What is a Normal lens?

Absinthe · Mar 18, 2008

If you like formulas here you go. Just enter the values in the white areas and read them the way you like in the colored areas.

JBrunner · Mar 18, 2008

To me, "normal" approximates human vision for given format excluding periphery. IDK how that relates to "normal" according to calculated norms of normal, (probably calculated by some guy named Norm) that just always how I have thought of it. I guess there is "official normal" or maybe "accepted normal" and "personal normal" as illustrated by the 47mm on 4x5 suggestion.

thuggins · Mar 18, 2008

There are some remarkable examples of obfuscating a simple question here. A normal lens is "normal" because it approximates the perception of the human eye. This is often regarded as the field of view of a single eye, but this is a pretty confusing way to look at it. The human eye cannot view a static image - that's not the way the eye/brain combination works. The eye actually scans a scene, while the brain constructs an image from the scan. If you try to perceive the limits of your field of view, your eye will involuntarily begin scanning a larger field of view, making the exercise impossible.

A better way of looking at it is with reference to perspective. A wide angle lens exaggerates perspective, causing near objects to be proportionately larger and far objects to be proportionately smaller. A telephoto lens compresses perspective, which is why the pitcher and batter appear to be the same size when watching baseball on TV. Even though the pitcher and batter are about 60 ft apart, the camera is a lot further away than that and using a telephoto lens. A normal lens produces normal perspective. Olympus lists 8mm - 35mm as wide angle, 40mm - 55 mm as normal, 85mm and longer as telephoto. (These are OM lenses).

Absinthe · Mar 18, 2008

Ok, that makes sense, but why is the relationship the same between the focal length as the diagonal?

Removed Account · Mar 19, 2008

I've realised that this may be a more appropriate thread for the imaginary conversation I made up concerning normal lenses. I wish to say that I was thinking 24" Artar, I'm not even sure there is a 48", and in my defence I have had a lot of caffeine today, along with a slight fever!

(there was a url link here which no longer exists)

- Justin

cotdt · Mar 19, 2008

a human eye would make for a great digital camera

David H. Bebbington · Mar 19, 2008

Absinthe said:
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.

The "normal" focal length for a given format is the one which BY CONVENTION gives a pleasing perspective - the lens in question will have an angle of view of ABOUT 46° and will have a focal length APPROXIMATELY equal to the diagonal of the format in question. Your figures corroborate this - for example, "normal" for 8x10" is 12 inches or 300 mm. Practical users may well deviate from this - an 8x10" studio photographer, for example, will always prefer a 360 mm (14") for its extra coverage, if he does portraits as well as still lives, he might well economize and just buy one 450 mm (18") lens.

Cameras originally intended for press use were deliberately given semi-wide lenses, e.g. original Rolleiflex 75 mm instead of 80, most 4x5" press cameras 5 or 5 1/2" (127 to 135 mm) instead of 150 mm. Large format reflex cameras in the days before retrofocus lens design always had longer lenses just to accommodate their large mirrors - a 4 x 5" reflex would have a 7 or 7 1/2" lens, for example.

35 mm photography is a story of an endless series of arbitrary numbers - the film is 35 mm wide because it originally was 2 3/4" wide Kodak film split in half, after sprocket holes were punched for cine use, the widest possible picture area was 24 mm. Barnack is supposed to have used a 50 mm "Kino Tessar" lens for his early experiments, this was designed to cover the cine frame of 18x24 mm, was no good for the double-size frame Barnack wanted, so he had to design his own lens but kept the 50 mm length. Most SLRs cannot accept a non-retrofocus lens of less than 55 or 58 mm focal length, the first SLRs simply had standard lenses of this length, later 50 mm or even 45 mm lenses were produced to retrofocus designs to make them more "normal".

"Normal" lenses are not always the right choice, this depends only on the personal choice of the photographer.

Regards,

David

PS: Among my older photographic books I have one from before World War I whose author is obsessed with the idea that a lens twice "normal" length always gives corrective perspective (e.g. 90 on 35 mm); he repeatedly shows examples of pictures taken with "normal" lenses and ridicules their "disturbingly distorted" perspective.

Absinthe · Mar 19, 2008

What is "retrofocus"

David H. Bebbington · Mar 19, 2008

Absinthe said:
What is "retrofocus"

Retrofocus lenses are ones where the optical length (the focal length) of the lens is shorter than its physical length. With SLR lenses, a wide-angle lens (24, 28, 35 mm) will almost always be around the same physical size as a standard (50 or 55 mm lens) because the rear of any lens cannot be allowed to project inside the camera (it would hit the mirror). RF camera lenses can be "straight" or non-retrofocus type - these are much smaller and easier to design to give high quality. Retrofocus is the opposite of telephoto, which is a lens physically shorter than its optical length - in older books, retrofocus is sometimes called "inverted telephoto".

