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The SMC Pentax-M 100mm f/2.8 is an old lady from the late autumn of the 20th century, a reasonably fast prime 100mm telephoto lens with pretty ok all round performance that can be easily boosted in post processing.

This lens is pretty much spot on for use as a converging tube lens with an infinite objective for extreme macro, and using a 10x lens will give you 5:1 with superb quality.

Pentax-M 100mm f/2.8, one of the lenses I use as a tube lens with infinite objectives.

A lively old gal from the late autumn of the 20th century with a buttery focus throw to dream of, it is a pretty decent tube lens that can be easily boosted in post processing

The build quality of these old ladies is outstanding, and the rock solid all-metal and glass construction will probably outlive all of us.

About the Pentax-M 100mm f/2.8

The SMC Pentax-M 100mm f/2.8 has all the usual Pentax M-series characteristics: compact, light, sharp, solidly made. It is not as directly contrasty as many later Pentax lenses, but the contrast is easily boosted with image processing.

The lens only has 5 elements in 5 groups, and 5 aperture blades, nevertheless there is plenty of quality there and Pentax glass has a dedicated and fanatical group of enthusiasts.

potential for purple fringing issues in high contrast situations

Resolution tests published by Yoshihiko Takinami show it not reaching maximum resolution until f/16, at which point it competes with high end Pentax primes. Back room testing rated the lens at about 74 lp/mm average at f/4 and 84 lp/mm at f/8.

about 74 lp/mm average at f/4 and 84 lp/mm at f/8

Used in the field, the lens has a potential for purple fringing issues in high contrast situations, but I have not personally experienced this with extreme macro, probably because stacking largely eliminates purple fringing.

The fact that it is a Pentax-M series lens rather than aperture-linked Pentax-A series lens does not make the blindest bit of difference for extreme macro use as a tube lens.

Use as a Tube Lens

this isn't the world's worst purchase

For Pentax users looking for a cheap tube lens, this isn't the world's worst purchase, as it is relatively cheap to find and the contrast deficiencies are easily rectified. It is also a compact lens with elements close together which is what we look for in a good tube lens.

I purchased this because the front element on the 100mm f/4 macro was recessed too far and this has proven to be a perfectly adequate replacement. About US$50-US$100 on eBay (2012) should get you a good copy although prices of old Pentax glass such as this, is, ironically, rising rather than falling.

SMC Pentax-M 100mm f/2.8 used as a tube lens from the Pentax manual of the era. The lens elements are close together, beneficial for a tube lens.

Pentax-M Series Lenses

The SMC Pentax-M series of lenses were launched in the very late 1970s, a compact successor range of lenses to the first multicoated plain SMC series. This was the second series using the then revolutionary K mount. There is no auto aperture with this lens series which differentiates it from the successor SMC-A series of Pentax lenses. It is thought that the -M designation maybe a nod to the then Asahi factory in Mashiko.

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