Relevance to a potential K-mount FF camera
Pentax haven't committed to launching a K-mount FF camera, and if they do some of its characteristics are currently uncertain. Here are some reasons to be cautious about the results of my tests tests:
- Targets and walls are different from normal subjects!
- The image circle needed by a sensor with Shake Reduction is larger than that needed by a 36mm by 24mm frame. (This mainly affects the corners).
- Film and digital sensors behave differently when light doesn't fall on them perpendicularly. (This mainly affects the edges and corners, typically with wide-angle lenses, and depends on the sensor design).
- Pentax may choose to restrict the permitted image circle for particular lenses by inflexible cropping.
First, a comment about my Pentax Z-1p camera. This has been crudely stored in a box (without battery) for 9 years since I bought my first digital camera, the Pentax *istD. I put in a fresh battery, attached one of these lenses, and it autofocused. It continued to work without any apparent fault throughout the tests. I had forgotten how noisy it is compared with (say) my K-5IIs!
Second, a comment about my potential use of a Pentax FF camera. When I buy a new camera, I don't dispose of my previous camera. It typically becomes a back-up camera in case of problems with my main camera. But it could simply become an alternative camera. Suppose that I bought a Pentax FF camera while my Pentax K-5IIs was still my main camera. For any shoot, I would choose which to use and mix-n-match with my available lenses for the best combination. For example I might use a Pentax FF camera for studio work while using my Pentax K-5IIs for action. In effect, I would not only have cameras with interchangeable lenses but also lenses with interchangeable cameras.
I will buy the Pentax 1.4x Teleconverter when it becomes available. It is expected to be weather resistant and operate both SDM and screw-focusing lenses. So I want to know how various combinations of my lenses and that teleconverter might work with a Pentax FF camera. If Pentax launched a 420mm f/5.6 lens, there would be lots of interest in whether it would work on an FF camera. A Pentax 1.4x Teleconverter plus a DA* 300mm f/4 lens is equivalent to that lens.
I already have a Kenko P-AF 1.4x TELEPLUS MC4 which gives me some clues. On my Pentax K-5IIs it can't autofocus any lens with SDM, whether or not that lens also has screw-focusing. The camera tries to use SDM, but this Kenko teleconverter won't transmit the signals. But the Pentax Z-1P plus this teleconverter can autofocus all of my lenses that have screw-focusing, because the camera doesn't attempt to use SDM.
A teleconverter works by expanding the image circle. As long as it expands the whole of the image circle, rather than just the centre of it, a 1.4x teleconverter makes a lens 1.4 times less cropped. So a lens that by itself only works on a 1.5-crop sensor should work on a 1.1-crop sensor in combination with a 1.4x teleconverter. A lens that by itself is marginal at FF should become reliable at FF with a 1.4x teleconverter. This has the potential to enable more Pentax lenses to work with an FF camera when they are used in combination with the Pentax 1.4x Teleconverter.
I care a lot about image quality, including print quality at A3+ and beyond. I use Lightroom (and Photoshop). Any lens faults that can easily be cured by the basic Lens Correction tab in Lightroom (distortion, chromatic aberration, and mild vignetting) don't matter much to me, especially if there are lens profiles to do so automatically. I want to show people high quality prints, not show them unprocessed raw files.
A lens can't be properly evaluated in isolation. It also needs to be evaluated as part of a system including the camera, a teleconverter if used, and the basic software used by a particular photographer for every image.
Brief summary of my analysis
These tests refute the myth that DA and DA* lenses are designed with a small image circle to match an APS-C sensor. Of my lenses, only the DA 17-70mm f/4 shows that restriction. The APS-C sensor dictates the minimum image circle, but the other lenses have a larger image circle in some or all circumstances.
Every lens has to be judged on its own merits, and zooms vary across the focal length range. Things improve when stopping down. There is no single crop size that applies to all these lenses, and some are fine for 35mm use.
The impending availability of a Pentax 1.4x Teleconverter has to be taken into account when judging the market for a Pentax K-mount FF camera. It adds extra options that will make a significant difference to some (certainly not all) photographers that will make a Pentax FF camera more useful to them.
I didn't test those of my lenses which were designed for FF: