If you decide to get seriously into 6x6 macrophotography, then by all means look for an auto bellows and a longish lens (180 or more). The longer lens will reduce distortion (the source of many old "guess what this is?" pictures of match heads and the like) and the large extension is in proportion to the lens focal length, so the magnification won't be unwieldy. The dedicated macro lenses are, of course, superb optically, but I found that there is at least one macro combination (C lens and variable extension tube) that is mechanically incompatible; a C lens and automatic bellows works fine.
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There is no lens that can't be used on the variable extension tube.
The variable extension tube was intended to be a focussing mount for the 135 mm S-Planar/Makro-Planar (C and CF version respectively).
The other macro lens in the range is the 120 mm S-Planar/Makro-Planar (C, CF and CFE/CFi versions).
135 mm would be the longest lens i would use for photomacrography, because the extension needed is indeed proportional to the focal length. (An 80 mm lens on a bellows covers a range from 0.8x to 2.6x. The same extension combined with, say, a 180 mm lens will cover 0.3x to 1.3x.)
So for 'serious' magnifications, shorter lenses are preferred.
However, the shortest lens in the Zeiss/Hasselblad range that produces good image quality in the macro range is the 80 mm Planar.
(Planars also are the lenses that keep performing well over a wide range, into the macro-zone, so a 120 mm or 135 mm Planar would also be the lens to use if you want a lens that's a bit longer.)
Longer lenses do not keep image quality up when getting close, and also not evenly over the entire field of view. The center sharpness is less than that of lenses like the Planars, and they produce colour faults and unsharpness towards the edges and corners of the frame.
For 'really serious' magnifications, special lenses even shorter than the Planars are used, such as the great Zeiss Luminars (the most used on MF for macro are the 16 mm, 25 mm and 40 mm ones. They don't have a shutter, so require a camera that does have one, or mustbe mounted on a shutter. With the long exposures you will run into, the rear auxillary shutter in 500 series cameras will often do.)
True distortion is something that also depends on lens design, and again the Planars are the best to use.
Perspective does indeed vary with working distance, which given a chosen fixed magnification varies with focal length.
But we are not used to see tiny things in any particular perspective, so you will not run into an exaggerated perspective situation, the way you do when photographing something from too close that we do see everyday in another perspective.
And even using longer lenses, lens to subject distances will still be small.
So all in all, longish lenses would be the worst choice for macro:
- too much extension needed for the same magnification
- not the best image quality in the center of the frame
- uneven image quality across the frame