The mode dial is the thing on the top of the camera that has a bunch of weird icons, and the following letters: P, A, S, and M. or P, av, Tv, and M if you're a canon shooter.
Mode Dial on a Canon 5Dmk3
PASM is an abbreviation of 4 modes for controlling how the camera behaves.
Canon cameras have "PTVAVM", or "MAVTVP", either way it doesn't roll off of the tounge as well. We will use 'PASM'. Same thing.
Camera's have --three-- two main ways to control how much light gets into a camera. Shutter Speed and Aperture.
We'll talk about the third, ISO, later. Aperture and Shutter Speed affect not just exposure but the look of an image. ISO is not used to creatively affect the look of an image, so it isn't as important to understand yet.
P for Program
P is the new auto mode. Now more green camera or A+ or green rectangle mode. It's P mode for you from NOW ON. P mode handles getting the image to look right for you, but allows you to - if you want - control other settings.
Settings like focus modes, drive modes, exposure compensation, and more fancy-schmancy camera talk that we will get back to later.
S (or Tv) for Shutter priority (or Time-Value)
This mode gives a new control for our camera, a dial (probably by the shutter button, on the top. Maybe on the back), that lets us control what the shutter speed is. How long the camera lets light in for. We'll talk about shutter speed later. Just now that it is measured in seconds, a unit of time; and S mode gives us control over it.
Canon calls it "Tv". Everybody Else just calls it "s".
A (or Av) for Apeture priority (or Aperture-Value)
Or Av for Aperture-Value.
A stands for Aperture. This mode gives you total control over the size of the hole in the lens, and the camera adjusts everything else to get the shot right. Otherwise, it's the same as P mode. We'll be talking about what this means later, just know for now that A mode gives you more control.
M for Manual Mode
This mode lets us control both the aperture, and the shutter speed. Also the ISO, which We'll get to later. Manual mode means the getting the "right" amount of light is up to us, and the camera won't make any decisions.
This is often important when you want to shoot consistently - like when stitching images together - and the camera's decisions can fluxuate slightly. It's also super useful when we want to take photos that are too dark or too bright.
Later you'll be out and about shooting in ONLY manual mode. We aren't there yet. Relax. Everything is going to be okay.
Mode Dial on a Canon 70D
Burn The Flower
No more flower ("macro") mode. No more person-running ("sport") mode, and no more mountain ("landscape") mode. These are just variations of program mode that perform better in certain situations.
Sport mode, for example, shoots with a faster shutter speed so shot's are less likely to be blurry. From now on, if you had to shoot a sporting event and didn't want blurry shots, you'll go into S mode and dial up the correct settings yourself.
This advanced control is important because while cameras are really really good at what they do, they're still limited by many assumptions - the camera doesn't know what it's pointed at. Only you know what you are trying to capture and emphasize. It's also important for learning how these things work!
Becoming a photographer means taking control over these decisions.
The Camera is a Calculator
Let's go over how What The Camera Does is being changed by the different shooting modes.
So the camera measures the scene and comes to a conclusion about how much light we need. Let's pretend this is some single unit number, (Perhaps we call this an "exposure Value) and let's pretend that it arrives at the number 10. We need 9 light for the scene.
Let's say the ISO gives us 3 units of exposure value, the aperture gets another 3, and the shutter speed gives us the final 3 to get to 9 imaginary units of light. These 3 elements of the camera all factor into how much light there is.
Now, in reality, these elements are probably being multiplied by each other. Like Shutter Speed x Aperture x ISO = Exposure Value. Just... shh. Relax. Relax. Everything is fine. Math is easy. This part doesn't matter right now. What matters is that all three of these elements contribute to the exposure of the camera. Our camera, on P mode, does all of the math for us. It decides how many units of shutter speed, aperture, and ISO to combine together to get the exposure.
On program (p) mode, the camera does it all for us.
On Aperture-priority mode, (A or Av mode on the dial), we control the aperture. We say, okay. I want an f-stop of 1.8. I want an f-stop of 8. We are deciding this unit. Like a recipe, I'm going "I want this much butter no matter what" and the camera is taking the ISO and the Shutter speed (the flour and the salt?) and changing them so our equation works out, and we get the right exposure.
Shutter priority (time value) mode is the same thing, but we control the shutter speed, we lock in this one unit of our equation, and the camera controls the other units. It's still trying to do the same thing, to get the right amount of light into the camera.
There is no ISO priority mode.
Manual mode, we control the shutter speed and the aperture and the ISO. Fun fact, is many cameras can have the ISO set to a specific number or on 'AUTO' on all of these modes. So we can have ISO+Aperture locked in on aperture-priority mode and the camera changes the shutter speed, or we can be in manual mode and putting in totally wrong settings with both the aperture and the shutter speed, but the images look okay because the camera is changing the ISO for us. Then we walk around like "aww yeah I'm shooting manual and I'm nailing the exposure every time" and that's not really true, but I'm not going to take that from you. You can keep that little victory.
Why no ISO priority mode?
The first reason is because the ISO doesn't really affect creative decisions. It doesn't change how an image looks, other than grainyness or noisiness. The higher the ISO, the "worse" the image, but it's otherwise the same. One always optimizes for a low ISO, and the camera will just do that for us.
So the first reason is because we don't need one. It would be silly.
Besides, you can go to program (P) mode and still lock the ISO to a specific number on most cameras, which then is basically ISO-priority mode.
The second reason is because of film. Before digital cameras, ISO wasn't something you could control with the camera at all. It came from what film you decided to buy, with different films rated to different ISO's. Different films were more or less sensitive to light. You would tell the camera "Okay, I have ISO 400 film in here now" and the camera wen't "okay" and then you uses your different shooting modes.
Digital cameras came around and we didn't add an ISO shooting mode, we just added a control that let one change the ISO amounts, or set it to auto. Cameras still behave the same way, otherwise! Neat!