What I Know Of: Aperture-Priority Mode

One Degree Of Control

Okay, now things get interesting because the next set of modes expect you to control one or more of the critical exposure parameters.  In aperture-priority, a.k.a. "A", mode, you set the aperture and the camera chooses a suitable shutter speed to get a photo that it believes is the correct exposure.  (BTW, Canon and some other manufacturers call this mode aperture value or "Av" mode.  A slightly different name but it's the same thing.)  This is a semi-automatic mode since everything is automated except the aperture setting.  If you recall from my previous post about exposure, aperture affects depth of field (DoF) as well as exposure. So you'd usually use A mode when a particular depth of field is critical to the photo you have in mind, but you're fine with letting the camera figure out the shutter speed to use in order to get a proper exposure.  Some examples of this would be:

• You're shooting a portrait and you want the background to be out-of-focus to draw eyes to the subject.  In this case, you'd want a large aperture which results in narrow depth of field.
• You're shooting a landscape and you want everything from the rocks in the foreground to the distant mountains in focus.  Here you'd want a wide depth of field so you'd use a small aperture.
• You're at your daughter's piano recital and you want to photograph her performing but don't want to disrupt things with a flash.  In this case, maybe the DoF is not important but you might want to use a large aperture anyway to maximize the amount of light hitting the sensor.
• You're shooting inanimate objects (such as a still life or a product photo) with a tripod and you don't have to worry about camera shake or subject movement.

I use aperture-priority for about 75% of my pictures.  I often shoot subjects in which controlling the depth of field is fairly crucial, but freezing/blurring movement isn't as much of a consideration.  So A mode works for me in quite a lot of situations.

Shallow DoF Using a Large Aperture (f/1.8)

So How Does This Thing Work?

When I talk about a "large aperture" I literally mean that the diaphragm inside the lens is opened wide.  What might be counter-intuitive is that the numbering system used to denote aperture size (called f/stops) uses smaller numbers to represent larger apertures, and bigger numbers to represent smaller apertures.  (Some math geekery:  This is because the f/stop is actually the ratio of the focal length to the effective aperture diameter, or put another way, the focal length divided by the aperture diameter.  As the diameter increases, the resulting number, i.e. the f/stop, gets smaller.)  So the f/stop system looks like this:

Now, the settings above are "full stops", which means each setting allows half the amount of light through the lens as the previous setting. In modern cameras, aperture settings are usually sequenced in half or one third stop increments, so you usually see a lot of numbers in between the ones above as you scroll through the aperture settings on your camera, but the same idea holds -- smaller numbers mean larger apertures (narrower DoF) and vice-versa.

Depth of field is also affected by things other than aperture (such as lens focal length, sensor size, and distance to the subject) but generally speaking on a DSLR, an f/stop of f/2.8 or less is considered a "large aperture" with narrow depth of field.  A medium setting would be around the f/4 to f/5.6 range, which will throw the background out of focus a bit but give you a wider area of sharpness for getting more a a subject (or a group of subjects) in acceptable focus. And a large aperture with wide DoF would be f/8 or above.  These are approximations and the resulting DoF can change a lot depending on the other factors that I talked about.  The best way to get a feel for how your camera and lenses affect DoF is to take a lot of photos of the same scene using different aperture settings and compare the results.  As you might imagine, aperture-priority mode is perfect for this.

Some lenses provide smaller apertures than f/22, but frankly I never use them.  Depending on the camera and the lens, apertures smaller than f/11 and f/16 start exhibiting noticeable diffraction which manifests itself in a lot of ways but most obviously as reduced sharpness.  With my camera and lenses, f/11 or f/16 is about the limit of what I consider acceptable sharpness (I'm a bit of a stickler on this).  On the other hand, for macro photography the depth is field is often so shallow as a result of the subject distance being so short, that tiny apertures are required to get enough of the subject in focus and diffraction is simply a necessary evil. Once again, experimentation is key here because different gear, different subject matter, and different personal tolerances for image softness are going to yield different results.