Why is Focal Length in mm (cm and inches), rather then Angle of View?

Kirks518 said:

I shoot multi-format, but I just recently started in MF. That's when I realized it was a PITA to remember that Xmm in one format will give one result, and a different result in another format.

It dawned on me that rather then putting 50mm on the outside of the lens of the MF, they could label it an 81° lens (the lens is made for a specific format, so only that format's AoV would need to be on the lens). Or they could put something like; 50mm/81°. It's not so a user can use it across different formats, its simply so that the user will have an idea of what the lens will give them without ever having to mount it on a camera.

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And so, what should I engrave around the Schneider 90mmXL lens which I alluded to in my earlier post? It has a different AOV for 4x5" than it does for 5x7" frame, and I the lens maker have no idea which format that Wilt will be using my lens on?! Or more of a problem...the Schneider 150mm XL, which could be used on 4x5 or 5x7 or 8x10?!

Angle of view or field does not really give a usable reference without extra calculations or thought whereas field of view or image circle is all I need for relationships between lenses. E.G. I have 2 (at least) f5.6 150mm enlarging lenses. My El Nikkor will just cover 4 x 5. I have a Wild Reprogon which has an image circle of at least 14" and covers a 9 x 9 negative. Just for interest; One weighs 10 oz. and the other weighs about 12 lb. the Wild is designed for extreme flatness of field and has a COC of about 1 micron with almost 0" depth of field. Focusing with a microscope, .001" out of focus and the image is soft.

Kirk, you wrote:

I shoot multi-format, but I just recently started in MF. That's when I realized it was a PITA to remember that Xmm in one format will give one result, and a different result in another format.

It dawned on me that rather then putting 50mm on the outside of the lens of the MF, they could label it an 81° lens (the lens is made for a specific format, so only that format's AoV would need to be on the lens). Or they could put something like; 50mm/81°. It's not so a user can use it across different formats, its simply so that the user will have an idea of what the lens will give them without ever having to mount it on a camera.

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What you suggest has been done. It isn't as helpful as you imagine. I shoot several formats that fall in the medium format category. 2x3 (6x9 in metric) and 6x12. I can also shoot 6x6 (2.25" square in Imperial), I have the roll holder for it, and could get a 6x7 roll holder. The shortest lens I have for these formats, a 35/4.5 Apo Grandagon, is engraved "120 °". It just covers 6x12, has ample coverage for smaller formats. Are you seriously suggesting that Rodenstock should have engraved its angular coverage for all formats I might use it on? Are you out of your mind?

I don't look at the scene in front of me, whip out a protractor or a zoom finder (director's viewfinder) and measure the angle needed to encompass what I want to fill the frame with. Do you? I doubt it.

What I do is survey the scene, guess the right focal length, put the most likely lens on the camera and look at the composition it allows. If I guessed wrong -- practice helps but I don't always guess right first time -- I adjust the guess and try again. I do use one rule of thumb when I guess. I know my formats normal (diagonal of the gate) focal length and usually know whether I want a lens wider than normal, normal, or longer than normal.

There's a shortcut calculation to accomplish what you want without using trigonometry or thinking about angles. Re angles, people are very bad at estimating them. That's why pie charts are so hard to understand. The trick:

equivalent focal length in format A = (focal length in format B) * ((length in format A)/(length in format B)) Length can be the format's short side, long side or diagonal.

If you can't do the arithmetic in your head, there are always calculators including that wonderful analog multiplier/divider/exponentiator the slide rule.

lxdude said:

Yes. It is a de facto standard to people, because of the very wide use of 35mm cameras. Most of us know how to correlate FL to an image in 135 format. It's not a formal standard, just one most people understand.

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35mm cameras in wide use?

Bill Burk said:

Does light then emanate from the retina and travel towards the source?

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I know you are being silly, but Maxwell's equations are symmetrical with time, so there is a sense in which the answer to your question is "we don't know and we cannot meaningfully distinguish the difference"...

