Balancing all three is a difficult process — you want to keep a high shutter speed to eliminate blur, you want to keep ISO low to reduce digital noise, and you’re limited by your lens’s maximum aperture as to how much light it can let in.
Even though autofocus sensors in modern digital cameras are great, and even though lens adapters let you autofocus on older digital camera lenses, learning how to manually focus your camera and its lenses can be extremely helpful especially when you’re in low light and your camera can’t lock focus automatically.
It might sound obvious, but checking your exposure meter when you’re shooting manually — especially if you’re shooting in quickly changing light conditions — can be the difference between getting a clear and clean and crisply detailed image, and having to rescue a shot in Lightroom or another post-processing app.
Your camera’s selected shutter speed is the measurement in a fraction of a second that the mechanical curtain (or electronic ‘curtain’, in some cameras) slides out of the way to expose the digital sensor and allow light to hit it, capturing the electronic information that forms a digital image.
If you use a larger aperture to let more light in, you’re getting a theoretically brighter image, but that image will also have a shallower depth of field — the range in which objects in your image are in sharp focus.
When you’re shooting manually, you control your exposure with three key elements — shutter speed, aperture, and ISO.
Read more here: Gizmodo