This Lumix G ‘pancake’ lens is normally sold with the new Panasonic GF1 camera, but it’s also available separately if you’ve bought the GF1 with the alternative 14-42mm kit lens, or if you already own a Panasonic G1 or GH1 camera.

But who would want to swap a zoom for a fixed focal length lens? It’s not only more expensive, but more limiting too, surely? This is a topic we’ve covered before in lens reviews, but it’s worth covering again.

It’s true that a fixed focal length (or ‘prime’) lens is, by its very nature, less flexible than a zoom, but prime lenses bring other advantages that are sometimes overlooked. One is optical quality. Prime lenses are simpler than zooms and therefore don’t involve as many optical compromises. They produce little or no distortion, less chromatic aberration and may even prove a little sharper, too. Prime lenses usually have a wider maximum aperture, and this means you can shoot hand-held, in lower light levels and also produce attractive shallow-focus effects.

There are some less obvious benefits, too. Working with a fixed focal length lens means that you have to do a bit more work when taking pictures. You have to spend more time exploring your subject, moving around it and looking for angles that work. This can actually make your pictures better. Zooms can make you lazy. It’s too easy to stand in one spot and just zoom in and out and think you’ve done enough.

These will be familiar arguments to older photographers who cut their teeth on 35mm SLRs at a time when they were sold with fast 50mm f1.8 or f1.7 ‘standard’ lenses. And this pretty much what the Lumix G 20mm replicates, though its actually equivalent to a 40mm lens, so it’s a little bit wider and, as a result, a bit more useful. If you had to work with just a single focal length, this is probably the most versatile in everyday terms.

Its f1.7 maximum aperture will a bit of an eye-opener to anyone used to zooms, where the usual maximum aperture is f3.5 – a whole two stops slower. This shows up even in casual photography where you might be using the camera’s program AE mode; you’ll find you can shoot in much lower light than usual without risking camera shake or having to use a higher ISO.

The danger is, though, that you often end up shooting at wider apertures than usual and hence get less depth of field than you’re used to. This shallow depth of field is a big selling point for a fast lens like this one because you can use differential focus to really make your subject stand out. However, you do need to stay in control of it and there will be many instances where you want good depth of field instead.

With a lens like this one, then, you may be better off in aperture-priority mode, where you choose the lens aperture directly and hence decide whether you want shallow focus or depth of field. More than once, the Panasonic GF1 we were using caught us out during our tests by choosing apertures of f2.8 or f4 even in good light. This lens is good enough to produce extremely sharp detail even at these apertures, as our test chart shows, but you don’t get much depth of field, so the background can often end up out of focus when you wouldn’t have expected it to.

We do have to mention this lens’s sharpness. To a degree, the resolution figures we get from our tests will depend on the camera we’re using, its sensor and how it processes its images. Nevertheless, the peak resolutions we got from this lens were higher than anything we’ve seen from anything other than a full-frame digital SLR. Our chart shows it peaking at over 2000 line widths/picture height, but these figures are an average of the centre and edge definition; the centre definition was higher still.

It’s not perfect. The edge definition is extremely good, but doesn’t catch up with the centre until about f8. Our lens was also slightly sharper at the right-hand edge than the left, a phenomenon you will sometimes see with lenses, but far from serious in this case (we averaged the left/right edge values for our chart).

It was interesting, too, that the resolution held up even at the smallest lens apertures. We’re used to seeing resolution fall at f11 and f16 because small lens apertures produce diffraction effects, which lower the definition, and this ought to be even more pronounced with a Micro Four Thirds lens because of the smaller sensor format. Not this time, though. Even at f16, the Lumix 20mm was still producing good levels of sharpness. To get such a consistently high level of definition across such a wide aperture range is really rather rare.

Using this lens isn’t quite like using an old-fashioned prime. The focus ring has no distance scale and, obviously, no depth of field markings as a result. This means that manual focusing has to be done visually and you can’t use old zone-focusing techniques to manage depth of field. In every other respect, though, this is a classic prime lens. It’s compact, simple, offers a huge aperture range and optical quality to match.

Final Verdict The Lumix 20mm f1.7 shows how good prime lenses can be and how they can enhance your photography in unexpected ways. OVERALL

Our lens reviewer, and technical expert, Rod is a veritable photographic encyclopaedia. His illustrious CV has seen him write for many mags, websites and journals.