f-stop / Aperature
Aperture refers to the lens opening, the smaller the opening, the lower the f-stop. F-2.8 in most standard lenses is the lowest f-stop. However, more expensive lenses can go down as low as f-1.2. Stops are most often expressed in partial stops, either half or third stops. It is important to understand “whole stops;” whole stops are a halving or doubling of the light going into the camera lens. Full stops are as follows: 1.0, 1.4, 2.0, 2.8, 4.0, 5.6, 8, 11, 16, 22, 32, 45, 64.
Speed / Shutter
Speed controls how fast the shutter opens and closes. Here you are controlling three things. First in a situation where nothing is moving speed controls whether or not your body movements blur the image or not. Before the invention of IBIS (in body image stabilization) and lens stabilization typically most people using a proper physical stance and a 50mm or wider (35mm, 24mm, 18mm) lens could use a speed of 1/60 and not have their body movement blur the image. If you were to use a more telephoto lens then you would have to keep increasing your speed the more telephoto the lens. The third thing it controls like the aperture is the amount of light going into the camera, the faster the shutter speed the less light enters the camera, and the slower the most light enters the camera. Like f-stops, shutter speeds are expressed in partial speeds but it is important to know the “whole shutter speeds,” as each whole speed is either a halving or doubling of light entering the camera. Whole speeds are as follows 1/8000,1/4000, 1/2000, 1/1000, 1/500, 1/250, 1/60, 1/30, 1/15, 1/8, 1/4, 1/2, 1sec, 2 sec, 4 sec, 8 sec and 16 sec.
https://aneejian.com/handheld-shutter-speed-calculator/
ISO
ISO controls how much noise is in your shot all cameras are designed to produce the cleanest images at their native ISO, most often this is an ISO of 100, although Nikon calibrates their ISO lower, and Fuji film higher. Noise is the result of heat coming from the sensor and over the years much has been done to decrease the impact of noise. Initially going over 400 ISO often generated a lot of noise in newer cameras ISO of 1600 is often fairly clean. Noise can be corrected in post-processing but this will reduce the resolution of the image. Like aperture and speed, noise has “whole ISO settings” increasing the ISO by a whole setting doubles the light getting to the sensor and decreasing by whole setting halves the light. Whole ISO settings are as follows: 12, 25, 100, 200, 400, 800, 1600, 3200, 6400, and 12500
Manual Use of All Three Settings
Capturing light is simply a matter of adjusting all three settings to meet the lighting conditions the camera finds itself in. You can see in the chart has two columns under each of these three adjustments. If you look at f 5.6 in the gray column, the next stop up is 4.0; moving the f-stop from 5.6 to 4.0 doubles the amount of light coming into the camera. So if you move a full stop in any direction, you are either doubling or halving the light going through the lens to the sensor. Adjusting the Speed of the Shutter using the speeds in the gray area, either up or down, will also double or half the light coming into the lens. The ISO numbers in the grayscale effectively do the same thing, making the sensor double or half its sensitivity to light. The chart gives you partial adjustments in the white columns as modern cameras allow you to adjust your settings by thirds.
The following triangle is another way of looking at the triad, illustrated by Duja Dudjic, along with some detailed information on the DIYPHotography.net
The Four Shooting Modes
Manual Mode
In Manual mode, the user fixes the aperture and shutter speed values. The ISO is also set to manual separately in the menu system. In this way, all three settings are adjusted to meet the needs of the image and lighting conditions.
Shutter Priority
In Shutter Priority mode, the shutter speed is set by the user and the aperture is determined by the camera system. If light levels change, the aperture will change as determined by the camera system. When the aperture changes the depth of field changes. The larger the camera sensor, the more dramatic this change will be.
Aperture Priority
In Aperture priority, the aperture is set by the user and the speed is determined by the camera. The controls for f/stop in most cameras are called Aperture and in most cameras, a dial on which you will find the letter “A” (unless you have a Canon camera which calls it AV). On some mirrorless cameras, the actual f-stop is written on the lens and can be changed by just turning the dial on the lens. When the speed automatically starts to slow down low light motion in the scene may become blurred or the whole scene could become blurred from micro tremors in your hand.
Program
In Program, Auto or any of the Digital Vari-Program modes the camera system determines the aperture and shutter speed values. As light levels change, the camera will alter shutter speed and aperture values to ensure the correct exposure. When the shutter speed required to achieve the correct exposure is beyond the shutter speed range of the camera, and the aperture cannot be adjusted any further by the camera system, ISO Auto will increase the sensitivity.
Other Modes on the PSAM Dial
I find the Mode or PSAM dial on cameras can be detrimental to learning to use your camera, so my suggestion is to start by using the Manual mode. This will, I believe, accelerate your learning process. You will notice on the cameras C1, C2, C3, C4, C5, and C6. These are programmable, so once you have mastered the triad, you can have your camera memorize your favourite settings for different types of photography. I recommend leaving these alone until you are comfortable operating your camera.