With the larger negative size the field of view of medium format lenses is much wider than the field of view with the same focal length on 35mm. To make a comparison with 35mm film SLR fields of view multiply the focal length by 0.48. For instance, 90mm is ~44mm in 35 film SLR terms.

I do not see a consensus on which lens series is the best. The non-C single anti-reflective coating lenses seem to be the “worst”, but the optical difference between the K/L and C lenses is not so clear. Some commentators have indicated that the C series had poor quality control, whereas the K/L lenses–being optically the same–are better by virtue of better quality control.

Original

Single anti-reflective coated lenses released in 1970 with the original Pro

NB

These lenses are optically identical to the original lenses, but have a newer barrel design (NB) that matches the later C lenses. They only were produced briefly and are comparatively rare on the market.

C

C, apparently, stands for “Design Improvement”. These lenses are multi-coated. Released with the Pro-S in 1974. Has a “C” on the front name ring.

K/L

Most recent. These lenses are, apparently, a slightly newer design with better coatings than the C lenses. The K/L is a code to indicate which bodies the lenses are compatible with. K indicates the Pro/Pro-S bodies and L indicates the Pro-SD bodies. KL lenses all shipped with an adapter attached to the lens so that they can be mounted to Pro-SD bodies. This adapter should be removed to mount to Pro/Pro-S bodies. This adapter is a fairly simple rubber O-ring for light sealing rather than for any mechanical reason. I, personally, have not had any issues using C lenses on my Pro-SD body without the small adapter ring.

Has a “K/L” on front of the barrel.

L lenses

Compatible with Pro-SD *only*; these are the 75mm Shift lens and 500mm APO only.

APO/L

Apochromatically corrected lenses. Sometimes known as A lens. More expensive, heavier, but chromatic aberrations are better corrected.

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Lens Accesories

Auto Extension Tubes

Two extension tubes–for use in close up photography–were produced, the No. 1 (45mm) and No. 2 (82mm).

Extension tubes were produced for the Pro and Pro-S and a different model for the Pro-SD. The Pro-SD extension tubes fit the larger lens throat of the Pro-SD body. Because of this the Pro-SD extension tubes do not fit on Pro or Pro-S bodies.

Bibliography

Mamiya Corporate Website