Most cameras whether they are mirrorless or single lens reflex and some compacts have a variety of shooting modes.
The most common ones are Shutter Priority, Aperture Priority, Manual, Auto and Program modes.
There is no one mode that it the only one you should use, but you need to understand the differences between the different modes that your camera can shoot in.
In Auto mode the camera will decide the shutter speed and aperture and possibly the ISO speed to use, when you press the shutter button the camera will auto-focus and then take the shot, simple. A lot of people will not use Auto mode because the lack of control, but in fact if you don’t have time to change and adjust settings it might be the difference between capturing an image or missing it altogether.
In Shutter Priority mode the photographer sets their desired shutter speed and the camera will adjust the aperture automatically to get the correct exposure. This mode is useful when you are trying to capture sports or any other high speed action. But you can also use it at much slower shutter speeds as well.
In Aperture Priority mode the photographer selects the aperture they wish to use and the camera automatically adjusts the shutter speed to get the correct exposure. Changing your aperture doesn’t just change the amount of light reaching the sensor/film, but more importantly the depth of field of the resulting image.
With Aperture Priority mode you need to be careful that the shutter speed selected doesn’t become too slow that you end up in a camera shake situation. You can compensate for this by increasing the ISO speed to bring the shutter speed back up again for your selected aperture.
In Manual Mode you will have control over all three parameters, shutter speed, aperture and ISO speed. The camera is just feeding you the current meter reading for the exposure, so you just have to be aware that the metering mode will influence the value and the photographer can adjust the settings to suit the image they are capturing depending on the variation of the contrast in the image.
Program Mode if it is offered is a fixed curve that exists within the camera that lets you adjust the shutter speed and aperture in a fixed relative ratio to each other. Have a look at the manual for your camera to see how that setting works, they aren’t all the same.
You will also find other modes offered such as automatic depth mode, where you focus on the furtherest subject and then the nearest subject and the camera will select the appropriate aperture for that required depth of field.
Your camera might also have a number of ‘creative modes’ where the settings are biased in a particular way depending on the category of image you are taking (landscape, nighttime, sports, macro etc..) Again check your camera manual for the details of these, as some of them will lock out certain functions on the camera when these modes are selected.
What mode do you typically use the most?