How to Use Aperture Priority with Auto ISO
If you’ve figured out how to focus consistently and have a bit of an understanding of how your camera exposes a scene, you might be wondering how to leave Program mode behind and take greater control over your images.
Aperture priority – often A or Av on the command dial of your camera — allows you to take control of one aspect of the exposure triangle.
A feature that has become increasingly common on cameras in the last ten years is Auto ISO.
ISO is the final part of the exposure triangle, and by setting it to Auto (usually in your ISO selection, just past ISO 50 or ISO 100), you are letting your camera do all the work while you can concentrate on composition and focusing.
Why & When to Use Aperture Priority
Perhaps the best way to understand how this mode can be used effectively is to imagine yourself shooting portraits during golden hour.
For a portrait, you typically want to isolate the subject of your photograph as much as possible. One way to create a sharp subject and a blurry background is to use as large an aperture — i.e., as low a number — as possible.
[Related guide: best lens for portraits]
For example, I often use my nifty fifty, an affordable 50mm lens with a maximum aperture of f/1.8 – as in the photo below:
Shooting wide open at f/1.8 allows me to focus on my subject’s eye (made easier using my Sony a7 III’s eye autofocus).
Other parts of the image which are further away from the lens consequently fall out of focus, drawing the viewer’s attention to my subject.
This pleasing effect also has a cinematic feel, giving your subjects a heroic, Hollywood vibe.
If I were to use Program mode, there would be no way to ensure that my camera is choosing the right aperture. Switching to Aperture Priority mode allows me to dial in f/1.8 to give me the shallowest depth of field possible.
The camera is selecting the right shutter speed. I could then decide to choose the ISO myself, perhaps taking some test shots and checking my histogram to see how my exposure is doing, and changing my ISO if I haven’t got it right.
Alternatively, I could scroll my ISO setting to “Auto ISO” and let the camera do all the work.
Suddenly I’m shooting at f/1.8 and all I need to worry about is focusing, composition, and posing my portrait subject.
If you’re shooting a landscape, you typically want to do the opposite: shoot with a relatively small aperture, perhaps f/11 or f/16, in order to give yourself greater depth of field. This keeps as much of the image in focus as possible.
Although the camera is doing most of the work for you, there are somethings that you need to keep an eye out for.
For example, for that landscape shot, using a small aperture can mean that you end up with the camera choosing a very high ISO, causing lots of noise in the image.
The same might happen with your golden hour portraits. If the light is slowly dropping, the camera may automatically choose a shutter speed that is so slow that you can’t avoid camera shake. Fortunately, there are ways to solve this.
Take Control of Your Shutter Speed
At this point, you have a few options. One is to switch to Manual mode which, hopefully, is no longer such a scary prospect.
If you’re shooting your portraits on your 50mm lens, you have your aperture set to f/1.8, you can switch to Manual and simply set your shutter speed to something that will avoid camera shake, such as 1/125th of a second.
The camera looks after your ISO, increasing it as it detects that the light level is becoming lower.
Alternatively, you can take advantage of a feature that has only become common on digital cameras in the last ten years or so: setting the minimum shutter speed.
Unfortunately, many entry-level cameras don’t include this functionality so switching to that Manual mode might be the better option.
Different manufacturers tuck minimum shutter speed setting away in the menu system in different places.
Canon usually hides it under “ISO Speed Settings,” Nikon calls it “ISO Sensitivity Settings,” while Sony opts for the unnecessarily abbreviated “ISO AUTO Min. SS.”
This functionality can be equally useful if you’re setting up for that landscape shot without a tripod described above.
For handheld shots with a wide-angle lens, anything slower than 1/60th of a second will risk some motion blur, but this will depend very much on how still you can hold your camera (can you brace against a tree?) and whether your camera has internal stabilisation.
Making Use of Exposure Compensation
Cameras have made huge progress in the last decade when it comes to deciding how an image should be exposed, but that doesn’t mean that they don’t make mistakes.
Strongly backlit subjects can still be confusing, and, going back to our portrait example, you still need to watch your histogram to ensure that you have a well-exposed image.
If you’re shooting with Aperture Priority and Auto ISO and the results are looking a little dark (I.e., the histogram is stacked up against the left-hand side), you can scroll the Exposure Compensation dial to +1 and make the image brighter.
Similarly, if the image is looking bright (the histogram is stacked up on the right-hand side, blowing your highlights), scroll the dial to -1 to make it darker.
Auto Exposure Lock
Another useful tool in your arsenal when shooting Aperture Priority and Auto ISO is the Auto Exposure Lock button, often labelled AEL. This is a quick way to tell the camera to stick with a decision about exposure and not change until you press the button again.
For me, it comes in very useful when I’m shooting someone climbing. As they progress up the rock, I find myself pointing the camera upwards, often incorporating more and more sky as they get higher.
Thanks to this sky, the amount of light entering my lens increases, often confusing my camera, and making it choose a faster shutter speed and a lower ISO. This results in an image that exposes a perfect sky, but my climber is suddenly a muddy silhouette.
One look at my histogram shows that it’s stacked heavily on the left-hand side.
One option would be to keep turning the Exposure Compensation Dial, as described above, but usually it’s simpler to get the perfect exposure when there’s a lot less sky in the shot, and then lock that exposure in using the AEL button.
With one press, I’ve told the camera to hang onto the settings and not make any changes until I tell it otherwise.
If you want to get really clever, you could even lock the exposure with the AEL button and then make small changes using the Exposure Compensation dial.
Pro tip: remember to return the Exposure Compensation dial back to zero once in between set ups.
You Don’t Need the Manual to Go Manual
With Aperture Priority and Auto ISO, you’re really only one step away from shooting in Manual mode – something that you might have previously felt was reserved for professionals and those with lots of experience.
The truth is different. Professionals and experienced photographers use their camera’s automated features extensively so that the camera makes the decisions and frees them up to focus on everything else.
But what you might have noticed is that shooting in Manual mode with Auto ISO is only a small step from Aperture Mode with Auto ISO. Hopefully you’re now all set to start exploring outside of your Program mode and get really creative with your photography.
Feel free to leave any questions in the comments below. Good luck!
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