Sony A7R III: Auto ISO with Minimum Shutter Speed

I’ve had the A7R III for two and a half years now, and I’m still learning things. I shot Canon for over 25 years, so learning Sony’s menu system alone took a fair amount of time. Granted, I don’t like reading manuals, and tend to teach myself just enough to shoot the way I’ve always shot without really digging into any new stuff.


At some point I stumbled onto the Auto ISO with minimum shutter speed functionality, (Sony shows this as ISOASS on the camera, and that’s what I’ll refer to it as from now till eternity because I’m 12,) and it was a game changer for me. I rarely shoot in a studio, and only use artificial light when necessary, so most of the time lighting conditions are constantly changing during a shoot. Shooting manual with Auto ISO was my previous solution, but that still involved some fiddling whenever we moved from one spot to the next. If I want to control exposure when using these settings, I use the exposure compensation dial. Generally I like to underexpose by a stop.

Today, I accidentally learned another new setting that goes along with ISOASS. The camera can automatically choose the minimum shutter speed based on the focal length of the current lens. The details are in the Owners Manual, which I reluctantly referenced when I had some recent issues using ISOASS. Turns out, the camera was doing what it was supposed to do, but I had the max ISO set too low for the lighting conditions (which forced the shutter speed to go lower than I had set.)

Auto ISO with minimum shutter speed of 1/250

I’ve been manually setting the minimum shutter speed when using ISOASS. I like my shutter speed to be at least twice the focal length, so when I’m shooting with my 85mm, I set the minimum shutter speed to 1/250. If I’ve got the 50mm attached, the minimum is 1/125. But what if I forget to change the minimum when I switch lenses? Not much if I’m going from 85mm to 50mm, but if I’m going the other direction there’s a chance of introducing some camera shake. Especially if I’m balancing on one foot with a light panel in one hand, and the camera in the other. Somehow that scenario plays out more than it probably should. Light stands are a pain to pack around, and I don’t always have a VALS.

ISOASS with automatic minimum shutter speed

Anyway, in addition to setting your own minimum shutter speed, there are 5 additional modes that set the minimum shutter speed automatically.

  • Slower
  • Slow
  • Standard
  • Fast
  • Faster
Auto ISO with FAST shutter speed set

According to the manual, the difference in these settings is 1EV. I can’t find any specifics on the exact minimum shutter speed per setting, but Standard seems to grab a shutter speed close to the focal length, and Fast seems to get a little closer to my preference of double the focal length.

Location of the ISO settings in the Sony menu

Why is my shutter speed lower than the minimum I set?

This is why I referenced the manual in the first place. I expected that when I set a minimum shutter speed, the camera would respect that setting even if it meant underexposing the shot. In reality, the camera will bump up the ISO to stay at or above the set minimum shutter speed until it hits the maximum ISO you’ve set. Then it starts lowering the shutter speed.

So that’s it… If, like me, you hate reading manuals and didn’t know about the auto shutter speed settings, I hope this helped.

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