[glasbak]

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.

Nope, normal focal length is generally defined as the diagonal of the sensor format

sqrt( 24^2 + 36^2 ) = 43mm

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

If you look at the field of view differences between these lenses, you wil see a significant difference !
Personally I think that lenses are better specified by the angle they cover, than their focal length, then all those confusing conversionfactors for different sensor sizes would not exist.

About the weird focal lengts, they are mainly marketing or historical.

[Sandy Hancock]

Get over it. They are excellent lenses, and when someone looks at the photos no one cares a hoot what the magic number on the lens was.

[lytrytyr]

when someone looks at the photos no one cares a hoot what the magic number on the lens was.

That may depend on your cultural background.

In my case, I saw so many boring snapshots taken with "normal lenses" as a child
(typically 43mm for the 35mm film "point-and-shoots" of the day)
that a photograph with that field of view now has to be exceptional to get my attention.

When I got my first Spotmatic, with its 55/1.8 Takumar, I really appreciated the extra length.

[dadipentak]

Why 135mm?

Dunno but it's one of my fav FLs.

[Sandy Hancock]

I saw so many boring snapshots taken with "normal lenses" as a child
(typically 43mm for the 35mm film "point-and-shoots" of the day)
that a photograph with that field of view now has to be exceptional to get my attention.

Well I guess that means the FA31 would not push your buttons on APS-C. Its beautiful rendering might redeem it somewhat....
I don't know what it means for any of the other whacky focal lengths, including the FA43.

[Anvh]

Nope, normal focal length is generally defined as the diagonal of the sensor format

sqrt( 24^2 + 36^2 ) = 43mm

Nope it actually just is coincidence it's the diameter of the sensor/film so that's not the reason.

Wiki has it written down quite well.

In photography and cinematography a normal lens is a lens that reproduces a field of view that generally looks "natural" to a human observer under normal viewing conditions.

For still photography, a lens with a focal length about equal to the diagonal size of the film or sensor format is considered to be a normal lens; its angle of view is similar to the angle subtended by a large-enough print viewed at a typical viewing distance equal to the print diagonal; this angle of view is about 53° diagonally. For cinematography, where the image is normally viewed at a greater distance, a lens with a focal length of roughly double the film or sensor diagonal is considered 'normal'.

[paperbag846]

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

As we both well know, the difference between 15 and 21 is much larger (almost 40% FOV difference) than the difference between 31 and 35, which is in itself larger than the marginal difference between 43 and 50, and negligible difference between 77 and 85.of)

In any case, Pentax is one of the only companies that is (sort of) rethinking lens design for a crop sensor, but only sort of. Their lens line up does feel and behave rather arbitrarily (e.g., "true normal" for film - 43, but only "regular normal" for digital - DA 35,,, where's the 28?) I'm not sure why you would bother arguing this simple point. The oddball focal lengths exist partly to be oddball, because a lot of these lenses could have been designed at more traditional focal lengths... but they were not. E.g., the DA 21, which crops to approximately 31 mm, emulates their OWN oddball focal length, when they easily could have designed a super high quality 24mm for ASPC. But they think different, and produce a more expensive lens due to R & D and design limitations at the wide end in order to produce the only 21mm lens I know that is currently produced.

[lytrytyr]

But they think different, and produce a more expensive lens due to R & D and design limitations at the wide end in order to produce the only 21mm lens I know that is currently produced.

The Zeiss Z*21/2.8 is still offered in Nikon and Canon mounts.
The difference between 21mm and 20mm is minimal:
Nikon, Sigma, and Voigtlaender offer 20mm lenses.

[lytrytyr]

Well I guess that means the FA31 would not push your buttons on APS-C. Its beautiful rendering might redeem it somewhat....

The focal length is indeed one of several reasons
that I have not found the FA31 attractive for my own use on APS-C,
instead opting for the DA35/2.8 Macro in AF and ZK35/2 in MF.
It's great that we have these choices,
although sadly the Zeiss is no longer made in K-mount.

[paperbag846]

The Zeiss Z*21/2.8 is still offered in Nikon and Canon mounts. The difference between 21mm and 20mm is minimal: Nikon, Sigma, and Voigtlaender offer 20mm lenses.

Yes you are correct.... except these are all ultra-wide lenses for full frame... the DA 21 is meant for crop in order to give you (essentially) a slightly wider than 35mm FOV (instead of 24mm which would give you pretty much exactly that FOV)...

The REAL point is funky fixed focal lengths don't matter a smidgen... slight differences in perspective are not important in the long run. Owning a 40mm and 50mm lens, for example, is superfluous.

[builttospill]

I'm not sure why you would bother arguing this simple point. The oddball focal lengths exist partly to be oddball, because a lot of these lenses could have been designed at more traditional focal lengths... but they were not.

The REAL point is funky fixed focal lengths don't matter a smidgen... slight differences in perspective are not important in the long run. Owning a 40mm and 50mm lens, for example, is superfluous.

I think you're missing the point I'm arguing then. I also think the focal lengths are nontraditional, if even quite odd and I never disagreed with that sentiment. I just have a problem when someone, a newcomer to Pentax, asks a serious question about focal lengths and someone responds it's due to marketing or the lengths are randomly selected, when they have no idea what they are talking about. There was/is much more thought and planning in designing a lens.

It's not often we agree, but on your second point above I fully agree. Respectfully it doesn't matter if I use a 40, 43 or 50mm lens to get the job done, as long as I've selected the tool I believe will get the job done the best. Lenses are tools, each offering slightly different characteristics and we're lucky to have so many choices to get the job done. I still dream one day Pentax will release another FA Limited, a 125mm (between 100 and 150) or a 117mm (in the middle of 100 and 135) .

[lytrytyr]

Yes you are correct.... except these are all ultra-wide lenses for full frame... the DA 21 is meant for crop in order to give you (essentially) a slightly wider than 35mm FOV (instead of 24mm which would give you pretty much exactly that FOV)...

OK, if crop is a prerequisite for funkiness in our game,
how about the Samsung 20/2.8 for the NX?
On that sensor, its FF equivalent is . . . 31mm!
Samsung NX 20mm f/2.8 ( W20NB ) - Review / Lab Test

[Blue]

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.

The comes up from time to time but the M 40 (released in 1975) only shares the optical layout 5/4 with the DA 40 ltd. The lens elements are of different shapes and the rear element of the LTD (all Limited lenses) have Ghostless Coating. The limited has an aspherical element.

M40





Da 40 Ltd



Compliments Dimitrov

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).

The M40 was released in 1975.

[Blue]

Well I guess that means the FA31 would not push your buttons on APS-C. Its beautiful rendering might redeem it somewhat....
I don't know what it means for any of the other whacky focal lengths, including the FA43.

Hirakawa Jun designed the FA 43 and 77 Ltd lenses. The 43 matches the diagonal of the 135 film (full frame) making it the perfect "normal" lens. He chose 77 because of special meaning of 7. The target focal length was between 75 and 80 so he picked 77. He also designed the DA 40 Ltd perhaps inspired by the M 40/2.8

[creampuff]

The limited has an aspherical element

Are you absolutely sure of this? I have never heard of any of the Pentax 40mm lenses having any aspherical elements.