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

[Kirks518]

(Wasn't sure where to put this, as we don't have a 'Lens' Section, but since 35mm is the general public's standard, I figured I'd put it here.)

Now that I'm shooting MF as well as 35mm (and APS-C), this question has really been bugging me.

I understand that the focal length of a lens is determined by the distance of the focal point to the film plane, whether it be label in mm, cm, or inches. But when you change formats, what an Xmm lens captures changes from format to format. In other words, a 50mm lens on a 35mm format camera will capture ~46° of the scene. But a 50mm lens on a 6x7 will capture ~81° of the scene. Same focal point to film plane distance, but a completely different image results.

Now it can be said that the general public knows what a 'XXmm' lens will give them as a final image, but that's only because it became the standard way back when. I know for a fact that people new to photography have a difficult time grasping the difference between what they will get as a final between a 24mm, 50mm, and 135mm. Many just don't have any idea what that means, and I'm sure back in the day it wasn't any difference. There is no (IMO) logical relationship between 50mm and 46°, that can be (relatively) immediately understood by a novice.

But if I say to someone who has a basic understanding of geometry that your eyes see ~200° of a scene, and that lens A gives you an image that represents 81° of that scene, I think they might have a better comprehension of what the end result will be. They can take their hands, hold them up to their face, and make blinders that give a 81° view of the scene, and get an idea of what they'll get in their image.

I personally try and convert a 'mm' lens into AOV lens, so I can 'see' what I'll get without having to try different lenses. In other words, I'll look at a scene, and say to myself I want approximately 55° of what I am seeing to be captured, and then grab the lens closest to that.

Labeling a lens as a 52° AOV would standardize the entire lens lineups across formats. No matter what format you're using, a lens with an AOV of 52° will give you 52° of the entire scene. So, in my case, my 24mm (35mm format) lens would be labeled as 81° AOV, as well as my 50mm MF lens (Mamiya RB67). But the way it is now, I have two lenses that are named the same, but give 'wildly' different results.

Discuss....

 

[Kirks518]

Oh, and yes, a lens' AOV is on the diagonal, but it would still be a more viable way for a novice (or anyone) to have a clue as to what the FL would be on any format.

 

[Alan Klein]

The angle of view is different horizontally then vertically because of the format of the sensor or film that varies. Of course the same could be said of 135mm for example. Good question; interesting solution. Since I'm an old guy who shot in 35mm cameras for so many years, giving equivalent to 35mm cameras makes it easier for me. But I could see how new shooters won't glean anything from those comparison.

 

[snapguy]

rot

It is a rotten world full of dumb people. It would seem you want to ditch the present dumb system and have millions of pepole all over the globe learn your system. What is wrong with this picture?

 

[frank]

Angle of view is dependent on the film format, focal length is what it is, regardless.

 

[Kirks518]

@snapguy - No.... and judging by your comprehension of the original thread, I could see how you would come to the conclusion you did. I never said to change anything, but rather why did they chose to do it the way they did? But I'll tell you that odds are if you took a room full of 9th grade kids, and taught half of them lens focal lengths in mm, and the other half in AOV, there would be a considerable difference in comprehension, and those taught the AOV would have a better understanding than the others. To then move that over to the general public (the majority of whom do not have a clue what the difference is between a 24mm/50mm/200mm lens is), it wouldn't be as difficult as you think to change the mindset.

Interestingly enough, I came across this on the B+H website:

To eliminate all this confusion, perhaps it's time to stop thinking of lenses in terms of millimeters and instead identify lenses in terms of their angles-of-view. Angles-of-view (AOV) are a constant. An 84° AOV will always identify the lens as a wide-angle. On a DSLR containing a full-frame (24x36mm) sensor this would translate into a 24mm lens, while on a Leica M8 (1.3x) it would be closer to an 18mm lens, and a 12mm lens on a 4/3-system camera (2x). There will be subtle differences between each of the resulting images based on the sensor size, but the angle-of-view will always appear the same.

and a link to the page: http://www.bhphotovideo.com/c/find/newsLetter/Angles-of-View.jsp

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.

 

[Sirius Glass]

Angle of view is dependent on the film format, focal length is what it is, regardless.

In the US lenses where it was described by the focal length in inches. Europe and the rest of the world [ignoring the UK] used millimeters and centimeters, and later standardized with millimeters. Focal length is the constant that can be used against any format. What 300mm means in 35mm versus in 6x6 for 120 film versus in 4"x5" versus 5"x7" versus 8"x10" varies but the commonality is the universally understood constant of focal length.

