40mm lenses: why are they so small?

[DtheG]

Not even that, quite honestly. Arguments about the 'natural' field of view are meaningless unless you specify enlargement size and viewing distance [...]

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.
.

Three things that make the eye more like a scanner than a camera:
The 'sensor' has a small area of high definition (and colour) accuity only in the centre.
There is a blind spot where the optic nerve overlies the retina.
The eye becomes desensitised unless contantly receiving fresh stimulus. The image that forms on the retina is not what we see. The eye constantly scans the world collecting information about edges and patterns and movement. What we see is a synthetic model of the visual world created from that data.
So in human vision there is no optical depth of field or angle of view, just a limit on how much of the data we can keep in mind at once.
[In a way, abit like digital correction of lenses etc.]

 

[Mark C]

My 50 Summicron measures about 42mm from the flange. My 35 Summicron measures about 28. 42-28=14mm. Pretty close to the FL difference, especially when you allow for the fact that they have similar filter ring mounts on front. I don't have the 40 here right now, but suspect it would fall pretty much in line.

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.

 

[Godfrey]

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..... )

Tessar design is four elements in three groups. Triotar design is three elements.

 

[Mark C]

Tessar design is four elements in three groups. Triotar design is three elements.

And in case it hasn't been mentioned here already, the 40 Summicron is a 6 element lens that always seemed to me to be a slightly lengthened version of the 6 element 35 Summicron. It shares a lot of the distinctive image quality of those lenses, but with better edge performance which would be logical. The Rokkor mentioned in the OP was the same lens to the best of my knowledge, though I think the coatings differed. Still, all those, plus the 50 Summicron I mentioned earlier are basic double Gauss designs, but done to a fine turn.

 

[emiguevara]

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.

And that sounds extremely reasonable, thanks! It probably translates to the 35mm focal length for rangefinders, right?

 

[Roger Hicks]

Three things that make the eye more like a scanner than a camera:
The 'sensor' has a small area of high definition (and colour) accuity only in the centre.
There is a blind spot where the optic nerve overlies the retina.
The eye becomes desensitised unless contantly receiving fresh stimulus. The image that forms on the retina is not what we see. The eye constantly scans the world collecting information about edges and patterns and movement. What we see is a synthetic model of the visual world created from that data.
So in human vision there is no optical depth of field or angle of view, just a limit on how much of the data we can keep in mind at once.
[In a way, a bit like digital correction of lenses etc.]

All of this is, indeed, what most people tend to forget. Or, in some cases, never knew.

Cheers,

R.

 

[Roger Hicks]

And that sounds extremely reasonable, thanks! It probably translates to the 35mm focal length for rangefinders, right?

Hard to say. Nikon flange-to-film is near enough 46.6, Leica 27.8. The lens can of course protrude backwards into the film chamber, so you can use more-or-less symmetrical designs such as the 21 Biogon, but with a reflex, the flipping mirror is in the way, so the lens can't poke that far back into the chamber. In other words, it's not so much flange-to-film as rear element-to-film.

As a general rule, it's easier to make longer lenses than shorter ones -- some very long lenses are simple doublets -- so the trick is to make the longest lens you decently can. On the other hand, fast, long lenses tend to get quite big, quite quickly, so the trick is to make the shortest (reasonably fast) lens you decently can. These two ambitions tend to coincide at about 35-45mm regardless of whether it's SLR or rangefinder, but with a shorter rear-element-to-film distance, it's easier to go to the 35m end for RF and 40-45mm for SLRs. For extreme speed, you may need a longer focal length, hence 55mm, 58mm and 60mm lenses. Note that 'extreme speed' has to be seen in the context of the time: newer glasses and computer aided design allowed shorter fast lenses.

On top of all this, things are complicated by (1) the fact that film isn't as unhappy with strongly divergent rays as a digital sensor, which is why you can get away with the rear glass nearer to film than to digi-sensors and (2) there are two ways to build fast wides. One is as a lens that will give good central sharpness and to hell with edges and coma (cf. pre-aspheric 35 Summilux) and the other is a lens with a flat field and high sharpness all over (cf current Summilux).

Of course the latter type of design is bigger, heavier and more expensive and an unkind photographer might say that the pre-aspherics are for those who want to shoot real pictures and the current aspherics for those who want to shoot test charts. That's an over-simplification, of course, but it is not without merit as an argument.

Cheers,

R.

 

[jonmanjiro]

I thought it was as simple as the diagonal length of a 24mm x 36mm rectangle (approx. coverage of a 35mm film image or a full frame sensor) is approx. 43.3mm, so the closer the focal length of a lens is to 43.3mm the easier it is to make the lens more compact. Did I miss something?

 

[Roger Hicks]

I thought it was as simple as the diagonal length of a 24mm x 36mm rectangle (approx. coverage of a 35mm film image or a full frame sensor) is approx. 43.3mm, so the closer the focal length of a lens is to 43.3mm the easier it is to make the lens more compact. Did I miss something?

Yes. There's no magic to the diagonal. The factors in my post above are much more important.

