This is an interesting thread. I agree with Jack's comments that if one is looking for long reach with autofocus, image stabilization, etc. for any subject that actually moves, medium format is the wrong platform. However some of us - including myself - are looking at the longer focal length lenses for reach and/or compression for landscape/cityscape compositions (i.e., no moving subjects, locked down on a tripod) while retaining the benefits of the larger format. As pointed out here and in other past threads on the subject, options are limited beyond a certain focal length.
Looking at the pictures of the 645Z hanging off of the Pentax 600mm, I am not surprised that people have encountered vibration issues with this setup. I also imagine that a more stable platform might be found e.g. on a Cube sitting directly on a beefy carbon fiber tripod apex than on a ball head sitting on top of a center column, even fully retracted, but still that's a lot of camera dangling off of a lot of lens. Presumably issues with shutter vibration would be mitigated with the smaller Fuji or Hasselblad X bodies using electronic shutter.
A friend of mine, who is an avid bird photographer but whom I've managed to introduce to the wonders of medium format for cityscapes and macros (you're welcome, Dante), recently picked up a Nikkor 600/4 lens for BIF (I think it's the penultimate VR version). Even used it wasn't cheap and ~2-3x the OP's originally stated price range. It gives spectacular results on the Fuji 50R, although requires cropping ~square to remove the effects of vignetting at the corners so perhaps not what the OP is looking for in terms of cost as well as usability across the entire 33x44mm sensor. And it's a beast, requiring its own Pelican case for transport.
From all that I've read on the various forums and in talking to others, it seems that the Pentax medium format telephotos give the best bang for the buck, at least at the moderate telephoto focal lengths up to 300~400mm without teleconverters. In terms of the older Hasselblad V system lenses, in my experience the less expensive lenses are not up to the task when using today's high resolution sensors. MTF charts for the lineup can be found at Hasselblad Historical:
http://www.hasselbladhistorical.eu/HW/HWLds.aspx. YMMV in real life, of course. The 350/5.6 CF (which I owned) is literally not the sharpest tool in the shed and also suffers from significant longitudinal chromatic aberration. According to the MTF charts, the 350/4 FE appears to perform a bit better but nowhere near the Superachromats; ditto for the 500/8 Tele-ApoTessar (disclaimer: I have used neither).
This brings me to the 250mm and 350mm Superachromats, which admittedly are pricey as well as rare (especially the 350), and also limited in reach. But the results can be amazing on a full size 40 x 54mm 100 Mp sensor and I have no doubt the same can be said at 150 Mp. To offset the lightening of the wallet, these lenses are actually quite compact, albeit pretty heavy. Below is a photo of my 250 Superachromat, 350 Tele-Superachromat, and APO 1.4XE teleconverter all packed neatly in an f-stop small ICU. It sits on top of a medium slope ICU stuffed into an f-stop Loka backpack. If I know I won't be needing the reach, I can simply leave this part of the kit at home. So, that's 250, 350 and 490mm in packed into one compact package. There's no discernible degradation in image quality when using the 350 with the dedicated APO 1.4XE converter (the regular 1.4XE also gives good results, albeit with a touch of CA at the edges, easily correctable in post). In a pinch I can pack my 2x teleconverter for 700mm but with some loss in image quality (it's actually not bad). In terms of stability, using the electronic shutter I have found that I can hang my Cambo off the end of the the 350 + 1.4x converter and still manage to get sharp images with exposures ranging into the tens of seconds (I have not tried using the leaf shutters, so can't comment on whether they would cause vibration-induced artifacts). I've also set up the entire contraption to sit on a RRS long lens rail to minimize flex/sag at the various couplings but am not yet 100% convinced this is necessary. The rig also works quite well with my Hasselblad X1D. I'm attaching some pictures here for reference.
I doubt this long-winded post is at all useful to the OP given his stated use case but thought it might be amusing to those who enjoy going down these rabbit holes.
John
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With contents, of course! That‘s some serious tele glass right there. Awesome stuff!
