Many entry-level or mid-level digital single lens reflex (DLSR) cameras have a dial on top of the camera called the exposure mode dial. It directly controls how the camera and the user interface to expose the imaging sensor to light.

Sometimes the dial resides in the spot that in years past housed the film rewind knob, and as camera makers morphed from film to digital, placing a round dial where once was a round knob is, in my view, somewhat uninventive.

Other cameras, particularly the smaller DSLRs, make that area too small to house a control of any size, and moved the exposure dial to the right side. This also did away with the LCD display in top of the camera.

The exposure mode dial usually features an AUTO mode which literally takes over most of the camera’s functions. I like to call it Aunt Millie Mode, since if you wanted to let Aunt Millie take pictures for a while, you would set it to this mode.

AUTO has some kissing cousins on the dial, called scene modes. Represented by tiny pictograms, they are meant to hold your hand through specific kinds of photography, like sports, macro, portrait, low-light, and so on.

The four most important exposure modes, which are the only modes used by professional photographers, are represented by the letters P, A, S, and M. The letters are not always in that order, but it’s easier to teach with the word “pasm” than “maps” or, to really confuse things, “spam.” The letters represent basic exposure modes…

• P  Program: The camera picks the aperture and the shutter speed for you.
• A  Aperture Priority (Av for Canon users): You pick the aperture, the camera picks the shutter speed for you.
• S  Shutter Priority (Tv for Canon users): You pick the shutter speed, the camera picks the aperture.
• M  Manual: The most obvious of the bunch, and the one that has the best teaching potential; you pick shutter speed and aperture manually.

Of these four basic modes, professional photographers tend to gravitate to one or two of them, and for me, it’s aperture priority. For reasons that remain at least somewhat unclear to me, I have an awkward, difficult time expressing to students the way I use aperture priority, especially while shooting sports. Since I came up through the film era using cameras that offered aperture priority or manual exposure control, it seemed a natural extension in the digital era to take advantage of this mode.

Aperture priority is my way of maximizing the potential of the scene. If I am shooting sports, for example, I almost always set my aperture to the largest setting, thus letting the lens transmit the most light it can. This has the added advantage of rendering the depth of field as shallow as possible, a big plus for isolating subjects for greater impact. With the lens wide open, often f/2.8 or f/4, I can move ISO up and down to give me the best shutter speed for the lighting conditions. Baseball, for example, is usually in bright daylight, so with my 300mm f/4 wide open and a lowish ISO like 200, my shutter speed falls in the range of 1/1000th or 1/2000th of a second, which is nice and fast. Night football, on the other hand, might require an ISO like 3200 combined with an f/2.8 to give me something in the neighborhood of 1/500th of a second.

If, on the other hand, depth of field is a requirement, aperture priority is a good choice again. With a wide angle, for example, I can select f/11 and let my shutter speed fall somewhere in the 1/30th to 1/125th range, since I don’t need blisteringly fast shutter speeds for a lot of wide angle stuff.

So much of what I try to convey like this to my students really only comes to fruition with practice and experience.