Originally posted by vijaykishan Apologies that this might have become a split thread and might need to be taken elsewhere. At the same time, if we have solid evidence that the 645 300mm offers a narrower FOV on FF (135), then it would be legit option for people with K1.
Lol, we certainly do not. The focal length is an optical property of the lens, absolutely nothing to do with sensor size. As people have said multiple times, 300mm is 300mm, doesn't matter if it's on a 1" sensor or a 645 sensor. All the smaller sensor does is crop the image. A 300mm designed for full frame with only project an image large enough to cover the full frame sensor - it would vignette on a 645 sensor, and a 645 designed lens wouldn't. Put both on a K-1 and you will have the same magnification on the image, no difference at all.
If you only have one format, and even if you have multiple, I suggest you stop thinking about equivalence and just sort of learn what each focal length corresponds to on your camera - but a 300mm is always a 300mm.
In short, the 645 300mm on full frame will offer the exact same FoV of the FF 300mm, but it is a cropped view of the 645 300mm on a 645 camera - which isn't really relevant to this thread at all.
Also, there's no reason to get a 645 lens if you're shooting full frame, especially tele, it will be larger, require adapters, etc etc.