March 29 2012
DxOMark labs recently tested this premium wide angle lens in a compact package from Pentax.
Pentax Limited lenses are some of their premium quality prime lens collection.
Pentax lens designers love compact prime lenses in focal lengths ranging from wide angle to short telephoto. Because of their flat compact shape Pentax call them ‘pancake’ lenses. One of their earliest efforts was the K mount 40 mm lens in 1976. The pancake shape offers distinct benefits in making the DA 21 mm an easy to carry light weight lens.
The light weight comes from the small size, as this Pentax Limited lenses has solid all metal construction from the body right down to the lens hood and cap. This gives the DA 21mm a silky feel and classy look.
The small size and moderate wide angle focal length make the DA 21mm limited at home trekking around for landscape photos or unobtrusive candid street photography.
Larger cameras and “look at me” white lenses are designed to draw attention to the photographer and their equipment rather than allowing candid street photography.
The extreme compact size does come with a performance price. The most obvious is the modest maximum aperture of only f3.2. The common maximum aperture for prime lenses in this focal length range is f2.8.
The DxOMark tests confirmed my impressions of some performance shortcomings. The tests reveal a persistent Chromic Aberration at most common aperture settings. This shows up as coloured fringing around the edges of objects where there is a large contrast between light and dark. In the test photo we are using white chairs set for a wedding ceremony. Looking closely purple fringing is evident around the edges of the chairs.
Pinkish discolouration on the chair uprights from Chromic Aberration in this expanded section of the wedding ceremony photo.
After correction in post processing the discolouration has disappeared. There are a number of photo processing programs offering this feature including DxO Optics Pro.
This is the full photo of the white chairs set for the wedding ceremony with my favourite aperture setting of f7.1 for the DA21mm Limited lens. This puts the amount of Chromic Aberration in to perspective and it appears only a minor fault.
In photographs without this extreme contrast it is not apparent.
The comparison tests from DxOMark show that the similar focal length Nikon lens is worse than the Pentax DA 21 mm.
As the apertures get smaller, larger f numbers, the Chromic Aberration in the Pentax DA21mm reduces, while it increases in the Nikon. The much larger and three times more expensive Carl Zeiss lens has negligible Chromic Aberration at any aperture, helping to justify the high price.
The good news for owners of the DA21mm is RAW processing software packages, such as DxOMark’s Optic Pro, minimise Chromic Aberration in their digital photos.
The test show the Pentax DA 21 mmLimited does have some vignetting at wider aperture settings.
The DxOMark tests show that the vignetting disappears as the aperture is nears to f8.
Impact on Photographs
In a wide angle lens often used for landscape, or similar photography, the usual aim is to maximise the depth of field, so apertures usually start at around f8 and get smaller.
Particularly when using one of the newer low noise DSLRs such as the Pentax K-5 photographers could use a slightly higher ISO settings to allow smaller aperture settings in low light situations. The other mitigating factor is the sensor shift Image Stabilisation built into Pentax DSLRs allows 3 to 4 exposure stops advantage. Photographers can confidently reduce the shutter speed by this amount rather than opening up the lens to achieve similar exposure without blurring due to camera shake.
I use mine with a aperture of f7.1 and the DSLR in Aperture Priority mode and adjust the ISO if the shutter speed drops too low.
Adorama have the Pentax DA 21 mm Limited wide angle lens available for around $US650