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Auto ISO Mode: What, How & When?

What is Auto ISO?

The Auto ISO Mode is a feature on some cameras where the camera decides what should be the optimum ISO for the exposure. 

The camera reads the aperture and the shutter speed in use and then depending on the scene and the metering mode, dials in the ISO number it feels is right.

Typically, the photographer sets the ISO, but in some situations, it is best to hand over the decision-making process to the camera and switch to the Auto ISO Mode.

How to Setup Auto ISO (Tripod vs Handheld)

What we are concerned about is a way to automatically set just the ISO of your camera without tinkering with the aperture and shutter speed.

Can this be done? Yes.

But the question that we need to answer is why we should do it?

When to use Auto ISO?

Let’s take an example. Imagine you are shooting in a low light environment. When we speak of little light, we invariably think of slowing down the shutter speed.

Well, that’s one way to handle the situation. But that would only work if you are shooting on a tripod. If you are shooting hand-held, this could be a problem because image blur will creep into your pictures.

Ok, so the next way to counter low light would be to open up the aperture. But what if you are shooting a landscape or cityscape scene and you cannot compromise on the depth of field?

Auto ISO Settings at ISO 100 | Coins

The final option is to tinker the ISO number. Most old timers would hesitate to tinker with the ISO number.

They come from old school film photography, and there were limitations to using film. I am referring to film grain. With digital photography, this is no longer a problem. Digital photographers routinely adjust ISO to a higher number when the desired combination of aperture and shutter speed do not give them a blur-free photo.

That’s all good, except that changing ISO every other time can become a mundane routine. Every time your exposure changes, you will have to adjust the ISO of your camera manually. You lose valuable seconds while you take your camera off your eye and change the ISO number.

Related Post: Understanding the ISO – Aperture – Shutter Speed Triangle

Especially when shooting high speed, action or wildlife photography missed opportunities will make you wonder if there was an easier way to set up ISO automatically.

Thankfully, there is one such way.

Auto ISO is not set to ‘On’ by default on all cameras. You have to turn it on.

Otherwise, you can just press down the ISO button and then dial it in manually like any other setting when you are shooting in aperture priority, shutter priority as well as manual mode.

Auto ISO saves you from having to dial in the ISO number each time your exposure changes.

How does Auto ISO Work?

How does it work? Your camera will not trigger Auto ISO when there is a lot of light. Your camera will shoot at the lowest ISO setting possible on your camera when the light is right. Let’s say ISO 100 or ISO 200 whatever is the lowest ISO setting.

But as the light becomes dimmer and dimmer, Auto ISO will kick in.

Your camera will boost the ISO number to keep the exposure correct.

How to Turn it On?

Different cameras have different steps to turn it on. But they are all somewhat similar.

Like on my Nikon, I have to

  • Go to Shooting Menu
  • ISO Sensitivity Settings
  • Auto ISO Sensitivity Control and
  • Set it to ‘On’ mode.
how to work with auto iso
Use the ISO Auto Setting especially in Low-Light Scenes like in this image.

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Limitations or Parameters

Now, most cameras would also have an option wherein you can specify the maximum ISO number that the camera can automatically set.

You could also set the minimum shutter speed when auto ISO kicks in. If you define a maximum ISO number, the camera won’t select anything higher than what you set.

Why would you do that? Well, let’s say that you are shooting in a dark condition and the camera sets the ISO to some ridiculously high number – ISO 3200 or maybe even higher.

Needless to say, you will get a lot of noise in your image at such high ISO numbers. To avoid that possibility you can set the maximum ISO, and your camera won’t select anything beyond.

Safeguards to Using the Auto ISO Mode

Auto ISO will work somewhat differently in different cameras. I don’t mean only the setting up aspect but the actual working part as well. Modern cameras are better at Auto ISO adjustments than their older cousins.

Working with Auto ISO in Low Light Environments

One more thing to note: Never set an ISO number at which your camera is not performing well. If the high ISO and low light performance of your camera are not particularly great, don’t select the ISO too high.

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  • Rajib is an avid travel photographer and an overall shutterbug. He loves to test and review new photography gear. He has been writing about cameras and lenses for over 10 years now. You can consider him as your "master guide" here at PhotoWorkout.

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