ISO settings, and other ramblings

FingerPainter wrote:

phil from seattle wrote:

I tend not to use auto-ISO, preferring to keep ISO as low as possible.

But Auto-ISO does keep ISO a low as possible, for the lightness you told the camera you wanted.

Also, with auto-ISO, I found myself setting the max ISO frequently so why not just set the ISO directly - it's super quick to do on Olympus cameras.

Because, since you are not as smart as the camera, you will sometimes set it higher than it needs to be to get the target image lightness, and as a result your image will be noisier than it needs to be.

I believe you are confused. You need to study the exposure triangle to understand how this all works.  Exposure value (EV) is controlled by 3 factors: aperture, shutter speed and sensor sensitivity (aka ISO).

When you are using auto-ISO and set the camera to aperture or shutter priority, there are 2 variables that need to get fixed - ISO and aperture (if S-priority) or shutter speed (if A-priority).  The camera will attempt to guess a good ISO to for the remaining variable.

In shutter priority where you set the shutter speed, I believe auto-ISO can do a reasonable job of guessing an ISO for a proper f/stop.  Though, I mostly shoot in A-priority so haven't tested this theory.

However, in aperture priority where you select the f/stop, autoISO doesn't know what you are taking a picture of and what the best shutter speed should be. A good ISO for playing kids is going to be different from one for a landscape or night scene in a city. AutoISO appears to bias towards action shots. I.e, picks an ISO value that will allow a fairly fast shutter speed to take a decent picture of the playing kids. And, as a result, my tripod mounted landscape shot gets a higher ISO than it should. All my night shots of London Christmas lights would have been at an unnecessarily high ISO because the fantastic IBIS in my cameras allows me to take long hand held exposures. Maybe someday computational photography will get us there but it's not today.

So, you are wrong. I am smarter than my camera because I know what I'm shooting and it doesn't. ISO is not some mysterious voodoo. I pretty much can look at a scene and know what the right ISO should be. And if I'm wrong, I can quickly change it since it's super fast to set the ISO and slow to tromp through menus to set maxISO.