[Virginia Bill]

I'm fairly new to Pentax, coming from many years as a Nikon user. I finally got tired of lugging heavy gear around, sold my Nikon outfit, and went back to Leica. But a rangefinder can't do macro and long tele photography, so I've quite happily added a K5 to my gear closet.

As a newcomer to Pentax culture, I'm puzzled by the oddball focal lengths of many Pentax prime lenses. The long lenses are familiar -- 300mm, 200mm, 100mm macro. I'm comfortable with a selection of 50mm and 35mm primes. But the other primes feature odd focal lengths and maximum apertures: 15mm, 21mm, 31mm, 40mm, 70mm, 77mm. I know how the 1.5 crop factor affects these lenses, so I'm very pleased with the 21mm I just received and look forward to the 40mm which will arrive next week.

Am I ignorant of some element in Pentax culture or lens design philosophy? Is the company really rethinking lens design altogether to suit the crop sensor? In any case, I like the eccentric flavor of these primes and look forward to acquiring more of them.

[clutch]

The focal lengths come from a variety of sources - some have been around since the film era like the 31, 40, and 77. On some lenses like the 40, Pentax adapted film lens designs to the APS-C sensor but kept the original focal length. Others come from taking a common film focal length and adjusting it for the crop factor. The DA70 comes from 105 divided by 1.5, the DA15 is the equivalent of a 24mm, and the 55-135 is the APS-C equivalent of a 70-200 zoom. The FA43 is the true normal lens for 24x36 film - it is the length of the diagonal of 35mm film. Most manufacturers rounded it up to 50mm.

[vladimiroltean]

The weird focal lengths are a relatively new thing, I think starting with the FA series. Prior to that there were the normal focal lengths you would expect. I'd say someone from the RD department got bored and came with these odd numbers (31 and 43 are also prime).

[lytrytyr]

The weird focal lengths are a relatively new thing, I think starting with the FA series.

There was the K 30/2.8 back in the 1970s.
The FA weirdness just made 31mm out of that
(most focal lengths are only "nominal," and may vary by several percent anyway).

The screwmounts could also be screwy, like the 83/1.9.
Even the 58/2.4, although Topcon/Voigtlaender made 58mm respectable.

[Anvh]

well 43mm is the true standard, they came with 31mm because 28mm was to wide and 35mm was to narrow and 77 is lucky number.
About the DA limited, no idea but some are equivalent to popular focal lengths from 135 format.

[vladimiroltean]

There was the K 30/2.8 back in the 1970s.
The FA weirdness just made 31mm out of that
(most focal lengths are only "nominal," and may vary by several percent anyway).

The screwmounts could also be screwy, like the 83/1.9.
Even the 58/2.4, although Topcon/Voigtlaender made 58mm respectable.

I knew about the 30mm, but that would be even. I thought we were talking about odd and prime. Huh, I can just imagine Pentaxians bragging: "So, this is my 31mm prime lens. Get it?" (ok, math joke, don't throw stones )

[paperbag846]

Mostly marketing. Focal lengths aren't all that consistent anyhow (e.g., FA 50 and K 50 are off by a few mm, FA 77 is actually closer to 80mm)... Generally speaking, the fact that Pentax has oddball focal lengths is generally advantageous to users of cropped format cameras.

[johnha]

The 40mm is probably related to the M series 40mm pancake, which is probably an 'easy lens to make thin'. As mentioned some of these focal lengths started with the FA limited lenses intended for 135 film bodies, hence you've got wide-angle (31), standard (43) & portrait (77), all just a little different to the typical 28, 50 & 85's.

[lytrytyr]

I knew about the 30mm, but that would be even. I thought we were talking about odd and prime. Huh, I can just imagine Pentaxians bragging: "So, this is my 31mm prime lens. Get it?" (ok, math joke, don't throw stones )

The K lens is 3cm, which is also odd and prime.

[lmd91343]

Don't forget the 105mm M42 lenses from the '60s and '70's! why not 100mm?

Also the 120mm and 150mm lenses exist from the same period. Why those lenses with two 135mm models?

