Missing Pages: Program AE
The Missing Pages column is a collection of all of the information that should have been included in your camera’s Owner’s Manual—but somehow got left out. This is a hybrid assortment of short articles that delivers the know-how you need to derive the maximum enjoyment—and creative expression—from your equipment.
It’s sort of a juiced-up User Guide for creative people who are not necessarily technical. Each part will teach you how to use one of the camera features or functions that you previously ignored or left set on Auto. And each will include a Creative Project so that you can try some scripted experimentation.
We will explain complex technical subject matter a way that everyone can understand. And if you happen to be a technical expert yourself, we’re including “Nerds Only” sidebars just for you. That way you can dig in deep—or just straddle the edges—of the technological stuff. It’s your decision.
Installment II: Program AE
[slightly more extensive than the auto/green mode]
Definition: an auto exposure system in which the camera achieves correct exposure by selecting the shutter speed and aperture (f/stop) from a predetermined table.
To understand Program AE you must understand the fundamentals of exposure. The amount of light that reaches the imaging sensor is determined by two physical factors: the size of the lens opening (aperture, or f/stop) and the length of time the light is allowed to travel through that opening (the shutter speed). The sensor doesn’t care whether you use a small aperture for a long time or a large aperture for a short time, just so long as the net result is that the same amount of light reaches it.
In other words, it’s possible to achieve exactly the same precisely correct exposure using different combinations of apertures and shutter speeds. For example, you can shoot at f/2 at 1/500 second, or f/8 at 1/60 and either combination will yield correct exposure. However, which combination you choose can have a profound effect on your image.
Please take a look at the accompanying graph. The red line shows Equivalent Exposure. Shooting at f/16 for 1/8 second is equivalent to f/5.6 at 1/60. The green line is called a Program Slope. It shows how a typical Program AE system might automatically adjust shutter speed and f/stop settings in reaction to differing light levels. With most cameras, the steps are seamless and infinitely variable but follow the same slope—which means you may shoot at 1/131 of a second at f/4, for instance. In this example, Program AE forces the camera to use a high shutter speed as much as possible, and never drops below 1/60 of a second.
Program AE makes rational choices for you. In general, Program AE tries to shoot at a relatively fast shutter speed because it operates under the normally correct theory that most people have a hard time holding cameras still at slower speeds. Program AE delivers correct exposure and good results most of the time. But when you use this mode the camera is making two critical decisions for you, and whenever the camera is doing the decision making, you are losing creative control.
Remember, your creative ambition is to get what you want, not merely want what you get.
Look at the image of the young girl holding a snail. The hand is in focus but the girl’s face is blurred. The image was not shot in Program AE. It was shot at wide open at f/2.8 to obscure the human and emphasize the creature.
If you want to control depth-of-field you must control the aperture. Small apertures produce greater depth-of-field than large ones (all else being equal). If you want to control subject motion (e.g., freeze action at a sporting event) you must have control over the shutter speed. Slow speeds allow subject blur whereas fast shutter speeds do not.
Camera manufacturers are aware that not every program setting meets the needs of the shooting environment, photographer’s creative impulse and subject’s special requirements. So they have provided two effective ways to provide photographers with creative control. The first is called Program Shift.
To use Program Shift one simply changes the shutter speed or aperture. The other corresponding value changes automatically to keep exposure correct. For example, if the Program Slope calls for a setting of 1/250 at f/5.6 (as in our charted example) but the user wants more depth-of-field, the correct action would be to stop down the lens. If the user selects f/11 while leaving the camera on Program AE, the shutter speed will automatically drop to 1/60 second. Exposure is still correct, and camera is still set on P, but the user has exercised creative awareness—and control.
The second mechanism is given different labels by different camera makers. Canon calls them Shooting Modes; Olympus refers to them as Scene Modes. We’re going to call them Scenes and reserve the word Mode to refer to bigger chunks like Program AE Mode, Aperture Priority Mode (which we’ll discuss in a later installment) and other more all-encompassing camera controls. Typical examples include Landscape, Portrait, Sports/Action and Fireworks.
If you’ve digested the Program AE slope that’s displayed on the accompanying chart you can easily guess that when a camera is set on Landscape, the Program Slope shifts to deliver a smaller aperture. Conversely, when set to Portrait, a larger aperture is selected to limit depth-of-field and separate the subject from the background. Virtually all cameras go one step further and make coordinated adjustments to Saturation, ISO, Sharpness—whatever the camera firmware design engineers feel is appropriate. On most cameras, when you use the Landscape or Portrait setting—or any other contrived, situation-specific scene—you are shooting to suit someone else’s taste.
If the tone of this piece seems slanted against Program AE, let me clarify: like any other camera setting, it’s a tool. It has no intrinsic value and should be neither favored nor despised—instead it should be used when you need it and ignored when you don’t. Most people get better results using the P setting than they do using the camera on fully Auto, so it definitely has a place.
But to use it effectively you must understand it, and that means knowing exactly what exposure settings are made—which f/stop and what shutter speed—when your camera is set on Program AE. There are two ways to do this.
The easy way is to look in your owner’s manual—easy if and only if the data is actually contained in your OM. In many cases it is not, which is incomprehensible to me since it’s critical information. Of course, you can always contact the camera maker (by e-mail is probably best, since they’ll have to e-mail the charts to you, anyway).
The hard way is to do it empirically, through actual trail and error. If you take this route I admire your pluck and remind you to make sure that you keep the ISO constant through all of the trials. When you give up, call the manufacturer. The point is this: Program AE Slope is important information for creative photographers, and camera manufacturers should tell us what it is.
Okay, that was boring. Here’s something more fun. Set your exposure mode dial on Program AE (usually marked “P”). Grab your favorite human subject (figuratively speaking) and park them in front of a large textured background that’s at least 30 feet behind them—a stand of trees or colorful building is ideal. First, shoot at the setting selected by the camera. Next, use Program Shift to reduce depth-of-field by selecting a larger aperture (this may be impossible if you’re not using a DSLR). Finally, set the camera to Portrait Scene (or Mode) and shoot again. When you download the images, carefully note the exposure settings that your camera selected for the first and last shots, and compare them to the setting you triggered with Program Shift. Which delivered what you want?
The first camera to have a Program AE function was not the Canon AE-1 Program as is commonly thought. Do you remember which model had the function built-in but undocumented? The Minolta XD-11, the world’s first “Dual Mode” (Aperture Priority and Shutter Priority) SLR, which first appeared in 1977. When set to Shutter Priority, the XD-11 tried to achieve correct exposure using the shutter speed you selected. But if it was too dark (or too bright) it changed to something that worked. Although it was never advertised as such, the XD-11 had a Program Mode where the user picked the starting minimum shutter speed. Later, Minolta painted the “S” on the Mode dial green and encouraged users to set the camera on “Green for Go.” Lame, even for 30 years ago.
The Pentax K20D has the best Program AE option I have ever encountered. They call it “MTF Priority.” It gives preference to the best aperture settings for the lens you are using at the time. All lenses have a sort of “sweet spot;” an aperture that delivers the best combination of sharpness, aberration correction and DOF. This can be determined through exhaustive testing. Pentax knows lens performance characteristics as only a mother would, and they’ve coded that info into the K20D. I use this camera often and I use this mode almost exclusively when I shoot on Program AE.
Text ©2009 Jon Sienkiewicz.
Text ©2009 Jon Sienkiewicz.