What is a Normal lens?

Absinthe · Mar 18, 2008

As I understood it, the "normal" lens was based on the diagonal of the film format. However, when I calculate these, I never get a normal lens that I am used to hearing:

H W Diag ?Customary Normal?
24 36 43.26661531mm ------------ 50mm?
4 5 6.403124237" --------------- 7"?
8 10 12.80624847" -------------- 13"?
2.25 2.25 3.181980515cm ---- 31.49 = 80 mm?
6 6 8.485281374cm ------------ 80 mm?
2.25 2.75 3.553167601" (90.25) ------ 90 mm?
6 7 9.219544457cm ------------ 90 mm?
6 9 10.81665383cm ----------- 100 mm?
2 3 3.605551275" (91) -------- 90 mm?

Is there something you do after you come up with the diagonal to get to the "normal" lens or is it just easier to deal with the next size up?

Some seem close, while others are not so close.

Ian Grant · Mar 18, 2008

It's very rule of thumb, and the formats have different aspect ratios.

So:
35mm - 50 -55mm
6x4.5 - 80mm
6x6 - 75 -80mm
6x7 -90mm
6x9 105mm
5x4 -150mm
10x8 300mm

Ian

Michel Hardy-Vallée · Mar 18, 2008

Bear in mind that a nominal "50mm" lens could in fact be a 51.7mm or whatnot. Manufacturers round up numbers, which makes it easier for us customers to remember them more easily.

cotdt · Mar 18, 2008

i know some say it's the diagonal of the film format, but to me a normal lens is what i see with my eye. so it's 14mm for 35mm format and 47mm for 4x5.

still others say that a normal lens is any lens that is neither telephoto or retrofocus, that the lens center to film plane is the same as the actual focal length. so then you can talk about performance from a normal 360mm lens versus a telephoto 360mm lens that would be smaller. there are also normal ultra-wides versus retrofocus ultra-wides on rangefinder cameras. on SLRs all wide angle lenses are retrofocus and all long lenses are telephoto so this definition of normal lens is no longer useful in the modern SLR era.

Absinthe · Mar 18, 2008

So the factor is just coming to a "nice sounding" number?

Michel Hardy-Vallée · Mar 18, 2008

Well, do you think you would sell more 51.7mm lenses or more "50mm" lenses?

Nick Zentena · Mar 18, 2008

Pentax makes a 43mm.

But "normal" tends to take into account a few other factors. 35mm film is small so a little longer lens seems a better choice. Mamiya for the RB/RZ went longer. 127mm or 110mm. I wonder if the 127mm was chosen because it was a common LF length? OTOH Fuji went a little wider using 90mm for it's later 6x9s.

For sheet film if you use imperial then your calculated numbers are much closer. 12" on 8x10. 6 or so inches on 4x5. Factor in the film rebate and I bet it gets even closer.

jd callow · Mar 18, 2008

Nick,I think, but sure don't know, that SLR's tend toward a slightly longer 'normal' and RF's tend toward slightly shorter 'normals' as the camera designs and usage better accommodates the respective lengths.

Nick Zentena · Mar 18, 2008

I was thinking that for 35mm but wasn't sure how much it mattered for larger cameras.

Fuji's used a longer lens IIRC on the 6x9 that didn't have the fixed lens.

Absinthe · Mar 18, 2008

Well, there is a factor I missed, what is the actual nominal measurements of the different formats?

cotdt · Mar 18, 2008

My Olympus XA actually uses a telephoto 35mm focal length lens to get it super small and compact.

walter23 · Mar 18, 2008

Technically normal is a focal length equal to the format diagonal. For 35mm it's about 43mm, but 50 is typically considered normal. It's not set in stone, this is all just semantics, but the format diagonal rule of thumb will give a roughly normal perspective on any format. For 4x5 it's like 170mm or so, for 8x10 it's 320mm, etc.

