Define "normal lens"

yuri_wolf · June 11, 2005

a) and b) are mathematically equivalent:

Phi_diag = 2*arctan(1/2) = 53o if diagonal is equal to the focal length

It just happened to be traditionally defined as b).

c) is an urban myth. First of all, the "same magnification as the human eye" involves the viewfinder magification, that can differ on different cameras with the same lens. Second, the magnification of a human eye is variable.

paul ron · June 11, 2005

But for some magical reason it seems to apply in almost all cameras. WHY? Something going on here Yuri?

yuri_wolf · June 11, 2005

But for some magical reason it seems to apply in almost all cameras. WHY?

Says who? :-) Different people see very differently and differnt cameras have different viewfinder magnifications.

See for example <A HREF="http://www.photo.net/bboard/q-and-a-fetch-msg?msg_id=006Voy">this thread</A>; specifically posts by Axel Farr (Nov 18, 2003; 06:29 a.m.) and Oliver S. (Nov 18, 2003; 10:10 a.m.).

But there is no magic - a cone of 40-60o usually is our everyday "tunnel of vision" - so it works after all when you view the print.

yuri_wolf · June 11, 2005

But for some magical reason it seems to apply in almost all cameras.

I guess I need to make an additional remark on the "magic" part. I don't fully understand myself the engineering part involved (I hope somebody versed in real-life optics will chime in), but from the optical design point of view there are three classes of lenses: "wide", "normal" and "tele". "Normals" are those whose focal length is close to the diameter of the image circle (those magical 53 degrees of the angle of view, give or take some); "teles" are, by definition, significantly longer than that; "wides" are shorter. They are designed differently. Incidentally, this is why you can make a decent 10x zoom going from, say, 50 to 500 mm (normal to tele) but 18-180 (wide to tele) would be a disaster on 35 mm cameras - in the first case you compromise between two sets of requirements, the second will require to reconcile three of them.

Will somebody expand on that?

dan_fromm1 · June 11, 2005

Yuri, you're confusing long focus lenses -- lenses with focal length longer than normal for the format they're used on -- with telephoto lenses. Telephoto lenses have a group of lenses at the front -- in the classical telephoto design a doublet -- that are converging and a group of lenses at the rear -- in the classical tele another doublet -- that are diverging. The effect of this design is to make the lens' back focus is usefully shorter than its focal length.

Examples. I have a 12"/4 Taylor Hobson telephoto whose back focus is ~ 85 mm. That's back of rear element to film with the lens focused at infinity. And I have a 30 cm/9 (alright, 5 mm short of 12") Cooke Apotal whose back focus is ~ 300 mm.

One of the consequences of the telephoto design is smaller coverage. My 12" tele barely covers 4x5, the 30 cm Apotal is supposed to come close to 8x10.

To add to the fun, there are long focus lenses with "Tele" in their names. For example, I just sold a 500/5.6 Century TeleAthenar II. This lens is a great long pipe with a doublet objective lens at the front and that's all. There's a piece of plate glass inside the tube at the back whose only function seems to be to add flare. At infinity, the lens' front element is ~ 510 mm from the film plane. Absolutely positively not a telephoto lens. Positively absolutely a long focus lens.

paul ron · June 11, 2005

The "magical" correlation between the calculated diagonal method for "normal" and the visual "myth" of no magnification seems to work. Perhaps it's just a coincidental observation?

As for the magnification of cameras viewfinders being different...NO, they are virtually the same. Let me explain. I was researching corrective Diopters for my viewfinder not too long ago, since my eyes are changing drastically with age. I found most, if not all, SLR camera manufacturers adjust their viewfinders to focus at a "virtual" 30" using diopters to compensate thier prisms and screens to the surface of the ground glass/filmplane. This seems to standardize things a bit from one camera to another and lens to lens, thus the reason most people agree on the "visual" normal lens which just so happens to coincide with the calculated normal lens. It also makes a Pentax 50mm, a Nikon 50mm just as a Leica 50mm all the same 50mm. People's eyes may vary but are then corrected to the focusing screen via diopter, bringing them back to normal or you'll not be able to focus your camera properly regardless what your eye is.

