SMC Pentax-A 100mm f/4 Macro

by Johan J Ingles-Le Nobel
Last updated August 31, 2017

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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

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.

Comments (5)

Article: Pentax-A 100mm f/4 Macro
Phil says...
Do you know if close-up lenses (attachments that look like filters) can get the 100 f/4 to 1:1 or greater? I see the 49 No. 1 available on eBay. And they can be bought new in sets of three strengths. Thanks.
27th June 2014 5:43pm
Johan says...
Yes, closeup adapters will do this. But look for achromatic adapters, they tend to be very good.
12th July 2014 9:52am
Richard says...
Yes I completely agree, it's a lovely close-up walkaround lens, light and nice focus feel, but performance at infinity is only so-so. Then again I haven't come across many macro lenses so far that do fare particularly well at that range. The Makro Planar f2 100mm is one exception, but considering the price it ought to be!
10th November 2013 11:52am
Rejean says...
Can it be use with a Eos 10D with adaptor K to Eos? Macro 1:2 it is interesting versus the macro 1:1
26th May 2014 4:38pm
Johan says...
Hmmm, sorry Rejean don't know - I don't use Canon so have never tried this :). I don't know if these adapters give you aperture control - Pentax' aperture control is mechanical linkage
26th May 2014 5:30pm
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