One nice thing about the superb FA 77mm Limited is you can whack it on your K-1 as well with superb performance also. It is amazingly compact for its focal length, aperture speed, and imaging quality. It's bokeh (rendering of out-of-focus ares) is outstanding!
My comment regarding having to go to a lens of narrower FOV when wanting greater telephoto was meant to address how things work in the real world, which is what I think you are after.
Since you have both the KP and the K-1 (lucky you!) there is another word you have used interchangeably with FOV, which is perspective. They are two different things, but very much related. And this relationship is one reason why some people are inclined to acquire both cameras, as you have done. In the middle between a telephoto FOV and a wide-angle FOV is an area and related FL referred to as "Normal". But of course, a "normal" lens would have to be of different focal lengths when considering use with your FF K-1 or with your KP. The traditional "normal" lens from 35mm film use is the 50mm lens, although to be a stickler for accuracy, 50mm on a FF body is still slightly on the tele side in its FOV. As you've said, its equivalent FOV for your KP would be a 35mm lens. Now comes the relevancy of perspective. "Normal" also means using a lens where in the resulting image, things have a look in relation to each other that is very close to the same as would be seen being on location by the naked eye, when the image is taken at some distance, not a close distance.
This means objects in the background will appear to be of about the same size in relationship to the subject,, and therefore at the same distance behind the subject and foreground objects, as they appear when viewing the scene yourself while standing at the same spot.
With a lens going more into a wide angle FOV, objects in the background will appear smaller and farther behind the subject than in real life. Angled objects of some size- buildings, trucks, busses, etc. will appear elongated from front-to-back compared with real life. Vertical objets may appear to lean back or sideways unless one can manage to shoot from an absolute centered position. This is perspective distortion (not the usual linear distortion). But this is a price we must pay to get more into our frame. With a lens going into a more tele FOV, the opposite happens- the foreground and background are compressed, which is often used as an enhancement. Again, perspective distortion.
Now back to your two camera bodies. True "Normal" for a FF body is actually considered to be 43mm. Probably the reason for the FA 43mm Limited design. I have enjoyed this lens even more for 35mm film use (FF) than for APS-C, as I have found its FL and wider FOV more versatile, that is usable for a greater variety of scenarios. But the other thing is its ideal "normal" perspective. It renders say, a street scene in a way that has a very natural look, more so than any other lens, as well as being really fine in the usual sense of "rendering". So for FF use, this is a very special lens. That said, the FA 50mm f/1.4 s a fine lens and is close enough to "normal"perspective to be considered as a "normal" lens, though it does not get quite as much into the frame.
Although for APS-C one could effectively get a similar natural perspective by using a 28mm lens, there is no such animal as a 28mm f/2 that small, and that well-made, which has comparable "rendering" qualities. There are, however, very good alternatives for getting very close to ideal "normal"perspective with your KP. The top pick would be the FA 31mm f/1.8 Limited. This lens will have all the other "rendering" qualities of the FA 43mm LTD on the K-1 and then some- its bokeh is top-notch. even better than the 43mm. Its wide-open aperture performance is also better. Like the FA 43mm "normal" on your K-1 compared to a 50mm "normal" lens, it lets more into the frame with your KP than would a 35mm "normal" lens. However, it is quite expensive, with its ultra-fine Limited build quality. But it is also of course, fully compatible with your K-1 as a wide-angle fast lens.
As an alternative, there is my 2nd top pic, the FA 35mm f/2, which is of the same caliber of build as your FA 50mm lens, but somewhat better by having a good dedicated lens hood. Its performance is excellent, including very good booked. Very good, even at wide open aperture. It's price is reasonable, and it provides the FOV and perspective your 50mm lens provides on your K-1. A very good "normal" lens for your KP. Having been designed for film (FF) use, it can also serve as a very versatile moderate wide angle lens on your K-1. This means you get more into the frame than you would with your 50mm or 43mm "normal" lens, and since the wide angle is moderate, there is not a lot of perspective distortion.
