[Sandy Hancock]

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

Which I think is pretty much what parsons and I were suggesting....

[jwc77]

Baskin Robins - 31 flavors = some of the best ice cream ever!
Pentax - FA 31 = one of the best lenses ever!
coincidence - I don't think so!

[lytrytyr]

I think Pentax is the only company that allows itself to use unusual focal lengths

Quite often, the nominal focal length differs from the actual one.
I believe that the ZK 25/2.8 is closer to 26mm in focal length, for example
(which is why I prefer to think of it as a 1 inch lens ).

[jeffshaddix]

There is more here

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

Excellent read! This says that 77 was picked because they found that most people liked a short telephoto between 70-80, 77 is the angle of view when the naked eye is looking at a human, 77 was a lucky number, and it reminded them of the Chinese symbol "delight".

BTW, I'm sold on this lens...they optimized it for MF as well as AF, and the article has interesting things to say about abberation correction and design choices.

---

Last edited by jeffshaddix; 04-16-2012 at 06:52 AM.

[zapp]

43 because Pentax could not decide to make a 35 and a 50 mm lens (both focal length exist in Pentax land)
31 because Pentax could not decide to make a 28 and a 35 mm lens (both focal length exist in Pentax land)
77 because Pentax got so used to this number game so that 77 was just cool or they could not decide between 70 and 85 mm (both focal length exist in Pentax land)

Erwin Puts elaborated on this topic in his Leica compendium. None of the three lenses above would really belong to the series of focal lengths that he discusses. Look for multiples of aperture openings for a nice series of lenses - make sure to include half and third stops.

[gtl]

Excellent read! This says that 77 was picked because they found that most people liked a short telephoto between 70-80, 77 is the angle of view when the naked eye is looking at a human, 77 was a lucky number, and it reminded them of the Chinese symbol "delight".

BTW, I'm sold on this lens...they optimized it for MF as well as AF, and the article has interesting things to say about abberation correction and design choices.

Now all we need is a FF to use it at its intended FL... Sadly it doesn't look like that day woould ever come.

[DSims]

Now all we need is a FF to use it at its intended FL... Sadly it doesn't look like that day woould ever come.

They won't let us know until the last minute - just because there's silence now doesn't mean it's not coming ...


I wouldn't have believed it myself unless Pentax/Ricoh hinted that such a thing is coming (although it may not be quite what we expect).

[Zav]

Quite often, the nominal focal length differs from the actual one.
I believe that the ZK 25/2.8 is closer to 26mm in focal length, for example
(which is why I prefer to think of it as a 1 inch lens ).

I think that's it. Someone probably decided not to round the number. Otherwise, the 31 would have been a 30, the 43 a 45 and the 77 a 80mm. Nothing really uncommon.

[Anvh]

Some possible answers here

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

There is more here

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

Good read thanks =]
Now only the 31mm left i think.

[paperbag846]

31 = I don't know. It was perhaps a design choice based on an old 28mm f2 design that was updated to include better sharpness / bokeh, faster aperture, and as a consequence, more narrow FOV. Pure speculation.
43 = Perfect normal.
77 = Lucky number.

[eddie1960]

the idea that the 31 started as a smaller 2.4 lens and was then changed for speed by 2 new designers makes sense, the 43 of course made lots of sense being the perfect normal (and they were definitely proud enough of it that they had the LTM model released) the lucky 77 makes sense as well and compared to the faste bigger 85 it fits with the designers preference for compact elegant lenses

[grahame]

Pentax is the only camera/lens maker that covers half-frame (movie), 35mm full fame (135) and medium frame (120) formats. So the focal length of FA limited lenses were the diagonal measure of three film formats:

43mm is the diagonal measure of 135 film (24 × 36 mm),
77mm is (close to) the diagonal measure of 6x6 120 film (56mm × 56mm),
and 31mm is the diagonal measure of 24mmX18mm movie film.

The reason they pick 77 rather than 79, which makes exactly the diagonal measure of 6x6 frame, is 7 is a lucky number. Apparently, 77 Limited deserves the “double-lucky”.

[eddie1960]

Pentax is the only camera/lens maker that covers half-frame (movie), 35mm full fame (135) and medium frame (120) formats. So the focal length of FA limited lenses were the diagonal measure of three film formats:

43mm is the diagonal measure of 135 film (24 × 36 mm),
77mm is (close to) the diagonal measure of 6x6 120 film (56mm × 56mm),
and 31mm is the diagonal measure of 24mmX18mm movie film.

The reason they pick 77 rather than 79, which makes exactly the diagonal measure of 6x6 frame, is 7 is a lucky number. Apparently, 77 Limited deserves the “double-lucky”.

this idea would be a better story if they were made for each of the formats i think.

[grahame]

I know, all three lenses are for 135. I guess pentax just want to remind people they can make all three formats, in a interesting way.

[Anvh]

I know, all three lenses are for 135. I guess pentax just want to remind people they can make all three formats, in a interesting way.

Expensive way to do that then, designing 3 high quality premium lenses just for that...

And there is a little problem.
43mm is precisely correct but it's 30mm for the half frame and pentax did not have 6x6 so that leaves 6x4,5 with 72mm or 6x7 with 88mm
So the story doesn't really fit...

When you read the story about the 43 and 77mm most likely 31mm simply also falls in the sweet spot of most photographers preference?