Camera Lens Quiz: Focal Length, Prime vs Zoom

Focal length is measured in millimeters. It is the distance between the lens's optical center and the sensor when focused at infinity. Shorter numbers mean wider angles of view, and longer numbers mean narrower, more magnified views.

On a full-frame camera, focal lengths shorter than about 35mm count as wide angle, so 24mm is a common wide lens. 50mm is the normal lens, close to the eye's view, while 135mm and 200mm are telephoto.

Telephoto lenses make distant objects look stacked close together, so a street of buildings or a mountain range seems flattened. This compressed look really comes from standing far away, which a long lens makes practical. It does not change color or exposure time.

A lens bends light and focuses it onto the sensor. Different lenses bend light at different angles, giving different fields of view. Prime lenses (fixed focal length) tend to be sharper and faster; zoom lenses offer flexibility and convenience.

A zoom lens covers a range of focal lengths, such as 24 to 70mm, so you can change framing without changing lenses. The tradeoff is that zooms are not always sharper than primes and many have a variable maximum aperture.

A crop sensor is smaller than full frame, so it sees only the center of the image a lens projects. That narrows the field of view: with a 1.5x crop factor a 50mm lens frames like a 75mm lens on full frame. The lens's actual focal length does not change.

A 50mm focal length on a full-frame sensor produces an angle of view close to what the human eye perceives as natural, without the distortion of a wide-angle or the compression of a telephoto.

Wide lenses exaggerate distance. Objects close to the lens look large and things a few steps back look much smaller and farther away, so space seems deeper than it is. That is why a wide lens close to a face makes the nose look big.

Telephoto lenses magnify distant subjects, so you can fill the frame with an animal without getting close enough to disturb or endanger it. They also blur the background, which helps separate wildlife from busy surroundings.

Wide-angle lenses are the most prone to distortion, especially barrel distortion that bows straight lines outward near the edges of the frame. The wider the lens, the stronger it is. Telephoto and normal lenses show much less.

Image stabilization detects and compensates for rotational movement of the camera body, reducing the blurring caused by hand tremor. It works only on camera-induced blur; subject motion is unaffected and still requires a fast enough shutter speed.

A fast lens has a large maximum aperture, such as f/1.4 or f/2.8. It lets in more light, so you can use faster shutter speeds in dim light, which is where the name comes from. It also gives shallower depth of field.

Most lenses are a little soft wide open and get sharpest a couple of stops down, around f/5.6 to f/8. At very small apertures diffraction softens them again. ISO and white balance do not change how sharp the lens itself is.

Minimum focus distance is how close the lens can focus. The closer it can focus, the bigger a small subject appears in the frame, so it decides how well the lens can do macro and close-up work. It has nothing to do with exposure or color.

The lens you pick changes how wide the view is (composition), how near and far objects relate (perspective) and how much the background blurs (subject isolation). That is why lens choice is one of the main creative decisions, not just a technical one.