Camera Lenses: Mount, Focal Length and Aperture

Lens mount first: E, EF, EF-S, F and the rest

A lens fits one mount family, named in the title and the lens mount field, such as E, EF, EF-S or F. Mirrorless and DSLR mounts differ, and adapters exist only for some pairings. Lenses made for smaller APS-C sensors, such as EF-S or DX types, either will not mount on some full-frame bodies or will crop the image. Full-frame lenses work on APS-C bodies, with a narrower view by the crop factor. Confirm the mount before the focal length.

Zoom, prime, macro or telephoto: matching the lens to the subject

Focal length sets the view: 10-22 mm is ultra-wide for interiors and landscapes, 50 mm approximates natural vision, 85 mm suits portraits, and 100-400 mm brings wildlife and sport closer. Zooms such as 18-200 mm cover many situations in one lens; primes are simpler, often faster at f/1.8 or wider, and sharper for their size. Macro lenses focus close for small subjects. Look for image stabilisation, a quiet autofocus motor and weather sealing if you shoot outdoors.

Questions about Camera Lenses: Mount, Focal Length and Aperture

What does f/1.8 or f/4 on a lens mean?

It is the widest aperture. A smaller f-number lets in more light, allowing faster shutter speeds in dim places and a blurrier background. f/1.8 gathers nearly five times the light of f/4.

Do I need image stabilisation in the lens?

Lens stabilisation helps with handheld shots at slow shutter speeds and long focal lengths. Many bodies stabilise the sensor instead, so a stabilised lens matters most on cameras without it, though some combinations work together for extra steadiness.

What is the crop factor and why does it matter?

A smaller sensor sees less of the lens's image, so the same lens looks more zoomed in. On APS-C, multiply the focal length by about 1.5, or 1.6 on some systems, to get the equivalent full-frame view.