The “P” of PSA & M stands for the PROGRAM MODE, or on some cameras the “Program AE mode”. They’re both the same: AE just stands for Automatic Exposure, which is a good reminder of what the “P” stands for: In program mode you’re telling the camera that you’d like IT to decide two of the three exposure controls–shutter speed and the aperture–for you. You may say, “Isn’t that what AUTOMATIC does? Yes, in fact Program mode is often called “Automatic for Experts”. There is a difference though: In Automatic your camera is deciding EVERYTHING for you, including your white balance, and whether or not your flash will fire. In Program mode, it’s deciding only your EXPOSURE settings, but even that requires some further explanation: In Program mode, the TWO controls that are found on the MODE dial–Shutter and Aperture–ARE made automatically. But what about ISO? If you’ll go to your ISO control–and you may have to refer to your user manual to find it–bring up the menu of ISO choices. There you’ll find that the list typically includes fixed values such as 100, 200, 400, 800, etc…. but there’s also a choice for “Auto”. If we choose Auto as our ISO setting while in program mode, then all three of our exposure settings ARE being made for us, just as they are when shooting in full Auto. So what’s the advantage of using Program mode? Eventually you’ll come to appreciate the fact that Program LETS us lock in our ISO settings! And as I mentioned, Program also lets us control our white balance if we’d like, and also whether or not our flash will pop up and be used in low-light conditions. AND there’s something else! Watch this:

If you have your camera with you, place your mode dial on the “P” setting. Aim at some object and meter the shot by pushing the shutter release button down halfway. Keep watching the numbers that appear in the viewfinder as you lift your finger from the button and spin the dial that you should find right next to the button. Do you see two sets of numbers that are changing as you spin it back and forth? Can you guess what they are and what they’re telling you? Right! One’s the shutter speed, and the other is the aperture. This is called “Program Shift” by some manufacturers, or “Flexible Program” by others. The reason they’re moving is that you’re being given a chance to select the shutter speed and aperture COMBINATION that suits your needs, with the assurance that each combination of the two should result in the same exposure exactly. As one moves to let in more light, the other one moves to let in LESS light by exactly the same amount. Spend some time studying how these two numbers move together. The ISO value doesn’t appear to be involved, but in fact what you’re seeing is a great illustration of how all THREE of the exposure controls relate to each other! It’s absolutely essential to understand this partnership between the three exposure controls. It’s also important learn the LANGUAGE that all three of them speak! Consider this:

Have you ever wondered how international airline pilots who speak different languages are able to understand instructions from the airport control towers around the world? You’re right! There has to be a common language that they ALL speak. Ever since World War II, English has been the universal language of the international commercial airline industry, and every pilot has to be proficient in English in order to fly into international airports.

Photographers working with the three exposure controls have a similar challenge: Shutter speeds are expressed in seconds and fractions of seconds. Apertures are expressed as the ratio of the focal length to the diameter of the aperture. And ISO is a simple number scale that quantifies relative film speed in film cameras, and the sensor’s sensitivity to light in digital cameras. How were photographers supposed to go back and forth between the three, making offsetting adjustments quickly in the field? What was needed was a single language that could be applied to each of the three. That language is expressed in “STOPS”.

A STOP is simply a doubling or cutting in half of light, depending on whether you’re increasing light or decreasing it. The three lists of settings above each express exposure adjustments in one stop intevals. Let’s say that you just took a picture of a child playing. You like the level of exposure, but you notice that the child’s arm is motion-blurred because your 1/60th of a second shutter speed was too slow. You decide to INCREASE your shutter to 1/120th – or one stop, which should be enough to eliminate the blurring, but you now fear that the image will

darken and become underexposed. In order to make up for the one-stop reduction in light from the faster shutter, you’ll have to either increase your lens’s aperture by one stop, or increase your ISO by one stop. Either one would work, but let’s use ISO. If it was set at 200 before changing the shutter, raising the ISO to 400 would double the sensor’s sensitivity to light, or increase it by ONE STOP. One stop LESS light coming in from the faster shutter, and one stop LESS light NEEDED… because of the more sensitive sensor, results in exactly the same exposure for both images.

I mentioned that Program mode is often referred to as “Automatic for Experts”, and it can be a good general purpose “walk-around” setting, for times when you’re not sure what shooting situation you may encounter. Program doesn’t guarantee good exposure results, but it should at least provide a good initial settings combination that you can fine-tune after seeing your results, either by using Exposure Compensation, or by transferring the shutter and aperture values to the Manual mode and adjusting them directly yourself. We’ll discuss that technique in our chapter on Manual mode soon.