Regards,

David

Dan Fromm · Mar 19, 2008

David, didn't you mean to say that a retrofocus lens is one whose back focus (back of rear element to film distance) is longer than usual for its focal length? Your explanation about lenses' physical lengths (you mean length from from of barrel to rear of barrel, don't you?) doesn't work at all for my 1.75"/2.8 Elcan. According to Elcan's data sheet for it, it is an inverted telephoto type and is 3.51 inches long; this is twice the Elcan's focal length and much longer than my 50/1.8 Nikkor-E and twice the Elcan's.

I think your description of telephoto is off a bit too. The key thing is that a telephoto lens' rear node is in front of its rear node, rather than behind as is usually the case. This makes the back focus shorter than usual for the focal length. I think you're in a 35 mm frame of mind and are confusing the focusing mount/tube that holds the glass with the glass. A telephoto lens in barrel, as for a large format camera, can easily be longer physically than a lens of the same focal length and of normal construction in barrel. But the tele will focus to infinity with less extension.

Cheers,

Dan

Absinthe · Mar 19, 2008

<off topic>
Dan, for some reason everytime I see your name spelled out I keep thinking "Art Frahm", and for the life of me I can't tell why...
</off topic>

Ok, so
telephoto = lens longer than focal length
retrofocus = lens shorter than focal length
right? or do I have it backwards

David H. Bebbington · Mar 19, 2008

Dan Fromm said:
David, didn't you mean to say that a retrofocus lens is one whose back focus (back of rear element to film distance) is longer than usual for its focal length? Your explanation about lenses' physical lengths (you mean length from from of barrel to rear of barrel, don't you?) doesn't work at all for my 1.75"/2.8 Elcan. According to Elcan's data sheet for it, it is an inverted telephoto type and is 3.51 inches long; this is twice the Elcan's focal length and much longer than my 50/1.8 Nikkor-E and twice the Elcan's.

I think your description of telephoto is off a bit too. The key thing is that a telephoto lens' rear node is in front of its rear node, rather than behind as is usually the case. This makes the back focus shorter than usual for the focal length. I think you're in a 35 mm frame of mind and are confusing the focusing mount/tube that holds the glass with the glass. A telephoto lens in barrel, as for a large format camera, can easily be longer physically than a lens of the same focal length and of normal construction in barrel. But the tele will focus to infinity with less extension.

Cheers,

Dan

Dan, you are striclty speaking correct - I was simply trying to give a simple explanation while avoiding the complication of terms like back focus and lens nodes. There's no confusion in my mind, but I was alluding to 35 mm lenses and using the term "physical length" to mean "physical distance from the lens to the film". Of course, with LF lenses which have no integral focusing mounting, the actual barrel of a telephoto lens is likely to be larger (longer) than that of a "normal" lens of the same focal length.

Regards,

David

ilya1963 · Mar 19, 2008

Forgive me if I am off subject

Am I wrong in saying that each given lens has different "Focal lengths" depending on how close or far the subject in focus is?

isn't it true when a term "Focal Length" is used to describe a lens it describes focusing at infinity ?

I always wondered what the focal length of a "normal" 12 inch lens is when the bellows are fully extended on my 8x10.
Is it 12 inches plus the length of the bellows or different rules apply somehow?

In both cases I am working with 12 inches of diagonal...

ILYA

Dan Fromm · Mar 19, 2008

David, thanks for the explanation.

What you call physical length seems much like what most of us call flange-to-film distance. Useful thing about flange-to-fill distance is that it is set by the camera body, not by the lens. This is why wide angle lenses for SLRs, which are all retrofocus, can be very useful reversed for closeup work; at the same magnification, they give more working distance than lenses of normal construction.

Michel Hardy-Vallée · Mar 19, 2008

Absinthe said:
Ok, so
telephoto = lens longer than focal length
retrofocus = lens shorter than focal length
right? or do I have it backwards

Backwards. A 300mm telephoto sits closer to the film than a 300mm long lens. A 35mm retrofocus lens is farther from the film than a non-retrofocus 35mm.

On an 8x10, a lens around 300mm focal length (a so-called "normal", to stay on-topic) needs to be 300mm from the film plane to focus at infinity. If you want to focus on a closer subject, you need to move the lens even further, so you need a longer bellows. But if that 300mm were to be of the telephoto kind, you could be focusing at infinity with, say, 200mm of bellows, and have ample bellow for close focus.

Zeiss currently sells two different 35mm focal length lenses. The Biogon and the Distagon. The 35mm Biogon is non-retrofocus, so the lens needs to be close to the film, something which is easy to do with a rangefinder. So the Biogon is a non-retrofocus lens, designed for cameras like the M-mount rangefinders. In comparison, the 35mm Distagon is a retrofocus design, and sits farther from the film plane, as is necessary in an SLR design like the F mount Nikons.

