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# Thread: What is a Normal lens?

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

2. 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.

3. Originally Posted by walter23
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.

4. 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.

5. Originally Posted by cotdt
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...

6. Originally Posted by cotdt
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.

7. Originally Posted by Ole
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.

8. Originally Posted by Ole
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?

9. 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.

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

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