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For many photographers Auto ISO is a recent addition to their camera’s list of exposure modes. There was no such thing in the days of film photography, as changing the ISO setting is only possible with digital cameras.
Some photographers don’t like using Auto ISO because they think ISO is the most important variable in the exposure triangle. In other words, you set ISO first, then adjust the aperture and shutter speed afterwards. Automatic exposure modes like Aperture priority and Shutter priority lock you into this way of thinking.
But that would be a mistake as Auto ISO is so useful in many situations.
What is Auto ISO?
Auto ISO is an exposure mode that lets the photographer set both aperture and shutter speed. The camera then selects the ISO setting that gives the best exposure, according to its built-in meter.
Most cameras also let you set the highest ISO you want Auto ISO to use. This gives you the peace of mind of knowing that your camera isn’t going to set a higher ISO than you’re comfortable with.
Every camera’s different, so check your manual for details.
Why Auto ISO is different
Auto ISO is different because it gives you complete control over the creative elements of the exposure triangle. Here’s what I mean by that.
Aperture is a creative setting because it controls depth of field.
Shutter speed is a creative setting because it controls movement, by either freezing it or letting it add blur to the photo.
ISO isn’t a creative setting* because it sets light sensitivity.
* But there are some situations where you could experiment with using high ISOs to add a grain-like effect to photos. Photographers Sarah Moon and Robert Farber did this with film back in the 70’s.
The advantage of Auto ISO is that you can set the aperture and shutter speed you need for the amount of depth of field and motion blur you want in the photo.
This works in many situations because the high ISO performance of modern digital cameras is so good. It frees you up to think about the creative side of exposure settings (controlled by aperture and shutter speed).
Auto ISO examples
Here’s some examples of when Auto ISO is useful.
With a portrait you might want to select a wide aperture like f2.8 to blur the background, and a shutter speed like 1/180 second to prevent any motion blur. Using Auto ISO lets you keep those settings, and adjusts ISO if the light levels change.
Close-up and macro photography
In close-up or macro photography, you can set a small aperture for more depth of field. Or (as in the photo below) you can experiment with a wide aperture for lots of background blur. If hand-holding the camera you’d set a faster shutter speed, like 1/250 or 1/500 second, to prevent camera shake.
It’s the perfect reason to use Auto ISO, and let the camera set the optimum ISO, especially when working at small apertures.
Experimenting with motion blur
Let’s say you’re making photos of a waterfall. What shutter speed is going to give the best effect? It might be 1/30 second, 1/2 second or even 1 second. It depends on how fast the water’s moving and you don’t know until you try it.
This is another situation for Auto ISO. Put the camera on a tripod, set the aperture to something like f11 or f16 (for good depth of field) and the shutter speed to something like 1/30 second. The camera sets the ISO for you, and adjusts it when you change the shutter speed.
By this, I mean the sort of photos you make when you’re out and about in a new place with your camera, making photos of interesting things that catch your eye.
Auto ISO is great for this. It lets you set a shutter speed that prevents camera shake, like 1/180 second, and an aperture that gives good depth of field, like f8.
The benefit of Auto ISO in this situation is flexibility. Let’s say you’re working at f8, but decide you want to open the aperture to f2.8 for a selective focus effect. All you have to do is adjust the aperture, and the camera takes care of ISO.
Or let’s say you want to experiment with motion blur, so you set the shutter speed to 1/15 second instead. Again, the camera takes care of the ISO so you don’t have to worry about it.
This is quicker than adjusting two exposure settings, which is what you would have to do if you were using Manual mode. It’s also quicker than switching from Aperture priority to Shutter priority modes, or vice versa.
What are the disadvantages of Auto ISO?
Great as it is, Auto ISO isn’t the best exposure mode for every situation.
For example, some photographers prefer to use the camera’s lowest ISO setting whenever possible. If you’re a landscape photographer, or you’re using studio flash, this is likely to apply to you. In that case, you’d be better off using Shutter priority, Aperture priority or Manual mode, depending on the situation.
Another disadvantage is that you can find yourself in a situation where the camera can’t give you a good exposure.
This is most likely to happen in bright sunlight, and when you’ve set a shutter speed of around 1/180 second or slower, and a wide aperture, like f2.8 or f4.
In this situation you might need an ISO of 50 or even 25 to get the best exposure. But if your camera’s lowest ISO setting is 100 or 200, then that’s not possible. The result is that your camera sets its lowest ISO and overexposes the photo.
Because of this you can’t assume that your camera is getting the exposure correct all the time in Auto ISO. Remember to check the histogram to make sure your photos aren’t overexposed.
In the situation described here, all you have to do is set a faster shutter speed, like 1/500 or 1/1000 second, so that the camera can set the optimum ISO.
My ebook Mastering Exposure gives you the knowledge you need to get the correct exposure every time you take a photo. Reading this ebook helps you master your camera’s exposure modes and metering tools. You’ll learn why your camera gets exposure wrong, and how to put it right when it does.
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