40mm lenses: why are they so small?

[emiguevara]

I've been using a Minolta Rokkor-M 40mm with great pleasure in the last month. I also have a Pentax-M SMC 40mm f2.8 that sees a bit of use. I know of other pancake lenses with 40mm focal length. What is it that allows "perfect normal" lenses to be so compact? Thanks!

 

[emiguevara]

I've seen this thread already, but it concentrated on the latest DX Pentax 40mm and there was no technical answer given...

 

[tunalegs]

Well to put it very simply: they need less glass.

 

[Mark C]

The 40 Summicron or Rokkor is really about the same size as the similar 6 element 35 Summicron. The other 35 Summicrons, until the latest, are small too.

 

[sparrow6224]

The 40/2.8 and 40/2.0 pancakes and 45mm pancakes of similar speed that became popular in the 1980s and following were largely (I believe, and someone better informed will correct me if I'm wrong) Tessar designs, which have only 3 elements. This is certainly not true of the Summicron/Rokkor design; nor of the Rollei Sonnar, which I guess is the widest sonnar made (?). But a lot of them were Tessars and that allows the pancake design. (On the other hand I have an AiS Nikkor 50mm f/1.8 pancake that has 6 elements in five groups so I probably don't know what I'm talking about..... )

 

[Rob-F]

Another design in that focal length range is the 45mm Nikkor-P for Nikon SLRs and DSLRs. Definitely flat as a pancake! It is so small it's hard to get your fingers on the controls! Apparently the number and thickness of the elements dips to a minimum dimension in this focal length range. A couple of near-pancakes are the 35/2.5 and 15mm CV lenses.

 

[Spicy]

Distortion-free for the size of the image circle with minimum correction (more elements)

 

[ssmc]

Good question.

Both the Minolta MD 45/2 and Olympus Zuiko 40/2 SLR lenses are very small (I think the Oly is the smaller of the two), and they are 6 element / 5 group designs AFAIK.

SLR lenses ought to be bigger than the equivalent RF designs but I suspect that a lot of the additional bulk is due to the much larger diameter required to accommodate the auto-stop-down mechanism - look at a lot of classic SLR 50s and see how far the front element is recessed (at least until you start to get really fast) to get am idea of how short the optical cell is.

 

[David Hughes]

Hi,

It might have something to do with the fact that all SLR's at one time were the size of a Zenit TTL, Pentax K1000 or Praktica MTL3 until Olympus produced the radical and much smaller OM-1. Naturally small lenses would follow to make the package smaller and - from memory - the Pentax pancake 40mm came out with the new small Pentax ME, which was a Pentax OM-1 sized camera.

So it could just be sales driven and, of course, they had the 35mm lenses popularity, in place of a standard 50mm, to nudge them in the right direction and, perhaps, someone said why not a standard lens some way between the 35 and 50mm...

Just my 2d worth.

Regards, David

 

[btgc]

The 40/2.8 and 40/2.0 pancakes and 45mm pancakes of similar speed that became popular in the 1980s and following were largely (I believe, and someone better informed will correct me if I'm wrong) Tessar designs, which have only 3 elements.

I think Tessar is about 4 elements, like those 40-45/2.8 pancakes do but I'm not a technical person so take my words with a grain of salt....

 

[Roger Hicks]

I think Tessar is about 4 elements, like those 40-45/2.8 pancakes do but I'm not a technical person so take my words with a grain of salt....

A true Tessar has exactly four elements, and is a derivative of the Cooke triplet with one of the three glasses made as a cemented doublet. Full-aperture performance deteriorates with speed: an f/6.3 Tessar is magical; an f/4.5 is pretty good; an f/3.5 is quite good; an f/2.8 (with late glasses) tolerable. Anything faster is normally pretty bad, which is why they're rare.

Cheers,

R.

 

[nemo2]

The 40-45 mm focal length approximatelly equals the flange distance of most SLR camera designs ergo the design remains simple and without need for extra distance between lens itself and the camera mount. Plus no unusual view angles.

 

[NicoM]

They are probably the smallest because they are the closest to human perspective. I've read that 43mm is equivalent to what the human eye sees in the 35mm format. I'm guessing that 40mm lenses require less glass, or the glass required is not as large/curved as longer or wider lenses.

 

[Mark C]

They are probably the smallest because they are the closest to human perspective. I've read that 43mm is equivalent to what the human eye sees in the 35mm format. I'm guessing that 40mm lenses require less glass, or the glass required is not as large/curved as longer or wider lenses.

I think that is the answer. These are anthropomorphic lenses.

😀

 

[emiguevara]

late reply....

late reply....

Thanks for your answers! So, I'll try to summarize what I have seen as the most convincing pointers in a reasoned way so far:

1. they need less glass, as said by tunalegs.

Why?

2. because they tend to be simple Tessar designs, as pointed out by sparrow6224, although there are also Sonnar (Rollei) and Double-Gauss/Planar designs (Summicron and Rokkor).

3. because they are virtually "distortion-free for the size of the image circle with minimum correction (more elements)" as pointed out by Spicy.

4. additionally, in SLR lenses, because 40-45 mm focal length is very close to the flange distance of most camera designs, without need for extra distance between lens itself and the camera mount, as nemo2 said.

So, we still don't have a definitive and authoritative answer, but I think we should be on the right track... thanks again everyone! E.

 

[Griffin]

I've read that 43mm is equivalent to what the human eye sees in the 35mm format.

Perhaps the sharp, in-focus part is, but if you look straight ahead and stretch out your arms to each side, you will find that you have near 180 degrees vision.

 

[Roger Hicks]

Let's try another tack. Barring reverse-telephoto (Retrofocus) and telephoto designs, a smaller focal length translates directly into a smaller lens, for a given speed, format, vignetting, sharpness, etc. The smallest focal length you can easily put on an SLR without a Retrofocus design is about 40mm. QED.

Cheers,

R.

 

[efix]

Let's try another tack. Barring reverse-telephoto (Retrofocus) and telephoto designs, a smaller focal length translates directly into a smaller lens, for a given speed, format, vignetting, sharpness, etc. The smallest focal length you can easily put on an SLR without a Retrofocus design is about 40mm. QED.

Cheers,

R.

You are speaking my thoughts exactly, Roger, only that I was not able to phrase it so eloquently. Thanks for providing us with this very plausible answer.

 

[Rob-F]

Perhaps the sharp, in-focus part is, but if you look straight ahead and stretch out your arms to each side, you will find that you have near 180 degrees vision.

True! And paradoxically, the area that we can see in sharp focus at one time, without eye movement, is only the size of a dime held at arm's length! That's because the part of the retina (the fovea) that's packed with enough receptor sites for hi-res vision is very tiny.

 

[Roger Hicks]

Perhaps the sharp, in-focus part is, but if you look straight ahead and stretch out your arms to each side, you will find that you have near 180 degrees vision.

Not even that, quite honestly. Arguments about the 'natural' field of view are meaningless unless you specify enlargement size and viewing distance. You might care to look at http://www.rogerandfrances.com/subscription/ps magic window 1.html (about 'magic windows'). To quote:

In some forgotten album, ancient post-card or antique guide book, we come across a picture that seems especially immediate. It is as if we could go through the surface of the print in the same way that Alice went through the mirror into Wonderland, and find the photographer's world on the other side.

As for 'natural' perspective, and all the other ways of creating the illusion of depth in a picture, take a look at http://www.rogerandfrances.com/subscription/ps perspective 1.html

Rob's point about scanning the scene with our eyes is unanswerable, too, though I thought the area of sharp vision was slightly bigger. Then again, a lot comes down to maximum visual acuity vs. adequate visual acuity vs. being able to see moving tigers with peripheral vision.

Cheers,

R.