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

MattKing said:

Actually Dan, I think the advent of widely available 35mm cameras changed it first.

Even the resolutely digital shooters are used to thinking in 35mm "equivalents".

I had a strange conversation recently with someone who was presenting a mini-workshop on depth of field when I said that one had to take into account "sensor" size/magnification.

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Most digital shooters have the '35mm equivalents' way of thinking for two reasons...

• The manufacturers first came out with small APS sensor dSLRs because 24x36mm sensors were too expensive/not viable affordably, so to allow 135 shooters to think quickly in the smaller format, they had to invent the '35mm equivalent'.
• All the wide variety of sizes of 'dinky sensor' P&S cameras with zoom lenses made it far too difficult for 135 P&S shooters to think about FL except in the context of 'what I used to use on my 135 formal film P&S'

Before then, film shooters simply 'understood' the shooting of different format sizes, and learned to naturally pick up a certain lens without the 'crop factor' gymnastics.
Digital screwed that all up, simply because everyone was used to shooting 35mm film cameras, and the digital sensor sizes were all over the place (but not 135 format-sized, 10 years ago!)
If you think of it, any P&S shooter was largely blind to what FL they were shooting at...it was NOT displayed on the camera! So '35mm equivalent' was a totally irrelevant concept to their experiences! And a smartphone shooter has zero context for '35mm equivalent', too. So the only people for whom it has relevance is the 'hardcore traditional SLR film shooter', but most of those people who would move to APS-C digital from film have already done so! So the only reason for the term are the APS-C digital shooters wanting to upgrade to FF sensors. No one else in the world really cares, nor needs to care about '35mm equivalent'

Nodda Duma said:

talking about an "ordinary person" artificially limits the scope of the discussion to a small slice of imaging optical design.

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Yes, because I was referring to ordinary people's use of common non-imaging optics. And the original discussion was of ordinary people's use of lenses in photography, to which my statement was related. Most people are not concerned with optical design-just how they can get what they want from those optics.

The question is if aspect ratio is important to lens designers outside of photography lenses. My answer -- as a lens designer -- is that it is.

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I'm sure it is, but that's not the question posed by the OP. All I was saying is that those who use common non-imaging optics do not deal with aspect ratio, or any issues of format, so designating by AoV/FoV makes sense.

wiltw said:

Lets use a Schneider large format lens information sheet to illustrate...

Let's concentrate on the Schneider 5.6/90 XL for this discussion...

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wiltw,

Thanks for using what I'm looking for as the example for discussion. Now I have a shopping list!

wiltw said:

Most digital shooters have the '35mm equivalents' way of thinking for two reasons...

• The manufacturers first came out with small APS sensor dSLRs because 24x36mm sensors were too expensive/not viable affordably, so to allow 135 shooters to think quickly in the smaller format, they had to invent the '35mm equivalent'.
• All the wide variety of sizes of 'dinky sensor' P&S cameras with zoom lenses made it far too difficult for 135 P&S shooters to think about FL except in the context of 'what I used to use on my 135 formal film P&S'

Before then, film shooters simply 'understood' the shooting of different format sizes, and learned to naturally pick up a certain lens without the 'crop factor' gymnastics.

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What I was trying to point out was that before 35mm cameras became ubiquitous, the 35mm equivalence wasn't all that important.

When a "snapshooter" was just as likely to be using 120, 620, 127, 116 or 616 film as they were to be using 35mm film.

You know - when 35mm cameras were still known as "miniature" cameras.

I would say that the change occurred in or about 1970, when both semi-affordable 35mm SLRs and affordable 35mm point and shoot and rangefinder cameras became commonplace.

The problem was not the popularity of the 35mm format. It was the fact that when they introduced the DSLR they used the smaller APS-C sensor but yet using exactly the same lens line as for the 35mm camera. If they were to introduce at the same time a different lens line for the digital then people won't have that problem.

Why is the gas tank on my truck measured in gallons instead of how many miles I can drive?

Why is my house measured in square feet instead of how many years I can live here?

Why is film rated with an ISO number instead of the minimum EV I can shoot it with?

MattKing said:

I would say that the change occurred in or about 1970, when both semi-affordable 35mm SLRs and affordable 35mm point and shoot and rangefinder cameras became commonplace.

