Whilst I do agree on many of the points you guys have brought up (but not all, won't specify, I'm not interested in *equivalency* debates), I was more interested in this due to my want to eventually transition to a 645 body at some point in the future. I would just like this for a transitional stopgap solution, as whilst I could probably justify spending ~$2500-3000 on a high quality lens, the $6000+ one needs to pick up a digital 645 body is quite out of the question at this point in time, and adapting the lens to a K-1 body would ease the cost over a longer period of time.
As for the D-FA 100mm Macro, whilst I find this lens quite sharp, it does suffer quite strongly (compared to the D-FA 70-200 & 18-35 ART) when it comes to chromatic aberrations and other colour fringing, and for Macro work, any extra sharpness would be welcome, as would DC focusing and a focus limiter. As an aside, if they could also include tilt and shift capabilities (I know, quite the long shot, would probably have to provide two separate adapters in this case), it could make quite the formidable combination for professional macro/reproduction work!
That said, it would prove an expensive option, but for those more budget strapped, if the adapter would also function with the FA 120mm F4 Macro, it could prove quite popular.
---------- Post added 01-13-17 at 07:51 PM ----------
Originally posted by WPRESTO The Pentax 645 sensor has an area of 1452 square mm, the K1 sensor has 864 smm, all Pentax APS-C sensors have 370 smm. So mounting the lens on a K1 is equivalent to cropping off about 41% of a 645 image, mounting on a K3 is equivalent to cropping off about 75% of the 645 image. 645 lenses are designed to provide a larger image circle ("circle of confusion") which involves a trade-off in all parameters of IQ (resolution, color fringing, edge-to-edge uniformity, etc). A lens designed for optimum performance on a FF or APS-C sensor will almost always outperform a lens designed for a larger format. We are not generally tempted to adapt lenses designed for 4X5 or 8X10 cameras to FF or APS-C sensors, even if they are outstanding optics for those large formats.
Though I would agree that lenses optimized for a certain sensor size would perform best when utilized on the 'correct' sensor size, I will bring up a few points of discussion (and please, I promise I'm not trying to argue that I'm right, I just like scholarly debate!)
1 Consider that the 645z sensor is ~%59 (FF area/645z area x 100 [just to show working incase I've fudged anything]) larger than a K-1 sensor, whilst a K-1 sensor is ~42% larger than a K-3 sensor.
2) It would be safe to say that when it comes to quantifying things as photographers, we rarely work in anything smaller than 1/3 stop margin of error (this is my opinion, definitely not a rule!) Thus, the area difference between 645/FF (59%) and FF/APS-c (42%) is smaller than a 1/3 stop.
3) I frequently use D-FA lenses optimized for a FF sensor, with no apparent* disadvantages when it comes to sharpness, flare etc.
*definitely warrants further investigation
4) Seeing that the D-FA 90mm Marco lens is actually specified for a 55x44mm sensor (hence it not being issues as a DA lens), would it be safe to assume that utilizing it of a 645z sensor is already compromising it's resolving/rendering power?
5) If point 4 is corrected, then I would agree with your comment that this MF lens would perform worse than a FF equivalent lens, otherwise, all things considered equal*, I would expect this lens to perform equally well across different bodies dues to logical deduction.
*that said, things cannot be considered equal due to adapter mount tolerances, see Roger Ciala's post for more info:
https://www.lensrentals.com/blog/2013/09/there-is-no-free-lunch-episode-763-lens-adapters/ (though I expect you've read this already, you seem well informed)
Hopefully you'll reply in kind, I look forward to you response