 

[Kirks518]

Angle of view is dependent on the film format, focal length is what it is, regardless.

How can you say that? If you are saying that a 50mm lens on a medium format will give you the same scene as a 50mm lens on a 35mm format, or a 50mm on 4x5 format, you're completely mistaken. The final image captured with a (designated) '50mm' lens will give you completely different images across different formats.

But if I have a lens designated as an 81° AOV lens on medium format, and take a picture of a scene, what is captured would be the same as if it was taken on 35mm format with a 35mm lens designated as a 81° AOV lens. But to accomplish that the way it is currently named/designated, I'd have to use a 50mm MF lens, or a 24mm lens on 35mm format. So the FL is not the constant.

 

[RalphLambrecht]

(Wasn't sure where to put this, as we don't have a 'Lens' Section, but since 35mm is the general public's standard, I figured I'd put it here.)

Now that I'm shooting MF as well as 35mm (and APS-C), this question has really been bugging me.

I understand that the focal length of a lens is determined by the distance of the focal point to the film plane, whether it be label in mm, cm, or inches. But when you change formats, what an Xmm lens captures changes from format to format. In other words, a 50mm lens on a 35mm format camera will capture ~46° of the scene. But a 50mm lens on a 6x7 will capture ~81° of the scene. Same focal point to film plane distance, but a completely different image results.

Now it can be said that the general public knows what a 'XXmm' lens will give them as a final image, but that's only because it became the standard way back when. I know for a fact that people new to photography have a difficult time grasping the difference between what they will get as a final between a 24mm, 50mm, and 135mm. Many just don't have any idea what that means, and I'm sure back in the day it wasn't any difference. There is no (IMO) logical relationship between 50mm and 46°, that can be (relatively) immediately understood by a novice.

But if I say to someone who has a basic understanding of geometry that your eyes see ~200° of a scene, and that lens A gives you an image that represents 81° of that scene, I think they might have a better comprehension of what the end result will be. They can take their hands, hold them up to their face, and make blinders that give a 81° view of the scene, and get an idea of what they'll get in their image.

I personally try and convert a 'mm' lens into AOV lens, so I can 'see' what I'll get without having to try different lenses. In other words, I'll look at a scene, and say to myself I want approximately 55° of what I am seeing to be captured, and then grab the lens closest to that.

Labeling a lens as a 52° AOV would standardize the entire lens lineups across formats. No matter what format you're using, a lens with an AOV of 52° will give you 52° of the entire scene. So, in my case, my 24mm (35mm format) lens would be labeled as 81° AOV, as well as my 50mm MF lens (Mamiya RB67). But the way it is now, I have two lenses that are named the same, but give 'wildly' different results.

Discuss....

everyone likes a standard but it's usuallytheir standard.Yourproposal makes sense but you'refightingmillions of people who have gotten used to something else.newcomers will get ot eventually too.

 

[Kirks518]

frank, I think I misunderstood you. But the end result is what a photographer, or the general public is really concerned about. Yes, the focal point to film plane on a 50mm MF lens and a 35mm format 50mm lens is a consistent 50mm, but it doesn't directly relate to the final output of the lens. It's the AOV that the lens produces that is constant.

 

[Kirks518]

Ok, to re-iterate:

I'M NOT TRYING TO CHANGE ANYTHING, JUST WONDERING WHY THEY CHOSE WHAT THEY DID

 

[Sirius Glass]

Ok, to re-iterate:

I'M NOT TRYING TO CHANGE ANYTHING, JUST WONDERING WHY THEY CHOSE WHAT THEY DID

See post #7.

When I compare field of view from format to format, I, personally, use the horizontal axis. Other use the diagonal or vertical axis. The problem for me with the diagonal is that the angle of the diagonal and the proportion of the diagonal vary so greatly between formats. Horizontal versus vertical is just a preference.

 

[frank]

How can you say that? If you are saying that a 50mm lens on a medium format will give you the same scene as a 50mm lens on a 35mm format, or a 50mm on 4x5 format, you're completely mistaken. The final image captured with a (designated) '50mm' lens will give you completely different images across different formats.

But if I have a lens designated as an 81° AOV lens on medium format, and take a picture of a scene, what is captured would be the same as if it was taken on 35mm format with a 35mm lens designated as a 81° AOV lens. But to accomplish that the way it is currently named/designated, I'd have to use a 50mm MF lens, or a 24mm lens on 35mm format. So the FL is not the constant.

I guess I didn't make my point clearly.

