Asahi Pentax Auto Extension Tubes

An economical portal to the macro world

By PF Staff in Influential Photo Gear on Jan 6, 2013

This blog post is an introduction to the Asahi Pentax Auto Extension Tubes designed for K mount lenses with “Auto” aperture control (contrasting the manual M42 style apertures).  The Auto Extension Tubes have the aperture linkage built into each of the three extension tubes, so that it is possible to focus with wide open apertures which then stop down to the desired setting on shutter release.  The tubes I own were acquired through a common internet auction house, they can be found on the second hand market, and at independent photography equipment dealers who handle second hand gear.  They differ in quality from newer “K-mount Extension Tubes” which have no aperture diaphragm control, have low build quality, but are easily found new.  Trust me; you will want the authentic variety.  The build quality is superior and well worth the search.

Pentax K-r + DA 50-200mm + AF 540 FGZ
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Specification and Technical

The Asahi Pentax Auto Extension Tubes are a three piece set consisting of tube #1 of 12mm in length, tube #2 of 19mm, and tube #3 of 26mm.  Combined the three tubes have a total extension of 57mm.

Magnifying power will be related to the lens mounted on the front.  As was suggested to me by Pentax Forums member Rense:  shoot a graduated rule and determine the picture size versus the size of the subject.  The APS-C sensor in my Pentax K-r is 23.6mm x 15.8mm.  Using the following formula magnification can be calculated: _____

15.8mm_____  _or ______23.6mm
Subject Height______ _____Subject Width

I often use Pentax F 28mm f2.8, Pentax M 28mm f3.5, Pentax 35mm f3.5, Pentax 55mm f2.0, and Pentax A 100mm f4 Macro – of which I can reach 1x magnification at 40cm focus distance.

My K-r with Tubes 1+2+3 attached to A 100mm f4.0 Macro captured with Blackberry 9780

Adding extension tubes to a lens has an exposure factor which represents the relative change in Aperture due to the apparent change in focal length.  Needless to say, putting a tripod to use when shooting with extension tubes is strongly recommended as shutter speeds may be lengthened by as much as 2 stops – depending on the focal length prime, and the extension used, a lens stopped down to f16 may behave like a lens at f32 – beware of diffraction affecting resolution. 

Pentax K-r + Tube #2 + F 28mm f2.8 resulting in approximately 0.79x magnification


While I am relatively new to photography, I have been exposed to many different areas by reading various posts, and following certain threads on among several other resources.  The two most influential accessories that I have enjoyed in my photography is the extension tubes described above, and ND filters of varying density.  I selected Extension Tubes as THE most important accessory because of the versatility of shooting in all sorts of light, indoors and out.  The creative effect of such thin depths of field is undeniable.  The macro view of common things is also unique and open to so much artistic interpretation.

Pentax K-r + Tube #2 + F 28mm f2.8 resulting in approximately 0.79x magnification


Asahi Pentax Auto Extension Tubes provide an economical way to convert your film era prime lenses into effective macro tools.  The creative options are vast, and limited only by the imagination of the photographer.  With the assistance of a steady tripod, and some simple photo editing software, other worldly effects can be had. 

Pentax K-r + Tube #2 + F 28mm f2.8 resulting in approximately 0.79x magnification
Pentax K-r + Tube #1 + M 40mm f2.8 resulting in approximately 0.4x magnification - a guess because understandably I wasn't at min focus distance
Pentax K-r + Tube #2 + K 55mm f2.0 resulting in approximately 0.5x magnification 

- mattt

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