Make images as light or dark as you want
Many, perhaps most, professional photographers shoot in Manual Mode. Cameras in automatic and semi-automatic modes cannot guess what you want. If you want the image darker or lighter, or if you want to control the ISO, Shutter Speed, and Aperture all at once, then shoot in Manual Mode.
There are three ways of controlling exposure: ISO, shutter speed, and aperture. When shooting in Manual Mode, you can vary all three, making images as light or as dark as you want.
To make an image lighter, you can do three things: use a higher ISO setting, use a slower (longer) shutter speed, or use a lower aperture number.
To make the image darker, you can do three things: use a lower ISO setting, use a faster (shorter) shutter speed, or use a higher aperture number.
Remember, changing the ISO controls noise, changing shutter speed controls motion blur, and changing aperture controls focus blur.
Usually, shooting in Manual Mode means you will use the camera’s built-in light meter. To get an average exposure, change your settings to center the light meter. That is frequently a good place to start.
In general, changing the shutter speed one click on the dial is equivalent to changing the aperture one click. Either change means one click more light or less light, depending on which direction you go. Usually it takes three clicks to make an image twice as bright or half as bright when changing shutter speed or aperture. ISO is often different. Many cameras change ISO settings by a factor of two. It only takes one “click” to make the camera twice as sensitive to light or half as sensitive. That makes the image twice as bright or half as bright.
Often, there a many combinations of ISO, shutter speed, and aperture that will produce a well-exposed image. Let’s call these equivalent exposures.
For example, you can choose a slow shutter speed and a high f/stop. The slow shutter speed lets lots of light into the camera while the high f/stop lets little light into the camera.
Or you can go the other way. For example, using a fast shutter speed and a low f/stop. The fast shutter speed lets little light into the camera while the low f/stop lets lots of light into the camera
Since shooting in Manual Mode lets you change all three variables, which is most important? Which has priority? If you want to control motion blur, set the shutter speed first. If you want to control focus blue, set the aperture first. If you have a good idea what ISO you want to use, set it first.
Setting ISO to Automatic
One way of using Manual Mode is to set the ISO to Automatic. If you do that, you can set the shutter speed and the aperture while letting the camera automatically adjust the ISO. This will give you a good average exposure, but you will lose the ability to make images as light or dark as you want.