Just use pythagoras eq'n: ie, diagonal = sqrt(X^2 + Y^2). So for 6x6, it'd be diagonal = sqrt(60*60 + 60*60) = ~85mm. Of course I think the actual frame size is a bit smaller than the nominal frame size (ie, not exactly 60mm x 60mm), so it'd be a bit shorter, but that gives you the idea.

cotdt · Mar 18, 2008

walter23 said:
Technically normal is a focal length equal to the format diagonal. For 35mm it's more like a 43mm than a 50, but 50 is typically used. It's not set in stone, this is all just semantics, but the format diagonal rule of thumb will give a roughly normal perspective on any format. For 4x5 it's like 170mm or so, for 8x10 it's 320mm, etc.

and what makes the format diagonal so special, so normal?

most 50mm SLR lenses are double gauss designs which are retrofocus.

Ed Sukach · Mar 18, 2008

Far back, in the beginning of camera lens design, someone *arbitrarily* chose the ratio of diagonal (or, if appropriate, the diameter) of the film format equal to the focal length of the film (or plate) format as the optimum parameter to be used allowing the greatest mximum aperture, greatest resoution, greatest contrast ... in short, the most efficient lens design. Generally, this is a valid assumption.

Our perception is different. Normal human perception - where the "apparent" flatness of field agrees with human sight -is equal to a greater focal length to field ration - roughly 90 - 100mm in 35mm format, and 140 - 180mm in 120 ... usually thought of as "portrait" lenses. Anyone who has taken a portrait with
wide-angle lenses has encountered their "ballooning" effect ... not desirable in most cases.

Ole · Mar 18, 2008

cotdt said:
most 50mm SLR lenses are double gauss designs which are retrofocus.

Eh no - that's wrong on both counts. Most 50mm SLR lenses are either Tessar-derivatives or (very distant, for newer ones) Planar-derivatives; both of which may or may not be slightly retrofocus.

It's fairly simple to make a lens with good speed which covers the necessary 53 degrees needed to cover the diagonal. But since SLRs have a mirror box, the "standard" became a slightly longer focal length that ensured that the rear element wouldn't interfere with the moving mirror.

The first retrofocus SLR lens was a 35mm Angenieux Retrofocus, BTW...

JBrunner · Mar 18, 2008

cotdt said:
i know some say it's the diagonal of the film format, but to me a normal lens is what i see with my eye. so it's 14mm for 35mm format and 47mm for 4x5.

I'm glad you qualified your statement, because personally I can't fathom 47mm being anywhere near normal for 4x5. As a matter of fact none of my 4x5 cameras could even wear a 47mm lens without modification, or at the very least a recessed board. Movements would be nil, regardless of coverage. I consider somewhere around 150mm "normal" for 4x5. I think most other LFr's would as well. I assume your eyes must be set pretty far apart, and pointed left and right, respectively.

cotdt · Mar 18, 2008

Ole said:
Eh no - that's wrong on both counts. Most 50mm SLR lenses are either Tessar-derivatives or (very distant, for newer ones) Planar-derivatives; both of which may or may not be slightly retrofocus.

Gauss-type and Planar is more or less the same thing. The 50mm lenses from Nikon, Zeiss, Canon, Minolta, Pentax, etc. are all this type. I don't know any 50mm Tessars for SLRs, but Nikon does make a 45/2.8 pancake lens that is a Tessar. I don't see how I can be wrong when the manufacterers themselves say their 50mm lenses are Gauss-type designs. I know Nikon's 50mm is slightly retrofocus as well. So it's not a normal lens.

Ed Sukach · Mar 18, 2008

Ole said:
Eh no - that's wrong on both counts.

Wow!! That was a QUICK reply.

Most 50mm SLR lenses are either Tessar-derivatives or (very distant, for newer ones) Planar-derivatives; both of which may or may not be slightly retrofocus.

Yes .. NOW. Back at the beginning of lens design, "retrofocus" and "Single Lens Reflex" considerations were not exactly the most important.

It's fairly simple to make a lens with good speed which covers the necessary 53 degrees needed to cover the diagonal. But since SLRs have a mirror box, the "standard" became a slightly longer focal length that ensured that the rear element wouldn't interfere with the moving mirror.