Just as an added point of mythical interest... I notice in my part of the country there is a certain type of tree that turns it's leaves over just prior to rain. How does it know? The farmers have passed this myth along for generations without question or science to back it up but they live by it. I'm sure there is a city slicker scientist that can answer it more accurately with all sorts of equations and none the less wrong. But as the farmer's son, I say it'll rain 'cause the leaves are white side up. Does that make me wrong while we discuss all the reasons under the same umbrella?

kelly_flanigan1 · June 11, 2005

Alan; RE I hate to seem intransigent, but those who are claiming anything other than "B" are simply incorrect; remember that we are not talking about logic here, but about the definition of a technical term. In optical engineering; "normal lens" is a BS weasel word; with no exact meaning. There are HUNDREDs of photo books that even go back well before WW2 that use use the term "normal lens"; and they all use it in a different way. It is more of a marketing term than anything; or a dumbed down "technical term" for idiots. If one worships marketing BS then consider it a "technical term". In a real optical engineering sense; it is a weasel word that has a vast range of definitions; depending on the market or product. With movie cameras; a normal is usually TWICE the focal length of the diagonal roughly; a 2X error. Those want an exact answer to what a normal lens is is like asking what a normal void or BM is; and avoiding age; sex; a persons weight; activity; phyisical condition. Real old still cameras long ago had longer lenses; because simpler lenses were used; with way less angular coverage. "normal lens" is a weasel word; that has many definitions; and thus open to debate. Thus is is not really an engineering or technical term; but spineless amateur marketing term; that can have a wide range of meanings. In 35mm cameras; lenses from 40mm to 58mm have been called normal lenses; this is a 1 to 1.45 ratio. Thus those who worry about what a "normal lens" are worrying about a marketing weasel word; and are gullable; and need assurance in nebulous cloudy matters in life. What has been labeled as a "normal lens" amount all the still and movie formats varies over a 2 to 1 ratio in focal lengths to the film diagonals involved; and thus is not really a proper technical term; but a marketing term

yuri_wolf · June 11, 2005

Dan:

Yuri, you're confusing long focus lenses... with telephoto lenses.

I can easily be - I am not an optical engineer. :-) So you are saying that "telephoto" is a specific design (converging group in the front and diverging at the rear) and "long focus" lens is a generic name for the "longer than normal" focal length lens. Do I understand you correcly? Then there can be, at least in principle, "normal telephoto" lens or maybe even "wide angle telephoto" lens, right?

The effect of this design is to make the lens' back focus is usefully shorter than its focal length.

...effectively pushing the rear nodal plane inside the lens barrel, right?

I have a 12"/4 Taylor Hobson telephoto whose back focus is ~ 85 mm. That's back of rear element to film with the lens focused at infinity. And I have a 30 cm/9... Cooke Apotal whose back focus is ~ 300 mm.

Are they designed for the same camera format?

Absolutely positively not a telephoto lens. Positively absolutely a long focus lens

I think I've got it. What about wide angle, then?

Paul:

The "magical" correlation between the calculated diagonal method for "normal" and the visual "myth" of no magnification seems to work. Perhaps it's just a coincidental observation?

No, I guess it isn't a coincidence. I'd guess if you are reading this on a desktop or a laptop computer (not a hand-held device), distance between your eyes and the screen is about equal the diagonal of the monitor - most likely not twice longer or twice shorter. The "typical" behavior of a "normal" human eye in "normal" conditions roughly corresponds to a 40-60o "cone of attention" which covers the "distance=diagonal" case. The myth is that the "normal" lens is defined this way.

As for the magnification of cameras viewfinders being different...NO, they are virtually the same.

I haven't researched this thorougly, but I believe I see the difference in the viewfinder magnification between, say, Nikon F100 and F65 - with the same lens. Again, I refer to you to the earlier quoted thread - people clearly state that there is a difference.

It also makes a Pentax 50mm, a Nikon 50mm just as a Leica 50mm all the same 50mm.

I would disagree - what really makes them "the same 50 mm", is that they produce the same size of image with the same imaging format - regardless of viefinder magnification (with Leica being a rangefinder, viefinder can be really anything at all).

I'm sure there is a city slicker scientist that can answer it more accurately with all sorts of equations and none the less wrong. But as the farmer's son, I say it'll rain 'cause the leaves are white side up. Does that make me wrong while we discuss all the reasons under the same umbrella?

Yeah, right. Next time these scientists will tell you that umbrellas don't cause road accidents, despite the perfect observed correlation between the number of umbrellas on the street and the number of insurance claims. :-)

dan_fromm1 · June 11, 2005

>Yuri, you're confusing long focus lenses... with telephoto lenses.

>I can easily be - I am not an optical engineer. :-) So you are saying >that "telephoto" is a specific design (converging group in the front >and diverging at the rear) and "long focus" lens is a generic name >for the "longer than normal" focal length lens. Do I understand you >correcly? Then there can be, at least in principle, "normal >telephoto" lens or maybe even "wide angle telephoto" lens, right?