Kirks518 said:

So my question remains; why do you think they (I have no clue who 'they' are) decided to use the focal point to film plane distance as the 'standard' of measurement, when even then there were different formats that would result in different final images? And keep in mind, LF was around for a long time before 35mm became the 'every man' format. My guess it had more to do with the design/manufacturing process than anything else.

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No, it has nothing to do with the design/manufacturing process and has everything to do with the physical characteristics of the lens and the images it will produce. Specifically, it deals with Depth of Field. A 50mm lens produces the same DOF regardless of the film format. That is why consumer grade d*&^%@l cameras have an essentially infinite DOF - the sensors are so small that a "normal" lens is extremely short and the entire scene will be in focus even with large apertures. This is a major advantage of 35mm photography. Lens length can be varied within practical limits depending on the subject (e.g. landscape with a deep DOF vs portraits with a shallow DOF). This same feature allows you to expand or compress perspective.

Only someone who knows little to nothing about photography would think about a lens in term of an angle of view.

NedL said:

I know you are being silly, but Maxwell's equations are symmetrical with time, so there is a sense in which the answer to your question is "we don't know and we cannot meaningfully distinguish the difference"...

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I was being a bit silly but historically, it was once believed. I think it was a theory that tried to explain how animals can sense your presence when you look at them.

Dan Fromm said:

Kirk, you wrote:



What you suggest has been done. It isn't as helpful as you imagine. I shoot several formats that fall in the medium format category. 2x3 (6x9 in metric) and 6x12. I can also shoot 6x6 (2.25" square in Imperial), I have the roll holder for it, and could get a 6x7 roll holder. The shortest lens I have for these formats, a 35/4.5 Apo Grandagon, is engraved "120 °". It just covers 6x12, has ample coverage for smaller formats. Are you seriously suggesting that Rodenstock should have engraved its angular coverage for all formats I might use it on? Are you out of your mind?

I don't look at the scene in front of me, whip out a protractor or a zoom finder (director's viewfinder) and measure the angle needed to encompass what I want to fill the frame with. Do you? I doubt it.

What I do is survey the scene, guess the right focal length, put the most likely lens on the camera and look at the composition it allows. If I guessed wrong -- practice helps but I don't always guess right first time -- I adjust the guess and try again. I do use one rule of thumb when I guess. I know my formats normal (diagonal of the gate) focal length and usually know whether I want a lens wider than normal, normal, or longer than normal.

There's a shortcut calculation to accomplish what you want without using trigonometry or thinking about angles. Re angles, people are very bad at estimating them. That's why pie charts are so hard to understand. The trick:

equivalent focal length in format A = (focal length in format B) * ((length in format A)/(length in format B)) Length can be the format's short side, long side or diagonal.

If you can't do the arithmetic in your head, there are always calculators including that wonderful analog multiplier/divider/exponentiator the slide rule.

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The first bolded area: No.... re-read what I wrote. One AoV (or FoV) for what may be considered the 'most common' image size. In medium and large format, I realize there are a gazillion different sizes that a user may be shooting, but maybe as you say what was done with the Angulon - the AoV/FoV based on the largest area it will cover. I don't see the problem with a manufacturer saying - 'The AoV (of FoV) of our lenses designed for large format use gives X° on a 6x12' Could be written on the lens as something like 120°/612. For lenses that only will cover a 6x9, maybe something like 120°/66 (for a 6x6).

The second bolded area: And why are you able to make an educated guess as to what lens to use based on the scene? Experience and familiarity with the current system. If you had not been shooting for years, you wouldn't have a clue what lens would give you what area in your VF. You'd (or any new camera owner) probably say, 'well, 50mm is pretty small, and I want to capture a large area of the scene, so maybe a 400mm would be better'. Thinking that 400mm is bigger then 50mm. but if one lens said it will produce an AoV/FoV of X°, and the other lens gives and AoV/FoV of Y°, you'd have a better chance of making the right guess, even though you would have had no experience with either.


