Light travels in straight lines. This is basic to how an image is captured in a camera. Light is reflected from your subject and flows in a straight line from each point on the object, through the lens aperture, and onto the sensor.
The lens “focuses” the light to produce a sharp image.
The quality of the lens matters, better lenses allow more light. Why? “Fast” lenses, usually more expensive, allow more light.
“Lens speed refers to the maximum amount of light that a given lens can transmit in a given length of time. A lens that can transmit a lot of light is called a fast lens. Some photographers like to say that it lets in light fast into the camera. A lens that transmits considerably less light when opened fully is called a slow lens. It lets in light slowly into the camera.”
On every lens you’ll find numbers stating the maximum aperture of a lens. (1:2.8 means that the maximum aperture of that lens is ƒ/2.8). You could find a range of numbers on zoom lenses, 1:3.5–5.6, it means that the maximum aperture of the lens varies, from ƒ/3.5 at the widest angle dropping to 5.6 when you zoom out fully.
Thanks for reading, keep on shooting, and, as always I look forward to your continued support.