Hello Sorashira, Welcome to the Pentax Forum!
Yes, there is a lot to learn and many of the terms are confusing. But the ones you mentioned, exposure and depth of field (DOF) are pretty simple, if you look at it in a purely mechanical way.
The correct exposure is a result of the amount of light that passes thru the lens and the amount of time it is allowed to pass. We control both.
The amount of LIGHT is controlled by the "F-Stop" or aperture ring on most older lenses. Lenses for digital cameras have eliminated this ring, but the function is still available through (usually) a dial on the camera body. These are numbers like f/2.0, f/2.8, f/4.0, etc.
The amount of TIME we allow the light to pass is controlled by the shutter speed. Again, this was set by a marked dial on top of film cameras, now it is controlled by a dial on the front or rear (top) of digital bodies. Again, same function, the selected speed is shown on the LCD or in the viewfinder. These numbers are 1/30s, 1/60s, 1/125s, etc.
Now we know that 1/60s is twice as fast as 1/30s, right? Or 1/1000s is twice as fast as 1/500s. This is important to remember, because if the shutter is only opening for half the time, only half as much light will reach the film or digital sensor.
Imagine you have a bucket of sand. You are going to pour the sand into another container, through a funnel. If the funnel has a large opening, the sand will pour quickly. Say it takes one minute to fill the second container.
Now, you try a smaller funnel. This one has a smaller opening, exactly 1/2 the size of the first one. How long does it take to fill the container?
Right! 2 minutes.
Light reaching the film or sensor works in exactly the say way. If we close the aperture (opening) by one f-stop (allowing 1/2 as much light in), we must double the shutter speed (time) to allow the same amount of light to pass.
1/30s at f/2.8 results in the same amount of light reaching the film/sensor as 1/60s at f/2.0.
1/1000s at f4.0 equals 1/500s at f5.6.
All a bunch of confusing numbers, I realize. But what's important to understand is that reducing the f-stop not only requires a longer shutter speed (smaller opening, more time, right?) but it has a profound effect on the photograph.
Depth of field. This means the "deeper" the focus.
This may be easier to see than explain. The smaller the f-stop opening, the more objects in front and behind the main subject are in focus.
To help you visualize this, here's a series of photos taken from the same location, with the same lens. Only the f-stop and shutter speed relationship are changed.
SMC Pentax-DA* 200mm F2.8 ED [IF] SDM Reviews - DA Prime Lenses - Pentax Lens Reviews & Lens Database
Scroll to post # 10, by Matjazz.
Notice how, at f/2.8, only a small portion of the logs are in sharp focus. As we close the aperture (higher f-stop numbers), more detail in front and behind, come into sharp focus. More...
Depth of field.
Hope this helps,
Good luck!
Ron