IMO, the focal length designation does not tell anyone what the lens does. It is only a measurement of one aspect of the lens' design. It's sort of like calling a vehicle engine a 29" rather then a 302, because the engine block is 29" long. The length of the block has little to do with the power output, like the mm designation has little to do with the output of a lens.

ic-racer said:

35mm cameras in wide use?

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And how many cameras other than 35mm have reached the production figures of 35's?
Canon mad over 1M of the AE1. how many 6X7/6X9/LF/ box cameras have been made? Combined, maybe 1M?

Kirk: I resent being called an "analog idiot" just as you might resent being called a Wanker.

frank said:

Ich bin ein analogue idiot!

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+1*104

Bill Burk said:

Does light then emanate from the retina and travel towards the source?

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If you ever saw the looks my ex would give me, you would say yes.

Sirius Glass said:

If you ever saw the looks my ex would give me, you would say yes.

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L O L !

One AoV (or FoV) for what may be considered the 'most common' image size. In medium and large format,

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While angle of view of a lens depends upon the format in use the field of view of the lens does not. The field of view related how much the lens can cover without vignetting or the quality is too poor to be considered usable.

Bill Burk said:

Does light then emanate from the retina and travel towards the source?

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Actually, yes, when doing certain things, like projection, the light leaves the film and enters the room. Also when doing ray tracing, the ray is projected from the "camera" into the "scene" until it hits an "object" in the scene, then is projected towards the "light source." If it hits another object before it gets to a light source, then that point of the object is in shadow. Doing it backwards makes the ray tracing faster, as all those pesky rays that don't enter the camera can be ignored.

Kirk, why don't you just mount a lens and look through it? And why don't you stop theorizing and stop practicing?

Re Rodenstock and my little Apo Grandy of nothing at all -- do you even know what it is? -- the lens will produce a circular image with decent image quality 125 mm in diameter on 5" x 7" sheet film. On 4x5 it will produce rectangular images with clipped corners. It barely misses filling quarter plate with good image. The corners will have decent image quality on 6x12 and smaller formats. Rodenstock's claimed 120° isn't available on 6x9, the format for which I originally bought the lens. 120° by itself doesn't mean squat.

Re experience, when I bought my first interchangeable lens camera I bought a 50 mm lens with it. That was the done thing. The second lens I bought for it was a 200, the third was a 105. Learning how to reason from what the 50 saw to what the 200 would see took several minutes. Similarly for the 105. Its not hard. I don't see why you find the exercise so threatening.

Stop complaining and use your lenses. What do you have?
My 2x3/6x12 lens kit isn't the smallest possible. 35, 47, 58, 60, 65, 75, 80, 85, 90, 100, 101 (macro only), 105, 120, 127, 130, 135, 150, 158, 160, 180, 210, 240, 250, 255, 300, 305, 360, 420, 450, 480, 610, 900. They don't all travel with me and few get much use. A small difference in focal length means very little for lenses longer than normal (100 mm for 6x9, 125 mm for 6x12). Note that the focal lengths listed are all as engraved; actual focal lengths may differ from engraved by as much as 3%.

Focal length tells you several important things about what a lens will do. In particular, and this is very important for view cameras and similiar, for lenses of normal construction (retrofocus and telephoto lenses are not of normal construction) it tells you the flange-to-film distance when the lens is focused at infinity. This makes it possible to answer the question "can I even use the thing on my camera?" And for all lenses it allows one to calculate extension from the infinity position needed to get the desired magnification. This is critically important for closeup and macro work. As I've said, you don't know very much. This is not an insult, it is an observation.

Your engine example is poorly taken. An engine's displacement give a weak indication of its power output, the length of the cylinder block gives no indication of the power output.

Put your shovel away, stop digging, and start looking through your lenses. Figuring out which of the lenses ready to hand to use doesn't take years of experience.

Are we writers or are we photographers!

Have we stumbled upon a controversy akin to f/64 vs the pictorialists:munch:?

