognize that there is no risk of camera shake thanks to the tripod and the exposure time is completely secondary for static subjects, it would select an unnecessarily high ISO number in low light and thus give away the image quality possible at low ISO numbers.

The situation is different, of course, if you want to have sharp images of moving objects in the dark. In these cases, the ISO number must of course be high enough so that the exposure time is sufficient to freeze the movement.

Conclusion

The Auto ISO can definitely make photographic work easier in some situations. However, you should always keep an eye on the ISO number selected by the camera to avoid being surprised by unacceptable image quality.

As always, please feel free to leave a comment. I’m looking forward to it.