David H. Bebbington · Mar 19, 2008

ilya1963 said:
Forgive me if I am off subject

Am I wrong in saying that each given lens has different "Focal lengths" depending on how close or far the subject in focus is?

isn't it true when a term "Focal Length" is used to describe a lens it describes focusing at infinity ?

I always wondered what the focal length of a "normal" 12 inch lens is when the bellows are fully extended on my 8x10.
Is it 12 inches plus the length of the bellows or different rules apply somehow?

In both cases I am working with 12 inches of diagonal...

ILYA

Yes, focal length is lens to film distance at infinity.

At closer distances, this formula can be used:

1/u + 1/v = 1/f

u = Subject to lens distance
v = Image distance (lens to film)
f = Focal length

If we solve this equation for v = 600 (double extension on your 8x10") and a 300 mm lens, we get:

1/u + 1/600 = 1/300

Both sides x 600:

600/u + 1 = 2

Both sides -1:

600/u = 1

Both sides x u:

600 = u

Subject distance to lens is 600 mm. The focal length of the lens is still 300 mm (never changes).

BobbyR · Mar 19, 2008

Norm=Standard.

Bobby

ilya1963 · Mar 19, 2008

Thanks Dave
Got it... took me a bit thou

ILYA

haris · Mar 19, 2008

I never understood why need for determinate what would be "normal" lens. I mean if I want 21mm I will buy 21mm, if I want 50 mm I will buy 50mm, if want 135 I will buy 135mm lens.

Imagine entering into shop and: Me "Hello, I would like to buy some lenses" - seller "Hello, what lenses would you like" - Me "Well, give me 2 wide angle, one normal and three telephoto lenses, please"...

Question: Which focal lenghts would have lenses you just bought, if you buy them as in given situation?

Absinthe · Mar 19, 2008

If we are dicussing Infinity, why does the distance to subject matter? Well, I mean I understand why but, shoudl there not be a calculation for infinity, that goes from the closest possible subject to infinity?

cotdt · Mar 19, 2008

mhv said:
Zeiss currently sells two different 35mm focal length lenses. The Biogon and the Distagon. The 35mm Biogon is non-retrofocus, so the lens needs to be close to the film, something which is easy to do with a rangefinder. So the Biogon is a non-retrofocus lens, designed for cameras like the M-mount rangefinders. In comparison, the 35mm Distagon is a retrofocus design, and sits farther from the film plane, as is necessary in an SLR design like the F mount Nikons.

the retrofocus 35mm Distagon actually outperforms the 35mm Biogon, it's just such an amazing lens. maybe retofocus is not so bad after all.

Removed Account · Mar 19, 2008

haris said:
I never understood why need for determinate what would be "normal" lens. I mean if I want 21mm I will buy 21mm, if I want 50 mm I will buy 50mm, if want 135 I will buy 135mm lens.

Imagine entering into shop and: Me "Hello, I would like to buy some lenses" - seller "Hello, what lenses would you like" - Me "Well, give me 2 wide angle, one normal and three telephoto lenses, please"...

Question: Which focal lenghts would have lenses you just bought, if you buy them as in given situation?

Wide angle refers to the angle of coverage, so a wide angle can cover a greater film size than usual for that focal length. Normal could mean just about anything (usually lens design or standard focal length), and telephoto refers to a specific lens design.

Maybe short focus or wide-field, standard, and long focus would work better? (I'm sure somone will debate on the difference between wide-angle and wide-field )

- Justin

Dan Fromm · Mar 19, 2008

Hmm. There's nothing about normal lenses in DSM-V.

Nick Zentena · Mar 19, 2008

Justin Silber said:
Wide angle refers to the angle of coverage, so a wide angle can cover a greater film size than usual for that focal length. Normal could mean just about anything (usually lens design or standard focal length), and telephoto refers to a specific lens design.

Only if you stick it on a bigger film format. Stick my 210mm Fuji-W that covers 80degrees and 8x10 on a 35mm camera [Don't ask me how ] and it sure won't be a wide angle.

Or stick it on 5x7 and it's a normal lens. The fact it covers far more doesn't matter.

Now stick it on an 8x10 and it's a wide angle. Why? Because it's shorter then a normal lens

John Koehrer · Mar 19, 2008

Nick Zentena said:
Only if you stick it on a bigger film format. Stick my 210mm Fuji-W that covers 80degrees and 8x10 on a 35mm camera [Don't ask me how ] and it sure won't be a wide angle.

Gaffer tape & a bellows. Did it once for s&g with an old tessar & Nikon bellows.
Used the shutter on the lens for exposure, camera on T

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