Modifying the DA* 60-250mm F4 for Full Frame

Introduction

Over the last decade, while Pentax only offered APS-C DSLR cameras, almost every new lens carried the DA label and was, officially at least, only compatible with APS-C. With the advent of the K-1 full frame camera body, users have wondered if some of these DA lenses were in fact capable of creating a full frame image.

Some lenses, like the DA 40mm Limited, can indeed be used as a full frame lens, albeit with reduced corner sharpness. Other lenses are of course not usable. In particular, no DA zooms have been found to illuminate the full frame image circle at all zoom settings.

One such lens, the SMC Pentax-DA* 60-250mm F4, is interesting because of its fundamental design. On its original patent, the lens was labeled as a full frame lens. Tests performed with the K-1 show that, in its unmodified incarnation, using it on full frame results in black corners. The culprit was quickly identified: a metallic ring placed at the end of the rear baffle.

Left: restrictive APS-C baffle | Center: Optimized custom FF bafffle | Right: Baffle removed

In this article we will explain how to remove this baffle, how to either modify or replace it, and how the lens performs with the updated design.