It may be that it would be better to add some extra information to the commonly available information for lenses, but the two most important and invariable facts about any lens remain the focal length and the apertures available.

All the rest will vary with the needs and intended uses of each photographer.

MattKing said:

Have we stumbled upon a controversy akin to f/64 vs the pictorialists:munch:?

It may be that it would be better to add some extra information to the commonly available information for lenses, but the two most important and invariable facts about any lens remain the focal length and the apertures available.

All the rest will vary with the needs and intended uses of each photographer.

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Iffin it ain't brok, don't fixxit.

Kirk,
I think that you are complicating things for yourself. It should take you a very short time to become aware of what each of your lenses will give you on each camera by looking at the image on the ground glass. With a little familiarity, changing lenses becomes second nature and takes a couple of seconds.

Someonenameddavid said:

Can you imagine f stops in relation to angle of view: although if you measured the angle in radians rather than degrees you could have a pi/4 lens f/2

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I can't, because there is no a priori relationship between a lens' angle of view and focal length.

Chan Tran said:

Specifying angle of view doesn't make sense while specifying angle of field does. But the focal length must be specified.

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The convention as accepted in the optical engineering and design community is "Field of View".

You image in two-dimensional space (field), not one-dimensional space (an angle).

Look up MIL-STD-1241A, which captured the industry standard optical terms and definitions.

http://everyspec.com/MIL-STD/MIL-STD-1100-1299/MIL_STD_1241A_867/

thuggins said:

No, it has nothing to do with the design/manufacturing process and has everything to do with the physical characteristics of the lens and the images it will produce. Specifically, it deals with Depth of Field. A 50mm lens produces the same DOF regardless of the film format. That is why consumer grade d*&^%@l cameras have an essentially infinite DOF -

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I'm sorry but this is incorrect on several different levels. Recommend you look up the definition of depth of field. It is a function of the marginal ray angle in image space..that is, the f/# of the system. Yes, f/# is partially dependent on focal length, but the focal length alone does not determine depth of field.

By the way, the depth of field is not impacted by the imaging media. An optical system is a linear system, after all. The lens doesn't "know" what's doing the imaging. Regardless of whether you're using film or digital, optical parameters such as DOF are dependent on the physics of the lens...just as you say.

As Dan says, just mount a lens and look through it.

Only someone who knows little to nothing about photography would think about a lens in term of an angle of view.

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Now that's just uncalled for.

I'm done with this thread.

Dan - You're an insulting jackass with a closed mind. I picked up my first camera at age 9 in 1974 (Instamatic), and moved to Canon SLR's about a year later (thanks Dad).

Twice in this thread you have attacked me personally simply because you don't agree with me, can't see beyond your own opinions, and as I've said before, you're luddite. Of everything you've said, I already knew about 99%. You're nothing special.

We have an optical engineer posting in this thread, and people don't want to learn from him. That would be like not listening to PE in the developing threads.

When I started this thread, I was just wondering why the system in place is there, and told what prompted the question. I did not suggest changing it, but it seems most who have responded to this thread have the reading comprehension of a hamster, and believed I said it should be changed. And so, because of your inabilities to read properly, and maintain a level of decorum that one would expect in an adult conversation, you throw insults, make assumptions about someone's skill levels, and puff up your chest because you're so smart and great.




Ya know, I just renewed my subscription to APUG, and I'm starting to regret doing so. A close minded group is not a group I take pride in associating myself with.... Nothing personal Sean.

Rereading Dan's posts #92, 103, 115, I find each of them personnally insulting to Kirk...NOT!

Perhaps, at the same time I should be insulted at posting a number of very factual and objective posts, to which Kirk has responded to none in any attempt to justify his position on the topic when my information was posed?!

Dan, your response to my post 51 was prophetic, not insulting.

Kirk, if all you wanted to know was why focal length was chosen as a means of identifying a lens's optical capability, then the answer is simple: it is the most logical way to do so.

May the burning of bridges light your way.