Pentax K-Mount Teleconverter Guide
Features and Lens Compatibility
By PF Staff in Articles and Tips on Apr 28, 2013
On our forum we often get questions about teleconverters from new users. While the objective of the teleconverter - to extend the reach of a lens - is not in question, many other aspects of teleconverters may seem a bit overwhelming at first.
We will try to shed some light on this topic in this article. Our purpose is not to review the many available converters in terms of optical performance, but rather to explain the features and interaction with the camera's exposure and autofocus mechanisms. For product reviews and teleconverter options please consult the user reviews in the lens databases (Pentax teleconverters, Third-party teleconverters).
Pentax calls many of their teleconverters "Rear Adapters". We will use the more common term teleconverter or simply converter in this article.
A teleconverter mounts between the camera body and the lens. It is basically a tube with a number of optical elements which bend the light rays outwards thereby extending the effective focal length of the lens.
Teleconverters work best with telephoto lenses; don't expect stellar optical performance of a wide angle lens with a teleconverter attached! Some teleconverters may yield acceptable results with a 50mm lens, but try not to go any shorter than that.
All teleconverters degrade the optical performance to some degree, some more than others. They also magnify the optical flaws of the lens. Best results are obtained with "matched" teleconverters and lenses, i.e. some converters's are designed to work with one specific lens, or with a small group of lenses, and is optically optimized for that application. Generally speaking, 2x converters degrade optical performance more than 1.4x converters. If you go for a 2x converter, look for a top-of-the-line model.
Effective Focal Length and Aperture
Teleconverters are marked with their extension factor, typically 1.4x or 2x. A 2x teleconverter effectively doubles the focal length of a lens. Nothing is free, however, so a teleconverter also results in a loss of light (makes the effective aperture smaller). Usinag a 2x teleconverter results in a loss of two stops of light. Fortunately, the light meter of a K-Mount camera measures the light through the lens and takes the light loss into account when suggesting or setting the exposure.
The bigger issue is that of focusing. If the effective largest F-stop with a converter drops to a slow F6.3 or F8, the combination becomes difficult to focus with. The viewfinder becomes dark hampering manual focus, and autofocus (if supported by the converter) becomes sluggish or may not work at all.
Will It Fit My Lens?
Whether or not a particular teleconverter will fit a given lens or not depends on the construction of the converter (and in some cases also the lens).
There are two cases: teleconverters with protruding optical elements and teleconverters with recessed optical elements. Both converters in the table below are 1.4x converters, but they are of significantly different optical designs:
Even if a teleconverter fits mechanically it may not work optically, i.e. it may not yield acceptable image quality. Many teleconverters are designed to perform only with lenses in a certain range of focal lengths, for example between 70mm and 300mm. If specifications are not available one will have to resort to simply testing it out. If the converter fits mechanically there is no risk of damaging the lens by running such tests.
Camera Functions Retained or Lost with Teleconverters
This is the more tricky part and requires a close look at the mount of the teleconverter at hand.
The illustration below identifies the parts to look for. You don't need all to be present; that depends on the lens and which functions you are willing to forego. We use the Tamron-F AF Tele Converter 1.4x Pz-AF MC4 for this illustration since it has all possible features available. Click to enlarge.
All teleconverters we know of have features 1 and 2. None of the genuine Pentax converters have features 4 and 5, i.e. autofocus is not available with Pentax converters (except for the desirable odd man out described below).
Based on the above analysis we get the following compatibility table between a digital Pentax K-Mount camera, a teleconverter and a lens. Feature 1 can be ignored for digital cameras.
|Features||Compatibility with Pentax DSLR and K-01 cameras|
|2 + 3||
|2 + 3 + 4||
|2 + 3 + 4 + 5||
These images of the Tamron converter show the various features from another angle. Click the image to enlarge.
The Odd Man Out
When Pentax introduced autofocus way back when with the F-series of K-Mount lenses, Pentax also introduced a special adapter, the SMC Pentax-F AF Adapter. This is a 1.7x teleconverter with built-in autofocus capability, which enables autofocus even with manual lenses.
The intention was to provide a means for turning manual focus lenses into AF lenses, but this adapter can also be used with AF lenses in which case it takes over the autofocus job from the lens.
On the back of the adapter is a drive shaft with which the camera can move the optical elements in the adapter and thus focus the lens. On the front there is no drive shaft (and no SDM contacts), so any autofocus lens mounted loses its own autofocus capability.
The adapter has features 1 through 3 mentioned above plus built-in autofocus and 6 lens information contacts on the front so we get this compatibility table:
|Compatibility with Pentax DSLR and K-01 cameras|
|smc Pentax-F 1.7x AF Adapter||
This adapter can be a bit quirky to use since its "focus throw" is somewhat limited. It works best if the lens is pre-focused near the subject.
Unlike extension tubes for macro photography teleconverters become a part of the optical system and it is therefore worth while to spring for a more expensive converter, preferably one that is designed specifically for the focal length(s) of the lens(es) it will be used with.
All Pentax teleconverters and many great third party converters have been discontinued, but it might be better to hunt down a good converter on the second hand market than buy a cheap knock-off from new.
Pentax has never introduced a teleconverter that enables the camera to autofocus the lens proper. We speculate that it has its reasons: Achieving accurate focus with a teleconverter inserted into the optical system is challenging unless the converter is dedicated to and can be calibrated for one particular lens. When shopping for a non-dedicated teleconverter it is better (until technology improves) to forego autofocus and spend the saved money on a non-autofocus converter of the highest optical quality you can find.
Alternatively, forego the teleconverter and crop the image! This is in many cases a viable approach with today's high resolution sensors.