Choosing a standard prime

A standard prime — around 50mm equivalent — renders roughly the perspective your eye takes in when you concentrate on something. That is why it is the classic first lens after a kit zoom, and why photographs taken with one tend to look unforced.

Why 40–70mm equivalent

Below 40mm the frame starts including context you did not intend; above 70mm you are already isolating and reaching. Between them, the lens mostly stays out of the way.

On APS-C that means roughly 30–45mm, on Micro Four Thirds roughly 20–35mm. Turn on 35mm-equivalent values in the filters and the ranges line up across mounts.

What you gain over the kit zoom

Two or three stops of light, and a lens that is usually smaller than the zoom it replaces. A kit zoom is f/3.5–5.6; a 50mm f/1.8 is around two and a half stops brighter at the same focal length. Indoors and after sunset that is the difference between a usable frame and a blurred one.

The f/1.8 lenses are the sweet spot

Every manufacturer makes an inexpensive standard prime around f/1.8, and they are almost all good. The f/1.4 and f/1.2 versions cost several times as much for a stop or less, plus considerable weight. Start with the f/1.8 — if you find yourself wanting more, you will know precisely what for.