Some time ago I decided not to join the MTF Club. To the uninitiated, MTF or "Modulation Transfer Function" evaluates lens performance and is expressed in figures as line-pairs-per-millimetre for resolution and as a percentage of contrast. As far as I'm concerned though, a lens has either got it or it hasn't... and if one of mine hasn't then I get rid of it!

Two very soft lenses I did keep for some time were for Canon EOS and Pentax 67 cameras... two quite different systems. The first being 35mm, autofocus, fast and light... the second medium-format, manual, heavy and slow. They are as different as chalk and cheese yet both systems include expensive "soft-focus" lenses in their line-up.

To spend the equivalent of $1,500 dollars on both the Canon EF 135mm f2.8 and Pentax 67 120mm f3.5 Soft lenses, whose prime function is to produce images as fluffy as candy floss, was a hard decision. But the answer was convenience... these lenses are simple in design yet specialised in use. They guarantee the predictable results that photographers look for when they want something different... and soft-focus images are different.

The Canon EF lens for the EOS system uses a separate setting ring with three click-stop positions... "0" for normal with "1" and "2" for increasing degrees of softness. The Pentax 6x7 lens uses a common aperture and soft-control ring... although softness can be hand selected without taking the eye from the camera viewfinder the lens's working aperture also has to be taken into account. The image above of my wife Trish nearing her 60th birthday caught her softly despite the onset of MS affecting her faculties.

In general if any of these special lenses is used at the maximum or widest aperture the soft-focus effect will be maximised. But if the shooting aperture is around f/8, f/11 or smaller the soft-focus effect will disappear altogether and results will only show a slight degradation in resolution... which is not much of a desirable effect.