Pentax have a long established and fine reputation for backwards compatibility, and it is a tribute to the finely designed K mount that even today it offers a remarkable degree of functionality with lenses going back to even the 1950s. The mount has regularly updated, with the advent of electronics and AF adding contacts and levers to the mix, but still any Pentax K mount lens that has an “A” on the aperture ring will function fully and normally, without any special requirements or adapters. Earlier lenses will still function, but manually and with restrictions.

The arrival of the K-1 full frame DSLR, with its relatively small range of new lenses, created an upsurge of interest in film-era Pentax lenses, which, obviously, are full frame format. They offer a way to plug any gaps in the current lens range, as well as the potential to save some cash by buying second hand. The range of new lenses has now expanded somewhat, with more to follow, but still, the SMC Pentax-FA 20-35mm f/4 AL gives us a low cost, very compact lens that is a very attractive proposition against for example the much larger and very expensive new 15-30mm lens.

But is it any good on a modern DSLR? Let's find out.

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Pentax SMC P-FA 20-35mm f/4.0 AL Handling and Features

The lens was introduced in 1998 and stayed in production for around 6 years, so although not overly common, it is by no means rare and good examples can still be found. The first impression is that it is a very different beast to the new lenses. It is tiny by comparison, very light at 245g and makes a fine companion to the full frame 36MP Pentax K-1 or K-1 II. We are using the original K-1 for this review.

Starting at the front of the lens, there is a very substantial plastic petal lens hood. This bayonets on cleanly and positively and there is no risk of it becoming loose accidentally. Within the bayonet fit is a standard 58mm filter thread. Behind this is a thin rubberised manual focusing ring. This is not active during AF and in fact rotates during focusing, so fingers should be kept away from it when using AF. Fortunately its position is very well thought out and there should be no tendency to interrupt its rotation. To use MF, this should be set on the camera and then the focusing ring is free to move manually. It is quite light in operation, but positive enough. The distance scale is situated behind a clear window and clearly marked in feet and metres. Focusing is down to 0.3m or 1 foot, a maximum magnification of 0.16x or 1:6.3. This is about what would be expected and close enough for most uses. There are IR focus correction marks for both 20mm and 35mm.

The zoom ring is smooth and light in operation and poses no difficulties. Finally, the aperture ring has a click stop for the “A” setting, but can also be set manually from f/4 to f/22. We are now so used to using the camera to set the aperture that it seems easier to do so than to use the aperture ring. Both techniques can be used as desired.

There is a well made stainless steel mount, with the appropriate contacts for metering and other display information. This lens uses the screw drive system for AF, so although it is fast and accurate it is also slightly more noisy than current SDM and DC motor built in focusing systems.

Optical construction in 10 elements in 8 groups, with 1 glass moulded and 1 hybrid aspherical elements. It is a floating element design, so when zooming elements move in different ways in relation to each other, with the intention of maintaining close up performance. The diaphragm has 8 blades to improve bokeh by producing a more rounded aperture. In addition to the SMC (Super-Multi-Coating) the front element has a Ghostless Coating to reduce or remove flare.

It is an amazing lens to use, so easy to focus, responsive, and the light, compact construction is such a relief compared to all the huge, heavy lenses that we are now being presented with. However, new lenses are as they are for a reason, so we need to look carefully at performance to see if that compactness carries with it some disadvantage.