P-Mode, Let the camera be your guide
In the last post we introduced the 4 different exposure modes, Programmed Auto Mode (P), Aperture Priority Mode (A), Shutter Priority Mode (S), and Manual Mode (M). We also introduced the concepts of shutter and aperture. This time we will explore the first and simplest of the exposure modes, Programmed Auto Mode, or the P-Mode. Simplest from the photographer’s point of view as the camera makes all the decisions for you, but from the cameras point of view, there are many complex calculations going on under the hood of your camera.
As you know from our previous post, there are 2 items that are involved in creating proper exposure, shutter speed and aperture. The shutter speed controls the length of time that light is allowed to hit your film or digital sensor. Aperture controls the size of the opening of the shutter, or how much light is allowed to hit your film or digital camera during the amount of time that the shutter is open. These 2 values combine in a very fancy mathematical formula to create a value known as Exposure-value or EV.
Exposure-value (EV) is basically the unit that is used to measure exposure. Similar to how we use Miles-per-hour to describe speed. When you are traveling, your speed is made up of how far you are going and how long it takes you to get there. We combine these 2 items to get your speed or Miles-per-hour. Same thing with EV. EV is made up of shutter-speed and aperture-size which combine to get your exposure measured in EV.
I don’t want to bore you with the actual formula for EV, nor is it really important for you to know the formula at this stage in the game. “Yay, Scott said we don’t need to know about EV”! Wait a min, hold the phone, I DIDN’T say that 
I said you didn’t need to know the actual formula, but you definitely need to know some things about EV, even while you are shooting in P-Mode. This is because even though the camera is making the exposure decisions for you, it is important for you to know why and how those decisions are being made.
So what do you need to understand about EV? We already know that EV is made up of shutter-speed and aperture-size. The important thing to always remember is that these 2 items are inversely proportional. (Scott, you’re using those fancy words again). I know, bear with me a moment. Shutter-speed and aperture-size are inversely proportional, what does this mean? This means that they are opposite of each other.
Let’s say we have a certain shutter-speed of 1/125 s, remember shutter speed is measured in seconds or fraction of a second, and an aperture value of f/4.0, recall that aperture is measured in “f-stops”. These 2 values combine to create an EV of 11. If we change either of these values we will end up with a different EV. Such as if we use a faster shutter speed of 1/250 s. This will allow light in for half the time of the previous setting of 1/125 s, and we’ll end up with an EV of 12.
But something interesting and unexpected happens if we now use a larger aperture value of f/2.8, this allows double the light in of our previous aperture setting of f/4.0 and we actually end up back at EV 11. This is because, as we mentioned before shutter-speed and aperture are inversely-proportional. If you make one bigger and the other smaller you’ll end up back at the same place. There are many different combinations of shutter and aperture that will produce the same EV. This is known as reciprocity.
Reciprocity is an odd concept to grasp, but makes sense if we relate it to something we know. Let me take you back to Kindergarten…no it’s not naptime, it’s math time! Yeah naps are more fun..let’s take addition:
2 + 2 = 4
Let’s say the first 2 is aperture, the second 2 is shutter-speed, and the 4 is our EV. If we increase the shutter-speed…we get a different EV result of 5:
2 + 3 = 5
However, if we change the aperture…we get back to the original EV result we started with of 4:
1 + 3 = 4
We can use several different values to combine and achieve the same result. Same thing with exposure, we can use several different aperture and shutter values to combine to get the same exposure. This is important because, as you recall from our last post, aperture and shutter also control 2 other things. Shutter-speed controls just how “frozen” the action is in your scene, and aperture controls your depth-of-field or how much of the scene is in focus. This allows us to have many options when it comes to exposure. Allow me to explain…
We have our camera in P-Mode, and press the shutter button down half-way to focus and measure the light in our scene. The camera decides it will use a shutter-speed of 1/250 s and an aperture value of f/4.0. This calculates to an EV of 12. So we know that in order for our photo to be properly exposed we need to have an EV of 12. However, I forgot to mention that we are actually taking a portrait photo of a little kid and I want the background to be a nice blur, with a shallow depth-of-field. In order to do this, I want to use an aperture value of f/2.8. This will create a very nice shallow depth-of-field. But, by increasing the size of my aperture to f/2.8 I’m now allowing double the light of my previous aperture value of f/4.0 to obtain an EV of 11. This means our photo will be over-exposed because I need an EV of 12 for proper exposure. So in order to bring things back into balance I want to increase my shutter speed to 1/500 s. This will halve the time that the shutter is open and therefore bring me back to my proper EV of 12, while still retaining that beautiful blurring of the background which is so great for portrait shots.
When you’re in P-Mode not only does the camera have to think about all of these fancy exposure calculations and equivalents, but it also has to determine the type of scene that you are shooting. Today’s cameras are very sophisticated machines, with all kinds of sensors that can determine whether we are shooting a portrait by a lake or a fast moving baseball player as he slides into home. Of course sometimes, the camera can be fooled or get confused and choose an aperture value or shutter-speed that may not be correct for the type of scene we are shooting.
This is where the other exposure modes come in…next time we tackle Aperture Priority Mode (A).
In the meantime, go out and try shooting many different types of scenes. Take a look at the aperture and shutter values that your camera chooses for the particular scene you are shooting. Did the photo turn out the way you were expecting? Did the camera choose values that make sense? What would you change about the photo, should it be sharper? Should there be more or less in focus? Should the action have been more “frozen”? Ask these questions each and every time you take a photo in P-Mode. If you continue with this logic you will quickly find that staying in happy P-Mode where the camera does all the thinking for you may not be the best choice, and that you want more control over the finished look of your photos.
Feel free to comment and post some of those results. Until next time…