"normal" tele? No, because a telephoto lens has less coverage than a normal lens of the same focal length. A tele lens with 43 mm focal length can't be used on 35 mm still because it won't cover 24x36.

"wide angle" tele? See above.

>The effect of this design is to make the lens' back focus is usefully >shorter than its focal length.

>...effectively pushing the rear nodal plane inside the lens barrel, >right?

Sometimes farther forwards than that.

>I have a 12"/4 Taylor Hobson telephoto whose back focus is ~ 85 mm. >That's back of rear element to film with the lens focused at >infinity. And I have a 30 cm/9... Cooke Apotal whose back focus is ~ >300 mm.

>Are they designed for the same camera format?

Oh, no. The 12"/4 was made for aerial cameras that shot 2 1/4 x 2 1/4 on 70 mm film. The lens will just cover 4x5. The Apotal is a process lens, I believe that it just covers 8x10. 300 mm is the "normal" focal length for 8x10. The Apotal, like many (NOT ALL) process lenses, covers no more than its focal length.

>Absolutely positively not a telephoto lens. Positively absolutely a >long focus lens

>I think I've got it. What about wide angle, then?

A w/a lens is a lens that's shorter than normal for the format it is used on. That's all.

To get back on topic, "normal" is used in another context. A lens of normal construction is one that's neither a telephoto lens nor a reversed telephoto (retrofocus) lens. That is, its a lens whose back focus is approximately its focal length. This topic has been beaten to death on photo.net. Much heat, little light.

pvp · June 11, 2005

Kelly said: In optical engineering; "normal lens" is a BS weasel word; with no exact meaning.

Yeah, it's sort of become a weasel word, since nobody any more is willing to accept a definition unless it corresponds to the way they want to use the term. BTW, just because one subset of photographers normally uses a lens twice the diagonal, doesn't change the definition of the term. I more often choose a 90mm lens on my 4x5 as my first choice, but that doesn't make it a normal lens.

There are HUNDREDs of photo books that even go back well before WW2 that use use the term "normal lens"; and they all use it in a different way.

How about some citations? The term "normal lens" came about as a means of choosing lenses with roughly comparable fields of view when changing film formats. A few generations of our photographic predecessors managed quite well by defining a normal lens as one with a focal length equal to the diagonal of the film. This gave them a starting point even when comparing formats with quite different aspect ratios, and it gave them a point to reference the related terms "wide" and "long." As far as I've learned in the past 35 years that I've been doing this stuff, they pretty much agreed on that definition, so I need to see a few specific references before I can accept your claim of "hundreds of books" that differ with that definition.

In 35mm cameras; lenses from 40mm to 58mm have been called normal lenses; this is a 1 to 1.45 ratio. Thus those who worry about what a "normal lens" are worrying about a marketing weasel word; and are gullable; and need assurance in nebulous cloudy matters in life.

Likewise (in 35mm cameras,) lenses from 7.5mm to 35mm (a 1 to 4.67 ratio) are called "wide" and I've seen lenses from 70mm to 1200mm (that's a 1 to 17.1 ratio!) called "long." Now, I'm confused.

"Normal" lenses, looking across different film formats, could just as easily be called "lenses with roughly comparable angles of view." But they chose to use the term "normal" and they defined it in terms of the diagonal measure of the film format. It's kind of a weaselly way to compare lenses, I guess, but I've gotten over it.

kelly_flanigan1 · June 11, 2005

The usage of terms is always getting mixed. 120 was once a mostly amateur format; ie the Brownie format; named after a childs deal/toy?. a normal englarging lens long ago for 35mm was longer than 50mm/2"; because many common folks used 3 or 4 elment lenses. The 6 element 5cm F2.8 El Nikkor didnt come out until 1956. Before that a top lens for 35mm was a 2" 5 elment Ektar; which common folks usually didnt own. A "normal" car had 6 volt batteries; a generator; drum brakes; a radio with a vibrator for the B+ high voltage for the tubes; bias tires; rear wheel drive. Turn signals were optional in 1950; one had to pay extra for turn signals. the normal lens can also mean the one "normally sold" with the camera; ie a 127mm Ektar on a 4x5 speed graphic; 127mm Ektar on a 3x4 spped graphic; a 60mm on a 35mm microfilm camera; a 50mm On a kodak Ektra; a 25mm on a 16mm Kodak movie camera. With our process camera; one uses a longer lens than the diagonal; mostly for better more even illumination.

ron c sunshine coast,qld,a · June 12, 2005

Ob boy-another thread that shows the utter stupidity of internet opinions.

