Photography Corner – Aperture Priority Mode

This is the second installment in the Photography Corner series. In the previous article we discussed some of the basics of exposure and how aperture, shutter speed, and ISO settings affect your photos. In this article, we’ll look at what happens when we want to prioritize one of those variables.

Aperture Priority Mode

Recall that the aperture controls how much light is let into your camera and how much of your image is in focus. Wide aperture means lots of light and only a small sliver of your scene in focus, while a narrow aperture will let less light in, but more of your photograph will be in focus.

Aperture priority mode lets you select a particular aperture and the camera will decide which shutter speed to use in order to properly expose your photograph. You might recall from the previous article that the aperture you choose either opens or closes the aperture blades to allow more or less light to hit the sensor during a given time interval, which is determined by your shutter speed.

Often marked on your camera mode selection dial with an A of some sort (Canon = Av, Nikon & Sony = A), this mode requires you to set the aperture. The range of available values is determined by the lens you are using. Some lenses are f/4 wide open, fast lenses might be f/1.4, and some only open up to f/5.6. Typically, the lower boundary is the one you’ll run into more often than the upper boundary because finding enough light to keep your ISO low can be a challenge.

Notes on Shutter Speed

You must have a fast enough shutter speed to eliminate any shake from hand-holding the camera and any movement of your subject. If it is very well illuminated, chances are that you see 1/250, 1/500, or faster. If that’s the case, take the photo. But how do you know how fast is fast enough? A little math will help here… 🙂

WITHOUT IMAGE STABILIZATION

Full frame: 1 / (focal length) :: In our example, 1/100s

Crop sensor: 1 / (focal length * crop factor) ;; 1/(100*1.6) = 1/160s

You probably know if you have a full frame camera – examples are Canon 5d series, Nikon D8XX, Sony a7R III, and many, many others. Crop sensors come in cameras such as the Canon 7d line, Rebel line, Nikon DX series, etc.

If you have a lens or body that performs image stabilization (IS) for you, the numbers above will change significantly. Image stabilization is typically expressed in the number of “stops” for which it will stabilize an image. IS uses a lens stabilization group to counteract movement, effectively holding the lens “more still” for a photo. Detailed information can be found here. For our example, let’s assume you have a lens with 2 stops of IS.

WITH IMAGE STABILIZATION

Full frame: (1 / (focal length)) * (2^(stops of IS)) = (1/100)*(2^2)= (1/100)*(4) = 1/25s

Crop Sensor: (1 / (focal length * crop factor)) * (2^(stops of IS)) = (1/(100*1.6))*(2^2)= (1/160)*(4) = 1/40s

Wow, so even 2 stops of image stabilization get you a 4x reduction in the shutter speed that is required to eliminate camera shake when you’re holding it. 3 stops gives you an 8x reduction! Amazing technology, but remember, it doesn’t freeze your subject – so whatever you’re taking a picture of must remain still for the shutter speed duration to eliminate blur.

Subject motion blur

Also, these results show us that it is much easier to hand-hold a shorter focal length lens vs. a super long lens in low light.

Sample Aperture Priority Portrait Setup

Let’s assume you have a fairly fast (large aperture) lens such as a Canon EF 70-200mm f/2.8L IS III USM. In the description of the lens, you can see that it opens up to f/2.8 and has a variable focal length of 70mm to 200mm. Since many portrait photographers prefer the compressed look of a longer focal length shot, set your lens to around 100mm.

In this hypothetical situation, you are looking to get a really nicely blurred background (called bokeh). Set the aperture of your camera to f/2.8, which lets the most light in and gives you a nice smooth out-of-focus background. Set your ISO to 100, or whatever the base ISO of your camera may be. Line up the shot and press the shutter halfway down to meter the scene. What shutter speed does the camera display in the viewfinder?

For simplicity, let’s assume you are not using IS. Your lens is set to 100mm, so you should be shooting at 1/100s or faster on a full frame or 1/160s on a crop sensor. Much slower and you may see your subject move anyway. If your shutter speed is good, then click away. Don’t forget to use your camera’s review function and zoom in to verify focus. Make sure you don’t have any blown highlights (something we’ll discuss in the future) and you should be all set.

If your camera is reporting less than 1/100s (or 1/160s), you’ll need more light or more sensitivity. You can’t open the lens any wider since you’re already at f/2.8, so to the ISO settings we go. Start with small increases until your shutter speed is acceptable. Take your shots, and once again, review them carefully, always zooming in to check for focus. If you’re happy, you’re done!

Sample Aperture Priority Landscape Setup

Landscape photos are a completely different animal. You really should be using a tripod in most instances for best results. This is a good thing, because it takes camera shake out of the equation. Remember earlier in this article when we talked about minimum shutter speeds? Throw that all out the window when your camera is sitting on a tripod. That 1-2 second exposure is no longer an issue because your camera body won’t move a bit.

Long exposure photo example in low light, on a tripod. See those blown highlights in the sky? That’s why you should check for those things 🙂

But quickly, let’s assume for your photo that you’re using a Canon EF 16-35mm f/4 IS USM lens attached to a full frame camera body and you’re shooting at 16mm. We know that your shutter speed (removing IS from the equation) should be at least 1/16s for a handheld shot. Just something to mention if you’re out and about and don’t have a tripod or substitute handy. But since we’re using a tripod, let’s focus on getting that shot.

Aperture is a big deal, once again. When you were shooting that portrait, you wanted a nice & blurry background, but now that you’re shooting a landscape scene, we’d like the whole image to be in focus. This means that you’ll need to stop down, or choose a smaller aperture. f/8 or f/11 are usually good starting points to get everything in focus with your wide-angle lens.

You’ll want a clean, noise-free image, so set your ISO to base level (probably 50 or 100) to get that nice image. Since you’ve stopped down to f/8 and are running at ISO 100, chances are that your shutter speed might be pretty slow if it isn’t bright outside. That’s OK! The tripod makes sure your camera won’t move during the shot. Of course, you still need to make sure the shutter speed is appropriate for whatever your subject is. If there is movement you want to stop, you’ll need to make sure the shutter is fast enough to do so. If you want to slightly blur running water, then your shutter speed needs to be slow enough to capture that movement. Experience and experimentation will go a long way here.

Take the shot, using a 2-second timer so you don’t shake the camera, and then of course review it. If it looks good and you haven’t blown any highlights, you’ve potentially taken a nice landscape shot.

Final Thoughts

I hope this article helped you sort out how you can set up shots in aperture priority mode. This mode works really well for running around and getting photos of kids and pets, and some landscapes too. In the future, we’ll discuss shutter priority mode and manual mode. Once you have those three modes mastered, you’ll have the tools you need to really start getting creative. Thanks for stopping by!