Lens Specifications Quiz – Understand MTF Charts, Aperture, and More

An MTF chart plots how faithfully a lens reproduces contrast as a function of spatial frequency and radial distance from the center. High values across the frame at fine spatial frequencies indicate a lens that renders detail and contrast well from corner to corner.

On an MTF chart, height shows how much contrast and detail the lens holds, and the horizontal axis runs from the center of the frame to the edge. Lines that are high show a sharp, contrasty lens, and lines that stay high and close together show that quality holds evenly across the frame.

The filter thread size is the diameter, in millimeters, of the threads on the front of the lens. Screw-in filters such as polarizers and ND filters must match that number. It does not describe focal length, aperture, or weight.

Every glass surface reflects a little light. Multi-coating adds several thin layers that cut those reflections, so less stray light bounces around inside the lens. That reduces flare and ghosting and keeps contrast high, especially when shooting toward bright lights.

On a variable aperture zoom, the widest available aperture changes with focal length. At 18mm the lens can open to f/3.5, and by 55mm the widest it can open is f/5.6. You can still stop down to smaller apertures like f/8 or f/16 at any focal length.

ED and LD stand for extra-low dispersion and low dispersion glass. Ordinary glass bends different colors by slightly different amounts, which causes color fringing. These special elements reduce that spread so colors focus together, giving sharper, cleaner edges.

Chromatic aberration is color fringing at edges of high-contrast areas, caused by the lens focusing different colors at slightly different distances. It is most visible in wide-angle lenses and at the edges of the frame. It is easily corrected in post-processing.

Out-of-focus highlights take the shape of the aperture opening. More blades, especially rounded ones, make that opening closer to a circle, so bokeh highlights look round and smooth. Fewer straight blades give polygon shapes. Blade count has nothing to do with focus speed or filter size.

With internal focusing, only elements inside the lens move, so the barrel does not extend or rotate. The lens keeps the same length and balance, is easier to seal against dust and moisture, and a polarizer on the front stays in position. It also tends to focus quickly.

Vignetting is the darkening of the corners compared with the center of the frame. It is strongest at the lens's widest aperture and usually fades as you stop down. Editing software can correct it with lens profiles or a simple adjustment.