Yes ... not an extreme problem ... but ... see above historical reference. Lenses were thought of as "normal" far before SLRs existed.

The first retrofocus SLR lens was a 35mm Angenieux Retrofocus, BTW...

Hmmm ... used on the Exacta? Was there another SLR (possibly Graflex), with interchangaeable lenses before then?

Pinholemaster · Mar 18, 2008

Why worry about it? For a sports photographer a normal lens is a 300 mm or longer on 35 mm.

For an architectural photographer, a normal lens usually is a wide angle on 4x5.

For a portrait photographer, a normal lens is probably a short telephoto on medium format.

Most shooters I know who use rangefinders 35 mm cameras, prefer a 35 mm lens over a 50 as their normal.

So what's 'normal?' Whatever you want it to be for your way of seeing.

Ed Sukach · Mar 18, 2008

Wooop!!! My apologies to Ole -- I misread "Eh" as "Ed", and I thought he was taking issue with MY post!

David Brown · Mar 18, 2008

Why be normal? :rolleyes:

Ole · Mar 18, 2008

Ed Sukach said:
Wooop!!! My apologies to Ole -- I misread "Eh" as "Ed", and I thought he was taking issue with MY post!

No worries ed, you've never been an "Eh".

"Back at the beginning", most lenses for 35mm film were Tessar and Tessar derivatives - take a loot at the Leitz Elmars, for example. a really fast Tessar, say an f:2.8, is very inadequate at a focal length equal to the frame diagonal at full aperture. They're still not much good after reducing the stated coverage by a couple of degrees, but they are a lot less objectionable.

"Normal" lenses for other formats tend to be very close to the frame diagonal; only the 35mm 24x36mm frame size is an exception. This could be due to several different things: 1) The need for fast, sharp lenses caused a 50mm standard since a 43mm with sharp corners at full aperture was too difficult.
2) Due to the narrower aspect ratio, a slightly longer focal length gives a more "normal" width" to the resulting frame.

Or maybe 3) - the first lens Leitz made for the Leica was a 50mm...

BTW, "absinthe's" original calculations seem to have been made under the influence of his namesake drink: Please remember that frame sizes are not equal to nominal size, and that there are 25.4mm in an inch.

MP_Wayne · Mar 18, 2008

DISREGARD PREVIOUS POST - Incorrect info. - THIS POST IS CORRECT NOW.

The original poster just wanted a rough rule of thumb. Walter23 was correct in providing the Pythagorean Theorum formula for determining the diagonal for each format size, and as a result, pretty close to the focal length.

For myself, when using various formats (4x5, 6x9, 6x12) with my view camera, I find it helpful to have a table using Walter's formula, and then converting that mm value (I measure each format in mm - even 4x5) into a magnification value. For example, as 50mm is the standard for 35mm, a 200mm would have a magnification of 200/50 or 4x. By setting up similar magnification values for various formats, I have an almost "apples to apples" comparison for quick referral in the field (i.e. best lens choice for what I want depending on format being shot). I know there are more complex 35mm equivalent tables about, but the above concept has served me well.

Absinthe · Mar 18, 2008

Hmm, well I was assuming that "Normal" meant something. I am not asking what is your prefered lens, or default lens and bes tlens for a job. But what exactly is "Normal". The fact that someone decided it should be the same focal length as the widest part of the image (diagonal) suggests something. I am sure there will be some reference to circles of confusion and all such things, but there is something interesting abotu the focal length and the diagonal being somehow similar. I mean, does a 300 mm lens project a 300 mm circle at 300 mm away at some particular f-stop or is it all just arbitrary? Certainly an inappropriately short lens will not project into the corners of the image, though perhaps that is a matter of other things?

JBrunner · Mar 18, 2008

Ed Sukach said:
Wooop!!! My apologies to Ole -- I misread "Eh" as "Ed", and I thought he was taking issue with MY post!

Eh?

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