Lumix S 20mm F2.5 vs 40mm F2 vs 18mm F1.8: which lens for the LUMIX S9?

Specs side by side

The 20mm and 40mm are built as a pair: identical 69.4 × 40.9 mm barrels, 2 g apart, same 62mm filter thread. The 18mm belongs to Panasonic's larger F1.8 series, where five primes share one size and a 67mm thread.

Angle of view is diagonal on full frame. Prices are Panasonic US list prices in September 2026 and change often.

Size on the S9

This is the difference you feel every day. The S9 weighs 486 g with its battery, so the 20mm or 40mm makes a camera of about 630 g that still slips into a jacket pocket. The 18mm takes it to 826 g and more than doubles how far the lens sticks out.

20mm F2.5 vs 18mm F1.8: is a stop of light worth the size?

On paper these two do the same job. In practice the 2mm gap is small: 94° against 100° of view, a little more room at the edges of a landscape or a small room. The real trade is light against size.

Light. f/1.8 is about one stop brighter than f/2.5, so at night the 18mm can use half the ISO or twice the shutter speed. It also has a nine-blade aperture for rounder out-of-focus highlights.

Size and cost. The 18mm is twice as long, 2.4 times as heavy and roughly double the price. On a body as small as the S9 it turns a pocket camera into a bag camera.

Image quality. Amateur Photographer gave the 18mm 4.5 stars for sharpness across the frame, with mild coma wide open. Its 20mm review gave 4 stars: very sharp in the centre at f/2.5, corners that improve when stopped down, strong uncorrected vignetting at f/2.5 that the camera corrects by default, and unimpressive sunstars. The 20mm also has no way to fit a hood.

Verdict: if the S9 is your small camera, get the 20mm. Get the 18mm if you shoot a lot after dark or indoors, or already use the F1.8 set on a bigger LUMIX and want matching filters and handling.

20mm F2.5 vs 40mm F2: which focal length suits you?

These two are the same size and weight, so this is purely about how you see.

The 20mm shows the whole scene: streets, rooms, architecture, landscapes and you at arm's length in a vlog. It focuses as close as 18 cm, handy for food and details on a table, and distortion is corrected in camera.

The 40mm sits close to how your eye frames things. It is better for people, food and everyday moments, about two thirds of a stop brighter at f/2, and separates a subject from the background far more.

Both have a customisable Lens Fn button. Panasonic's S9 firmware 2.1, due 13 October 2026, adds more functions you can assign to it, including Crop Zoom.

Verdict: one lens, the 40mm. Mostly places rather than people, the 20mm. Because they share size, filters and handling, the pair is the most natural small kit on the S9.

What about the 18-40mm kit zoom?

The Lumix S 18-40mm F4.5-6.3 covers both focal lengths in a 155 g lens of almost the same size, also with a 62mm thread. The primes win on light: the 20mm F2.5 is about 1.7 stops brighter than the zoom at 20mm, and the 40mm F2 about 3.3 stops brighter than the zoom at 40mm, where it closes to f/6.3. If you already own the kit zoom, the 40mm F2 is the bigger upgrade.

Which should you buy?

One lens for everything: the 40mm F2.

Travel, cities and landscapes: the 20mm F2.5.

Vlogging and walk-and-talk video: the 20mm F2.5, wide at arm's length with quiet autofocus and no visible breathing.

Night streets, gigs and dim interiors: the 18mm F1.8.

A two-lens kit: the 20mm and 40mm together, 286 g in total on one filter size.

Already have the 18-40mm: add the 40mm F2 first.

Smallest possible budget: the 40mm F2, or see the $99 Viltrox AF 28mm F4.5 in our small lens guide.

Sources: Panasonic 20mm F2.5 and firmware announcement, PetaPixel 20mm specs, DPReview 40mm F2 specs, B&H 40mm F2 close focus, Panasonic 18mm F1.8 specs, Amateur Photographer 20mm F2.5 review, Amateur Photographer 18mm F1.8 review, and prices from Panasonic's US store.