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I disagree...I grew up learning photography in the 1960's. I own and have shot multiple formats professionally as an adult, an uncle even owned the half-frame cameras from Olympus back then when I was a kid. NEVER, EVER did I hear of the term '35mm equivalent' -- until digital APS-C digital bodies that accepted FF SLR lenses. Never, ever did I ever hear of a 'crop factor' or numerical multiplier to 'convert' FL in one format to the 'equivalent' in a different format.

In 1990 Sinar published a Large Format book which equated focal lengths to the frame dimensions, not to the focal length equivalents of any other format (much less 135, which was never a photographic 'standard' by which all other formats were defined). FL = short dimension of frame was the definition of a 'wide angle' lens for that format, for example.

Advance to the introduction of affordable digital SLRs, and suddenly the trade press and the blurbs from the manfacturers start to relate things to the 135 format as if it were a 'standard' of measurements!
Why would a format, which the photographic community was still trying to establish with an air of respectability even in the 1960's, be a 'standard'? It fell into it with the lay community, just as 'bokeh' is so wrongly used by the lay community!

wiltw said:

Why would a format, which the photographic community was still trying to establish with an air of respectability even in the 1960's, be a 'standard'? It fell into it with the lay community

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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.

I'm really disturbed that car mileage changed from miles per gallon to litres per 100 kilometers. I'm okay with the metric system but would have preferred kilometers per litre or kilometers per 10 litres.

Chan Tran said:

The problem was not the popularity of the 35mm format.

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It is the fault of the fauxtographers.

frank said:

I'm really disturbed that car mileage changed from miles per gallon to litres per 100 kilometers. I'm okay with the metric system but would have preferred kilometers per litre or kilometers per 10 litres.

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Yes, you live in a country which converted to metric in a manner which nearly crashed a jetliner full of people because of the conversion error about amount of fuel to load before a flight!

frank said:

I'm really disturbed that car mileage changed from miles per gallon to litres per 100 kilometers. I'm okay with the metric system but would have preferred kilometers per litre or kilometers per 10 litres.

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Leave it to the gummint to screw it up...

I like gallons per mile.

Sirius Glass said:

I like gallons per mile.

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In your off-road vehicle?

Of course photography is overloaded with relationships that don;t make sense. Paper is made in sizes that do not match film or sensor formats which do not match HDTV and computer screens. Cropping is a dark science that screws up the SOOC picture you took in the format of the camera that refuses to fit into print or display sizes unless you refuse to buy standard size mating and like using an Excel knife. So why shouldn't we continue with this madness with lenses?

Here's a suggestion. Print and inscribe on the barrel something like this: 135 FF: 50mm/28° MFT 2x: 100mm/56° APS 1.5x: 75mm/42° etc.

Sirius Glass said:

I like gallons per mile.

MattKing said:

In your off-road vehicle?

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Exactly

Alan Klein said:

Here's a suggestion. Print and inscribe on the barrel something like this: 135 FF: 50mm/28° MFT 2x: 100mm/56° APS 1.5x: 75mm/42° etc.

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All this nonsense to tell whether a lens is the right one to use for a shot? Why not just mount it and look through it. If the camera doesn't offer TTL viewing, use the right viewfinder.

I am baffled that this discussing is tolerated on a forum dedicated to film enthusiasts. It makes no sense in the context of cameras used to shoot film, addresses what only digital idiots and ignoramuses see as a problem.

Dan, there is no way that I can read your statement as anything other than an insult to me (OP), and anyone else that disagrees with you. You are baffled because you're an analog idiot, and without a doubt, a luddite. Remember the old SCTV (or was it SNL) skit - "And that's the way we liked it!"? That's what you sound like.

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.

A lot of the responses here lead to the fact that most already know what XXmm lens will result in, but that is due to one thing, and one thing only: EXPERIENCE. But the first time you had a camera in your hands, you had no idea what you were going to see in the VF; until you looked through it. If you knew what the AoV was, you would have had some inkling of what you'd see. Every hand someone one of your cameras with an ultrawide or a super-tele on it? They're astounded! They had no clue that a 400mm lens is going to 'bring them so close'. But if you said, "Here, this lens gives you a 7° view of what you're pointing at", they would probably have an idea.

The question (as I stated earlier) really had nothing to do with using lenses across formats. It was about using format specific lenses on their design specific format, and how the millimeter designation (in ANY format) is not a good representation of what the end result (what is seen in the VF) will be, whereas an AoV designation for that specific lens, on the format it is specifically designed for, would.

