A, Is for Aperture Priority

I don’t recall exactly how long it took before I was able to look at the letters on the menu dial of my camera and see anything other than alphabet soup. It certainly wasn’t overnight after first getting the camera, weeks at a minimum but probably closer to months.

In part this was due to the ability of the modern DSLR camera to auto select settings that produce good photos. Another factor was the menu of scene modes on my Nikon D70. When taking pictures of people, I used the Portrait setting. I also used the Landscape, Sports and Night settings. The names said everything I needed to know about their function and made it easy to explore settings other than Auto. Yes, I was getting all radical with my camera and trying different menu dial positions just weeks after getting it.

But the real experimentation started when I took the leap of faith required to dive into the alphabet soup settings on the menu dial: A, M, P and S. In addition to being an anagram for, spam, those four letters tease and temp a new photographer with hints of the unknown and mysterious. What happens when you choose, A? Will the camera even function?

The Command Dial on most DSLR cameras allows you to choose between a variety of scene and other settings, including Aperture Priority (A)

The Command Dial on most DSLR cameras allows you to choose between a variety of scene and other settings, including Aperture Priority (A) (Graphic used courtesy Creative Commons)

A, is for Aperture Priority, a setting where you manually select the aperture and the camera fill in the other settings. An aperture is an opening that allows light to pass through the lens and fall on your camera’s sensor. If you’re framing a shot at high noon on a sunny day, there’s a lot of light hitting your subject. This abundance of ambient light means your camera’s aperture can be small and still allow enough light to hit the sensor to capture a beautiful image with a short exposure. This brings us to the concept of f-stop or f/ratio. These terms mean the same thing, the ratio of the focal length of your camera lens to the diameter of the lens opening (aperture) allowing light to fall on the camera sensor.

Suppose your shooting with the kit zoom lens that came with your camera and the focal length (zoom) is set to 50 mm (millimeters). Put the camera in auto and take a picture of something. Then, play the photo and look at the information about that exposure. In particular look for the f-number. The number next the f tells you the ratio of the aperture to lens focal length. A picture taken with a 50 mm lens at f/10 had a 5 mm (1/5-inch) aperture. A picture taken with a 50 mm lens at f/4 had a 12.5 mm (1/2-inch) aperture or opening.

So, the smaller the focal ratio or f-number, the larger the aperture. In short, when you shoot with a small f-number, you’re letting more light into the camera to fall on the sensor. Or at least, your camera is trying to let in more light. When we started this discussion, we began by imagining a scenario of taking a picture at high noon on a sunny day. Let’s change the time from high noon to the golden hour, that first hour after sunrise or last hour before sunset when natural light is soft, warm and dramatic. The light quality is better but there’s not as much of it. So, in order for your camera to allow the same quantity of light on its sensor as a shot taken at noon, the aperture needs to be larger.

The begs the question, so what? Why should you care that the aperture needs to be bigger during the golden hour than at high noon? Well, aperture determines the depth of field in a photo. Depth of field is the range of distance within which objects or people will be in focus in a photograph. When shooting portraits, it’s often more pleasing to have a shallow depth of field. In other words, the person who’s the subject of the photo is in focus but anything in the foreground or background will be out of focus. This quality of an out of focus foreground or background is called, bokeh. Bokeh is a Japanese term describing the portion of a photograph that is obviously out of focus. When shooting a portrait, obvious bokeh in the background is typically very pleasing to the eye. The way to create bokeh in your shot is to shoot using a small focal ratio or f-number, the smaller the better.

Northern Arizona's summer monsoon calls forth brilliant yellow wildflowers near Flagstaff, Arizona. (Bill Ferris)

Northern Arizona’s summer monsoon calls forth brilliant yellow wildflowers near Flagstaff, Arizona. (Bill Ferris)

Above, is an example of bokeh. The subject is in focus in the foreground and the background is obviously out of focus. This transforms the background flowers into a pleasingly abstract tapestry of hues and textures complimenting the subject of the photo.

Suppose you’re shooting a landscape at sunset. A wonderful, warm light is painting the temples and buttes of Grand Canyon. For this shot, you’ll probably want everything in your frame to be in focus. You’re not trying to capture just one temple or butte. Everything in your frame needs to be in focus. In this scenario, a shallow depth of field will put much of the frame out of focus. To put everything from the most distant butte to the blooming foreground wildflower in focus, you need to shoot using a large focal ratio. Typically, I shoot at f/9 when doing landscape photography.

 (Bill Ferris)

The last light of day paints Desert View Watchtower on the South Rim of Grand Canyon a deep rusty red. Desert View overlooks the Palisades of the Desert and the Colorado River in eastern Grand Canyon National Park. (Bill Ferris)

This photo of Desert View Watchtower on the South Rim of Grand Canyon was taken at sunset. My Nikon D90 was mounted on a tripod. The camera was in aperture priority at f/9, ISO 500, a lens focal length of 19 mm and an exposure of 1/30-second.

This brings us to the final question of this blog entry. What impact, other than depth of field, will shooting at f/9 have on your photo? As we’ve discussed, the larger the f-number the smaller the aperture or lens opening. If shooting at sunset to capture a landscape illuminated by that gorgeous, soft light, you can compensate for the small aperture by boosting the ISO setting. The same scene that can be captured with an ISO of 200 at high noon, may require an ISO setting of 800 during that dusky time around sunset.

But there is a price to pay for taking this approach. Boosting the ISO to make your camera more sensitive to light adds digital noise to your photo. With higher end digital cameras, images shot at ISO 800 often look very good. But if you’re shooting with an entry-level camera or an older DSLR, an image shot at ISO 800 will often looking grainy and rough. Rest assured, however, there is another solution.

Use a tripod. For landscape photography or any photography where you want to capture sharp, detailed images with good depth-of-field, a tripod is essential gear. Mounting your camera on a tripod allows you to precisely frame the shot. It also allows you to shoot at low ISO’s of 400 or less during the golden hour and still capture great detail. The scene that was perfectly captured with a 1/500-second expose in bright sunlight may require a 1/10-second exposure at a time of day when the quality of the light is warmer, softer and more dramatic.

Now, 1/10-second may not seem slow but trust me, it is slow in photography. As a general rule when shooting handheld, you want exposure times to be no slower than the reciprocal of the lens focal length. OK, in English. Let’s say you’re shooting with a 50 mm lens. If you’re holding the camera in hand for the shot, the slowest shutter speed you’ll want to use is 1/50-second. Faster would be better. Any slower speed will allow the camera to capture the vibration or unsteadiness of your handhold. That vibration will soften details and produce an image that looks out of focus. If the camera is mounted on a tripod, that rock solid platform will allow you to take longer exposures producing crisp, focused photographs.

OK, I’ve rambled long enough. The next time you’re out with your camera, experiment with using the A or aperture priority setting. Shoot the same subject using different apertures. Compare the ISO settings and exposure times for the resulting photos. Most important, compare how the photos look. Which one looks best to you? Write down or make a mental note of those settings and spend the rest of the day shooting with them. You’ll learn a little more about your camera and you’ll get some great shots.

Get out and shoot!

Bill Ferris | August 2013

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