I agree with 672 also on this one, even though I still have the lenses, including the 600mm/5.6. I remember hauling it all over Alaska during film days and taking pictures of brown bears. However, with the 645D, resolution increased and suddenly the mirror slap became an issue. Then I thought perhaps a faster lens would help, and I got the 400/4 ED P67. Another great lens, but so huge considering the magnification it delivers. The 400mm/5.6 645 lens is actually quite a good compromise.
In the meantime I had gotten a m43 system for travel and everyday pictures and bought the fantastic Kowa 500mm with 350mm/850mm converters, which turned out to give me great results, even though it lacked autofocus. Obviously there wasn't the tonality and pixel/print density of medium format, but often 645D medium format telephoto images were blurred due to less than optimal lighting and resulting camera shake. Spoiled by the m43 system, I haven't really gotten back to MF telephoto nature photography. BTW - there are some folks on this forum (e.g. Thomas Janich) who'd proof me wrong. I guess I should once more go back and try my lenses with the 645Z.
In the meantime though I realized that autofocus and reach were the two limiting factors for many of the photographs I was interested in taking. I thus bought a Nikon D810 body (forgive) and a 400mm Nikon and a 150-600mm Sigma lens. Both of them are superb lenses and the Nikon D810 is just a fantastic body which comes very close to medium format.
Is it impossible to shoot great medium format telephoto images? Certainly not. Is it practical? Not sure anymore. For me that was different during film years. Medium format almost always had such a clear edge that I was often not happy with my 35mm system compared to what I got out of medium format. But today with sensors like those in the D810 I am just not sure about medium format anymore.
I know there are many here on the forum who like the 28-45mm, but the substantial barrel distortion at the wide end is certainly a turn-off for me. And the 25mm? So expensive and many lenses have mechanical problems. Mine are still fine, but how long? I guess I'd prefer Leica's lens portfolio over where Pentax is heading.
So what's left for Pentax and me? The 645Z is certainly a superior body and perhaps my most favorite camera ever. But with all the advances of competing systems I find that the strengths of the system for me are increasingly in the studio, for extreme macro photography, and for portraits using some adapted old-time portrait lenses. There the huge sensor still makes a difference for me.
Of course these are subjective mumblings, and I am sure others will disagree. I guess I have also learned that no system fits all needs. I used to find it economical to stick with one or two systems and get used to individual strengths and weaknesses. However, especially when using telephoto lenses, lens costs exceed body costs by a large margin, and the choice of the lens becomes more important than the body attached to it.
I never thought the 645 Pentax system would take a second place to anything in my areal, but I am afraid that time has come at least in terms of telephoto photography. Or perhaps I should just go against the stream, get more exercise, and start once more to carry my 800mm ED with teleconverter, 645 adapter, 645Z, and heavy tripod around
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