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I already have a Pentax 645 300 mm ED IF lens and use it for astrophotography, very satisfied with it and no reason to replace it. But one day there was a Pentax 645 FA* 300 ED IF lens for sale. That's the version they sell today. It has better anti reflex protection on the lens surfaces, the older lens has 5 lens elements and this have 8 lens elements. With that I couldn't resist to buy this lens. It's no sure that the extra elements is better when doing astrophotography, more surfaces can set down the contrast. But my plan is to use it for a medium format camera in the future and then the extra correction these lenses gives if to prefer. This new lens weight 1.68 kg compare to the old that weight 1.39 kg.
Here is the old Pentax 645 lens that I use today: Pentax 645 ED 300 f4 IF.
The hood is much bigger on this lens, the focus barrel is wider. The focus scale is protected under a window.
They have added a foot to mount the lens on a tripod, use a lens like this without a stable tripod is not good so it's welcome for the normal photographer.
Photos of my earlier Pentax 6x7 165 f/2.8 lens.
If I'm correct there should be 8 lenses inside here.
Have a look at the internal construction of the lens:
I'm not sure how I will use this lens, but I think I wait until I have a medium format camera. It's a lot of work to build everything around that's needed. I have started to design a medium format off-axis adapter. maybe this is the one I will use it for.