BTW the 105 is a FANTASTIC lens, one of Pentax's best. The 150mm is no slouch either! I have both.

[dadipentak]

I, for one, am delighted that it's nonsensical. Otherwise there'd be a gazillion tedious and contentious threads about it.

[lmd91343]

Come to think of it. What is the fascination with the 135mm FL lenses?

They made sense in the old RF LTM days. They were the longest lenses that the RF mechanism could focus. They were the longest non-mirror box LTM lenses Leica and Canon made.

That FL makes no sense in the 35mm SLR world and the APS-c universe. It is just a hold over from the RF day's of the '30s, '40s, '50s, and '60's. I would guess that 90% of today's digital camera users have never touched an RF, let alone used one.

Pentax made two for their 35mm SLRs. Even for one of their best zoom lenses has the longest length 135mm (50mm-135mm). Canon made at least three different 135mm lenses for their SLRs including their red circle 135mm/f2.0. Why 135mm not 150mm?

Why 135mm?

[builttospill]

Mostly marketing.

It has nothing to do with marketing.

To answer the question, when Pentax developed the 43 the designer spent some time doing research and found 43mm to be a "true normal" as opposed to the standard 50mm. I think the other FA Limiteds, the 31mm and 77mm, along with the 43mm were designed with focal lengths in mind to complete a nice small kit to cover a broad range.

The wide 31 is directly in the middle of standard focal lengths like 28 and 35, 24 and 40mm. The normal 43 is in the middle of standard focal lengths such as 35 and 50. The short telephoto 77 is between standard lengths of 70 and 85, 50 and 100. Instead of having a 24, 28, 35, 50, 70, 85 and 100 the three FA Limiteds are a nice set to adequately cover a good portion of this range for most people with extremely high quality lenses.

As far as the 21 I don't know where they came up with that focal length.

[paperbag846]

It has nothing to do with marketing.

I think you are wrong. Think different etc. 43 might be a "true normal" (whatever that mean in practice) but 31 and 77 were selected for largely arbitrary reasons... the K 30 of yore was just as arbitrary, and the 77 was selected because it is a lucky number. That is what we call marketing. Pretty much every manufacturer created this line of lenses for a long time:

20, 24, 28, 35, 50, 85, 100, 135.

Think of it as an industry standard. I don't think it is a coincidence that NONE of the limiteds are any of those focal lengths. Marketing. Aren't I different? I shoot Pentax.

The point is, as far as focal lengths are concerned, the practical difference between 43 and 50 is nothing, between 31 and 35 is nothing, and between 77 and 85 is nothing. It just sounds hip. Like vinyl, high tops, and mustaches.

[builttospill]

43 might be a "true normal" (whatever that mean in practice) but 31 and 77 were selected for largely arbitrary reasons...and the 77 was selected because it is a lucky number. That is what we call marketing.

A true normal replicates what the eye sees. When I hold the viewfinder of a film camera with 100% magnification, with the 43 mounted, to my eye it shouldn't look any different from when I remove it from my eye. This means the subject should appear to be the same distance behind the lens and to my eyes by themselves.

There's nothing arbitrary about the focal lengths selected. No company; Pentax, Nikon or Canon would pull a focal length number out of a hat for their next lens. Research is done. Lenses are designed. Your statement seems random and arbitrary and doesn't make sense. Lens design is not random, nor is marketing, and never has been. Picking a lucky number can't be called marketing. If marketing was involved a number of analyses would have been conducted, and may have included focus groups consisting of photographers discussing preferred focal lengths while marketers took notes. Marketing includes research and evaluation of data to meet a target audience. I can tell you with confidence the marketing team from Pentax never approached the FA Limited lens designer and informed him he had to create a 77mm lens because it's a lucky arbitrary number. Sounds silly doesn't it? It should.

The point is, as far as focal lengths are concerned, the practical difference between 43 and 50 is nothing, between 31 and 35 is nothing, and between 77 and 85 is nothing. It just sounds hip. Like vinyl, high tops, and mustaches.

Obviously the difference is enough because Pentax currently produces the DA 15 Limited and DA 21 Limited; members of this very forum own both.