[Anvh]

I'm curious how Pentax came with these focal lengths.
The 43mm is the real standard focal length for 135 format film but what about the 31mm & 77mm ?

[Adam]

Just to be different, I think. Everybody's got a 30mm and an 85mm lens

[MegaPower]

43 = diagonal of 135mm film?
I am thinking 31 and 77 are diagonals of some other format

[Anvh]

Just to be different, I think. Everybody's got a 30mm and an 85mm lens

I hope there is more behind it though...

43 = diagonal of 135mm film?
I am thinking 31 and 77 are diagonals of some other format

43mm is to real standard lens for 135mm film so you have an AOV of 53° diagonally with that lens.
I don't see why another diagonals of some other format would matter because the lenses were designed to be used on 135mm film

[wizofoz]

I asked a simmilar question a couple of years ago.

https://www.pentaxforums.com/forums/pentax-dslr-discussion/117201-sensor-size...l-lengths.html

The consensus seems to be.... "Err, we dunno".

[Caat]

I think that the 77mm isn't 85mm because 77mm is appreciably shorter, and Pentax has/had the FA 85mm Soft and FA* 85mm. The question of why is the 77mm not 75mm I suspect is probably because 77mm is a more unique focal length.

I would think the 31mm is 31mm for similar reasons

[Adam]

I think that the 77mm isn't 85mm because 77mm is appreciably shorter, and Pentax has/had the FA 85mm Soft and FA* 85mm. The question of why is the 77mm not 75mm I suspect is probably because 77mm is a more unique focal length.

The FA 77mm actually re-uses the optical design of the A* 85mm for the most part, so one could say that it's an 85mm in disguise. They just moved around the elements a bit to arrive at a different FL. You're right that they're different lenses, but the difference between 77mm and 85mm is just 2 degrees of FOV.

Adam
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[Sandy Hancock]

I have heard the focal lengths used are the heights of the three pixies, from whom Pentax obtained the all-important dust.

[parsons]

77 is close to the Japanese symbol for delight.
43 is that diagonal of film thing making it the 'perfect focal length' for that film.

[twitch]

Is 77/1.8 about the limit for a compact lens with a 49mm thread?

Same for 31mm f1.8 and 58mm thread?

[Sandy Hancock]

Is 77/1.8 about the limit for a compact lens with a 49mm thread?

Same for 31mm f1.8 and 58mm thread?

Seriously, I reckon this has much more to do with it than anything else. I suspect the focal lengths were determined by engineering constraints while fulfilling the brief to create high quality wide, normal and tele lenses which were also compact and fast.

[parsons]

I think that ashiflex knows the story behind them. His avatar is the guy who designed the 77 and 43. Not the 31 though cos he wanted it to more compact, and slower because of this. So two other guys designed the 31.

[gtl]

Some possible answers here

RiceHigh's Pentax Blog: Some "Lost Stories" About the Three FA Limited Lenses

[gtl]

There is more here

RiceHigh's Pentax Blog: Lens Technical Report on SMC Pentax FA77mm F1.8 Limited and FA43mm F1.9 Limited

[bdery]

As an optical designer, I have another hypothesis. I believe the designers at Pentax left the focal length as a variable when doing their designs. Sometimes changing the focal lengths*a bit in a design will allow you to correct other limitations, improve performances. I think Pentax is the only company that allows itself to use unusual flcal lengths, but the lenses are amazing because of that