Cheers,

R.

 

[unixrevolution]

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

1. Don't confuse Nikon's bull**** names for industry standards with industry standards.

2. The DA 40mm f/2.8 is the same optical design as the M series lens, and has been confirmed by numerous people on many occasions to cover FF perfectly.

 

[brbo]

These two ambitions tend to coincide at about 35-45mm regardless of format

So, why is a 4/40 MUCH bigger than 2,8/80 in medium format (lets say Hasselblad)?

 

[Roger Hicks]

So, why is a 4/40 MUCH bigger than 2,8/80 in medium format (lets say Hasselblad)?

Sorry. Sheer error and brain-fade. I meant 'regardless of whether it's SLR or rangefinder' and I've now amended my post to read this. With a bigger format (a) it's a wider-angle and (b) because the Hasselblad is a reflex necessitating a reverse-telephoto (retrofocus) design.

It is in fact possible to build tiny wide-angles for quite large formats, provided you don't want a wide aperture, and if they're not reflexes: the 65/6.8 Angulon springs to mind, covering 6x9cm (about 28mm equivalent on 35mm) though you needed f/11 or better still f/16 for sharpness. This is not a lot to do with my original error, though, for which I apologize again.

Cheers,

R.

 

[lxmike]

I must admit That I am really enjoying this thread very interesting, I must admit that I am a big fan of the 40mm focal length

 

[goamules]

Size, if you mean diameter of the lens, is precisely related to the speed of a lens. However, you have to add the diameter of the manufacture's particular focus and aperture mechanism, i.e. the mount. But a 40mm F2 would be much larger than a 40mm F3.5. Wider glass is needed to add speed, little else. The F-stop is calculated by dividing the focal length (from the nodal point, not back focus) by the apparent aperture looking inside the front glass. That is the circle of light you see holding the lens up with a ruler on the front. So the glass on a F2 would be about 20mm wide, an F4 would be only 10mm wide.

Size, if you mean length, is precisely related to focal length. But you measure focal length from the nodal point, usually around the center, between all the elements. A 50mm lens needs a little "bellows" which is created in modern lenses by a longer barrel to set the nodal point, at infinity, exactly 50mm from the film plane. A 40mm can be set back further, towards the film plane, making a shorter "pancake" barrel.

 

[Roger Hicks]

Size, if you mean diameter of the lens, is precisely related to the speed of a lens. However, you have to add the diameter of the manufacture's particular focus and aperture mechanism, i.e. the mount. But a 40mm F2 would be much larger than a 40mm F3.5. Wider glass is needed to add speed, little else. The F-stop is calculated by dividing the focal length (from the nodal point, not back focus) by the apparent aperture looking inside the front glass. That is the circle of light you see holding the lens up with a ruler on the front. So the glass on a F2 would be about 20mm wide, an F4 would be only 10mm wide.

Size, if you mean length, is precisely related to focal length. But you measure focal length from the nodal point, usually around the center, between all the elements. A 50mm lens needs a little "bellows" which is created in modern lenses by a longer barrel to set the nodal point, at infinity, exactly 50mm from the film plane. A 40mm can be set back further, towards the film plane, making a shorter "pancake" barrel.

Including, of course, speed in the corners, i.e. less vignetting.

Cheers,

R.

 

[goamules]

Here's a good example of what I wrote above, with almost 40mm lenses. Note that while the glass size of the slower lens is much smaller, Olympus decided to use the same mount for both, so the lenses feel the same. But they also mounted this same F2.8 lens in a smaller pancake mount....because they could. All three will have the same distance from lens node to film plane.

 

[emiguevara]

1. Don't confuse Nikon's bull**** names for industry standards with industry standards.

2. The DA 40mm f/2.8 is the same optical design as the M series lens, and has been confirmed by numerous people on many occasions to cover FF perfectly.

Oops... my bad! Sorry for the DX/DA slip... and it's good to know my Pentax-M 40mm will cover the upcoming (hopefully... when?) fullframe digital Pentax 🙂

And, lastly, thank you a lot Roger for your explanations. I think you are giving us a lot of nice pointers there; things that I kind of should know already, but you made it very nicely organised and clear. Cheers!

E.

 

[emiguevara]

Here's a good example of what I wrote above, with almost 40mm lenses. Note that while the glass size of the slower lens is much smaller, Olympus decided to use the same mount for both, so the lenses feel the same. But they also mounted this same F2.8 lens in a smaller pancake mount....because they could. All three will have the same distance from lens node to film plane.

I was thinking mainly of length of the 40mm objectives, but you make a valid point. And those two look wonderful? They are roughly equivalent to 50mm in fullframe, right?

 

[goamules]

I was thinking mainly of length of the 40mm objectives, but you make a valid point. And those two look wonderful? They are roughly equivalent to 50mm in fullframe, right?

The Olympus PEN-F lenses are fantastic, and small. I use them on m4/3 where they are like a lens of double the focal length. So they're like a 76mm. The 25mm one is like a 50mm.