And as for the argument that in MF and LF there are different image sizes (6x6, 6x7, 6x9, 4x5, etc), the AoV could be based on the most historically common size (probably 6x6 for MF and I dunno for LF (don't shoot it)).

The original question was also why did they finally settle on millimeters, and not centimeters or inches, both of which were commonly used in the earlier days of photography and lens design.

Ich bin ein analogue idiot!

wiltw said:

I disagree...I grew up learning photography in the 1960's. I own and have shot multiple formats professionally as an adult, an uncle even owned the half-frame cameras from Olympus back then when I was a kid. NEVER, EVER did I hear of the term '35mm equivalent' -- until digital APS-C digital bodies that accepted FF SLR lenses.

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I agree the small sensor in early digital cameras that cost a thousand bucks made it necessary for marketers to "explain what you are getting" in terms you could understand and they could sell.

But I think I've seen it explained in terms of equivalent per format long ago. Maybe in LIFE Library of Photography or other general photography teaching books. The need for the chart back then was to help the student who might be using sheet film or medium format in class when they had been accustomed to 35mm at home.

During my university years as an Electrical Engineering student, we were told that, by convention, electric current is the direction of positive charge flow. After this convention was set, it was discovered that the charge carrier in metals is in fact the electron, which is negatively charged. But us engineers continue to use the convention that electric current is the direction of positive charge flow. Where physics is involved (e.g. explaining transistor behavior), we use the electron in N-doped materials and the hole (absence of an electron) in P-doped materials, but otherwise the direction of current flow is the direction of positive charge flow (even though us engineers know the current is carried by electrons which are negatively charged).

I suspect that it's somewhat the same in photography - it's convention, nothing more. Angle of View makes a lot of sense for 35mm photography (and even "half-frame" since those lenses are designed for a specific camera mount), but I can see the merits of using focal length for medium and large format (LF especially, since you can use a lens intended for 8x10 on a 4x5 camera).

Here's a MF example, though, of why it might be a bad idea to use AOV, and why Focal Length is a good idea because it's a property of the lens regardless of format: The Lomo Belair 6x12 takes photos in 3 formats - 6x6, 6x9, and 6x12. If you purchase the optional 35mm back, you can take 4 formats with the same camera. A normal lens is 80mm for 6x6, so let's assume that the horizontal AOV for this lens is 50 degrees on 6x6 (just to make the math easy to do in my head and avoid having to deal with the Pythagorean theorem to get the length of the diagonal). On 6x9, the AOV is probably around 75 degrees, and on 6x12, it's around 100 degrees (again, assuming that the 80mm lens can in fact cover 6x12). But if you market this lens as having an AOV of 50 degrees, the 6x9 and 6x12 users may think that the AOV is 50 degrees for their format, when in fact it is much greater, leading to upset purchasers thinking they're getting a normal lens for their format when in fact they're getting a wide angle (yes, some people are really that dumb). Marketing lenses by focal length makes sense because it is a property of the lens, not a property of the lens + film format.

As an addendum, I've also seen P&S digital cameras with a focal length on them, but marked as an equivalent 35mm focal length, since so many people were familiar with 35mm film photography and the AOVs they got from those lenses (e.g. 28-105mm 35mm equiv.)

ME Super said:

During my university years as an Electrical Engineering student, we were told that, by convention, electric current is the direction of positive charge flow. After this convention was set, it was discovered that the charge carrier in metals is in fact the electron, which is negatively charged. But us engineers continue to use the convention that electric current is the direction of positive charge flow. Where physics is involved (e.g. explaining transistor behavior), we use the electron in N-doped materials and the hole (absence of an electron) in P-doped materials, but otherwise the direction of current flow is the direction of positive charge flow (even though us engineers know the current is carried by electrons which are negatively charged).

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

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

The original question was also why did they finally settle on millimeters, and not centimeters or inches, both of which were commonly used in the earlier days of photography and lens design.

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Most of the world use the metric system so using inches doesn't make too much sense unless the lens is for sale only in the US. It's common practice not using the Decimeter or Centimeter. Generally meter, millimeter, micrometer, nanometer in the 1000 factor. Among all of those the mm makes the most sense as most lenses focal length ranging from a few milimeters to a few hundred.

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