Great advice above. I shoot in shutter priority 95% of the time and use exposure compensation to fine tune the brightness of the image. I rarely shoot in Manual.
That said I do think it is important to understand the relationship between shutter speed, iso, and aperture (f-stop) and how they work together to expose the image.
If you want to use a faster shutter speed (to help eliminate motion blur on a fast moving subject), this will let in less light which means a darker image. So to compensate you either have to increase the ISO (the sensors sensitivity to light) or open up the aperure (lower the f-stop).
If you want a shallow depth of field and a blurry background you need to open up the aperture (lower the f-stop). But this will let in more light resulting in a brighter image so you would compensate by using a faster shutter speed (to let in less light) and/or lower the ISO.
It may seem a little confusing at first (especially f-stop numbers...smaller # = larger opening) but I would recommend watching some YouTube videos and familiarizing yourself with how these 3 components work together to expose the image.....
This chart helps you visualize how changing each component affects the image. Slower shutter speed = more motion blur. Higher ISO = more noise (grainy pic). Larger aperture (lower f-stop) = shallower depth of field (blurier backgrounds). It's pretty neat.....
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