I’m in a photo 1 class and talking about the program mode and flexible programming is confusing. If changing one setting changes the others to meet it, how is anything changing?
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Each of the three settings – shutter speed, aperture and ISO – control how much light gets in. If you increase one of them, you need to decrease one of the other to compensate, and vice versa.
However, each one also has a side effect. Faster shutter speed means it takes less time to take the photo, so if you want to shoot something moving, you should use faster shutter speed (unless you want to capture the motion blur). Larger aperture means ~~smaller focal length~~ shallower field depth, so you can get shots where a small part of the photo is in focus and the rest is blurred (good for portrait shots, for example). On the other hand, smaller aperture means more of the photo will be in focus (good for landscapes). Higher ISO means the photo is noisier, so normally you would want to avoid that.
To take a “good” photo, you need to strike a balance between 1) how much of a scene is in focus, 2) how long the sensor/film is exposed to the scene, and 3) how sensitive to light you want the sensor/film to be.
Say you want to photograph a pretty butterfly. To get a “good” photo, one where the butterfly isn’t too blurry or too dark/bright to see, these three settings need to be in a balance of sorts. When you’re picking a camera mode, you are essentially telling the camera what setting YOU want to control manually and what settings you want the CAMERA to adjust automatically to try to strike that balance:
Program Mode = I want to just point at something and take a photo. Camera, you automatically control EVERY setting and do your best to give me what you think is a good picture.
Aperture Mode = I want to control how much of my photo is in focus. Camera, you automatically control shutter speed and light sensitivity to give me the best picture you can.
Shutter Mode = I want to control how long I expose the camera sensor/film. Camera, you automatically control how much is in focus and light sensitivity to give me the best picture you can.
Manual Mode = I want to control EVERY setting about the photo. Camera, you’re benched; you control nothing, here have a Gatorade.
Let’s go back to our butterfly photo example (to simplify I’m going to use totally made up non-camera setting numbers here). Say you want to control how much of the butterfly is in focus, so you choose Aperture Mode. For your first photo, you want the whole butterfly in focus, so you choose the Aperture setting of 3. Based on this, the camera automatically adjusts the shutter speed to Medium and the sensor sensitivity to Normal. Voila, you have a great photo!
Now for your second photo, you want just the head of the butterfly in focus so you choose Aperture setting of 1. Based on this, the camera automatically adjusts the shutter speed to Slow and the sensor sensitivity to High. Snap! You have the photo you want, woo!
In both cases, what’s changing is the “behind the scenes” stuff that you told the camera you don’t care to control yourself in an attempt to give you a balanced and “good” photo. Under many circumstances, you probably won’t even notice the difference between the two photos aside from the one change you specifically made. I could go on about when you would notice the difference, but I believe that’s outside the scope of your question.
If you’re in a class you’re already paying for the instructor, so asking them should be higher on your list of ways to get an answer.
Background reading: [Exposure triangle](https://www.bhphotovideo.com/explora/photography/tips-and-solutions/understanding-exposure-part-1-the-exposure-triangle) and [program mode](https://digital-photography-school.com/understanding-program-mode-on-your-camera/)
Three variables contribute to the total exposure. If you increase one than one or both of the others decrease to keep the total. Aperture affects how thick of a slice is in focus (depth of field). Shutter speed affects how long the exposure is in time (motion blur or freezing). ISO or sensitivity controls how much final arises from the light input. A higher ISO can make a bright-enough image with less light, at the expense of more noise.
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