Idiots claiming to be experts,people thinking and talking inconsistantly,brainy guys who get caught up in their own theories,most trying to force their point of view ,too many arguing for the sake of it and too few really trying to learn.

dan_fromm1 · June 12, 2005

Ron, what's even more wonderful about it is the very strong sense of deja vu it generates. Not the first time this question has been asked, not the first time these answers have been elicited.

What is new is a sensible comment from you. Good progress, keep it up.

erick_lamontagne · June 12, 2005

I've done the "same magnification" test some time ago and on a 35mm camera, 50mm doesn't come close. 75mm is more like it, and I've read this same number quoted many times.

Matthew Currie · June 12, 2005

"I've done the "same magnification" test some time ago and on a 35mm camera, 50mm doesn't come close. 75mm is more like it, and I've read this same number quoted many times."

Depends on the camera and its viewfinder. On a Nikon F it's about 70-75. But on my Minolta X370 it's close enough to 50 that if you look through the viewfinder with both eyes open, it appears that you're looking through both eyes normally with a transparent camera in front of you. That observation may be meaningless in the context of this rather silly thread, but it's an amusing effect anyway.

I would have thought it more important to decide why we choose a "normal" lens, or what has led to its becoming normal, than how it is subsequently defined. I've always figured a normal lens is functionally normal because it renders perspective more or less as we see it normally. Defining it as focal length = film plane diagonal may well be accurate and convenient, but in terms of deciding what lens to take a photograph with, it's pretty useless.

pvp · June 13, 2005

Defining it as focal length = film plane diagonal may well be accurate and convenient, but in terms of deciding what lens to take a photograph with, it's pretty useless.

I don't know of any (authoritative) source that ever said it was. All the term "normal lens" ever was (or is) is a generalized rule-of-thumb to allow a photographer to guess the results he/she would get with a given focal length on a given size film. And to define the point where "wide" becomes "long" on a given format. It was never intended to mean "the lens I normally use."

I think the term "normal" is unfortunate, because most people seem unable, or unwilling, to accept a word being used in a manner inconsistent with their own preferences. Might be better if it was just called a middle-length lens or something, but I doubt if any three of us could agree on that either.

The answer is still "B" and I'm outta here.

alf_beharie · February 22, 2007

The answer is C, but note: The FL that gives the same magnification and/or FOV as the human eye - which defines a what is a "normal lens" - is not fixed...It varies with the film size or digital sensor size (the crop factor).

For instance on an SLR using 36x24mm film, or a full-frame sensor DSLR with a 36x24mm sensor a normal lens is ~50mm, but on an APSc sized sensor DSLR with a 1.7x crop a 30mm lens is a normal lens (30x1.7x=51mm)and a 50mm becomes an 85mm portrait/short-telephoto lens (50x1.7x=85mm), and so on.

Alf.

wideopeniris · May 23, 2007

Sorry Guys,

But the answer is "a, b and c are all correct" to a certain extent.

The basic answer is that the angle of view of a 'normal' lens is similar to the average angle of view subtended by the macula - this is the part of your eye that sees fine detail (a).

This happens to roughly coincide with the diagonal of the film (b) in many formats, but not all (try 6x17 for instance!).

If you present this angle of view then perspective is correct for a print viewed at normal viewing distances (this is the VITAL point - forget your viewfinders). The viewfinder attempts also to cover your macular field of view and thus has normal perspective when viewed through a 'normal' lens

The assertion about digital cameras needing 50mm standard lens is incorrect. In many APS-C digital cameras the angle of view of the viewfinder is small (and dont they look pokey! I much prefer my Canon A-1 viewfinder to my 10D) and this is partly an optical design problem. The viewfinder magnifications are thus different to 35mm cameras.

A print viewed at normal viewing distance shot with a 50mm lens on an aps-c sensor will have short tele perspective (which is why I use my 50mm 1.4 as a portrait lens - a bit cheaper than an 85mm F1.2 L on film!) - the 'normal' lens for an APS-C DSLR is about 30mm

Kevin.

wideopeniris · May 23, 2007

Just to make my point finally:

Normal Lens:

Normal means - gives normal perspective on a print viewed at normal viewing distances.

Nothing to do with - the normal lens for that camera or normally sold with etc etc

My favourite definition come fom dan stella:

..that a normal lens is second best for any particular purposes..

http://www.dantestella.com/technical/lenses.html

cheers

Kevin.

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