---

basically no good as a tube lens

The SMC Pentax-A 100mm f/4 Macro is a perfectly good macro lens for use in the field, but it is not the ideal lens to use as a tube lens.

Pentax-A 100mm f/4 Macro, an old film era lens, which covers the 35mm (fullframe) field of view. With a recessed front element like this it is basically no good as a tube lens. But as a macro lens, it shines.

About the Pentax-A 100mm f/4 Macro

The SMC Pentax-A 100mm f/4 Macro is an old film era lens, which covers the 35mm (fullframe) field of view.

As an old Pentax-A series, it has been corrected for colour fringing and its optical quality is high. Its maximum aperture is f/4 and its minimum aperture is f/32; it has a nice long focus throw with buttery smooth focusing and extremely nice bokeh.

being a macro lens it is optimised for close performance

Although it has just the 5 elements, it is still very sharp. In use, the focusing is very smooth with about 340 degrees of travel from infinity to close focus (0.45m). This allows very precise focus adjustment. For some subjects a 1:2 magnification is just perfect, for instance birds, butterflies, dragonflies and damselflies, which is where this lens excels.

Pentax-A 100mm f/4 Macro As A Tube Lens

The SMC Pentax-A 100mm f/4 Macro is really basically no good as a tube lens with an infinite objective, for two reasons:

birds, butterflies, dragonflies and damselflies, which is where this lens excels

Being a macro lens it is optimised for close performance. So, at infinity focus, as a telephoto lens, the results are not always quite as stellar as at close range. White highlights have sometimes been observed with this lens to have yellow fringing at infinity, a phenomenon not observed at close range.

Secondly, the front element of this lens is deeply recessed, making a lens hood unnecessary. This is great for field photography but not so great for use as a tube lens, because you cannot get an objective close to the front element.

---