I suggest you ask Chris: (there was a url link here which no longer exists)

 

[Ken Nadvornick]

How can you say that? If you are saying that a 50mm lens on a medium format will give you the same scene as a 50mm lens on a 35mm format, or a 50mm on 4x5 format, you're completely mistaken.

It will give you exactly the same scene (angle of coverage). It's just that the scene is being projected onto a differently sized piece of film.

If the piece of film is larger, the corners will be cut off. Large enough and the entire image projects as only a circle in the middle. Your 50mm lens is still projecting full coverage for a 24x36mm frame, but that "frame" is now buried in the middle of a 56x56mm medium format piece of film. Or nearly lost in a 4x5-inch piece of film.

If the piece of film is smaller, only the center portion of the projected scene is recorded, thus giving the net effect of a telephoto. Think cropping factors for digital sensors that are smaller than the 24x36mm of full-frame 35mm.

Ken

 

[Sirius Glass]

It will give you exactly the same scene (angle of coverage). It's just that the scene is being projected onto a differently sized piece of film.

If the piece of film is larger, the corners will be cut off. Large enough and the entire image projects as only a circle in the middle. Your 50mm lens is still projecting full coverage for a 24x36mm frame, but that "frame" is now buried in the middle of a 56x56mm medium format piece of film. Or nearly lost in a 4x5-inch piece of film.

If the piece of film is smaller, only the center portion of the projected scene is recorded, thus giving the net effect of a telephoto. Think cropping factors for digital sensors that are smaller than the 24x36mm of full-frame 35mm.

Ken

Well said. The focal length relates to the circle of coverage. It does not tell one if a lens will provide the coverage for a 4"x5" sheet of film or an 8"x10" sheet of film. That is another problem.

 

[Christopher Walrath]

And I think where everyone is going, Kirk, is that the system in place is the standard that is used and that is to ID a lens primarily by focal length over AOV. This may have to do with the fact that when photography came into being that there was relatively little consideration to different formats as there were very few. Lenses as they came to be were quantified by focal length as they were being used in a very limited number of formats. AOV was not much of a consideration. The lenses made merely had to cover the films/emulsions being used at the time. The system stuck.

As a photographer now, focal length based on format with some quick math tells me exactly that which I need to know.

 

[gzinsel]

well I think, if I understand your OP. That focal length is the distance between lens and film plane that would be focused at infinity. It says nothing about coverage. I.e circle of illumination. so a 47mm lens can cover 135, 120, and 4x5. HOWEVER due to the format the angle of view will change with each format used.

focal length is the distance between lens and film plane that would be focused at infinity. It is also interesting to note that a Schneider s 300mm is actually a 298.something ,something. I.e. effective focal length. what is actual and what is label are two different things. a 50mm lens can vary from 43mm (effective focal length) to something like 55mm depending on what the manufacturer makes) labeling such and such, but not the "real" focal length. all 35mm lenses in 135 format are not exactly 35mm from lens to film plane . they all vary,

 

[Kirks518]

Not that I'm trying to convert the system, but now that there are (essentially) an infinite number of formats (including cell phones), maybe it would make sense to revisit the FL system. That way, someone using a 81° AoV lens on camera A ormat will know what a 81° AoV lens would produce on camera B's format.

What I realize I haven't been getting across is that it is the final image that really anyone cares about. If I say to you 'This is a 65mm lens", you will have absolutely no clue what image would be produced with that lens until you know what format that lens will be used on. Whereas if I say "This is a 60° AoV lens", you will know that the final image will capture an area of the scene that envelops 60° of the scene, regardless of format being used. It is the final image produced that matters, not the format used to get there (specific merits of individual formats aside).

 

[NedL]

Focal length is a property of the lens itself. It is one of the most important properties of the lens and it is appropriate that lens be marked and characterized by it.

The field of view is a property of how the lens is used. You are proposing that all lenses sold must be only used for one format ( the one for the printed-on AOV). So, how would you characterize your 105mm MF lens, if it could be used for 6x4.5, 6x6, 6x7, or 6x9 format on 120 film? Usually it is best to label and characterize items by features that are inherent, rather than by assumptions on how they will be used.

It's certainly not uncommon to hear people talk of "35mm equivalent"... that gets to your idea too, I think.

I feel slightly sad that:
tangent = opposite / adjacent
is "complicated"!

 

[Christopher Walrath]

The AOV would tell you the coverage of a lens for a certain format. But if you have a xx degree AOV lens for 4x5 the AOV would remain the same. However, for a smaller format such as 135, the rendered subject area would be much smaller proportionally. So the AOV of a lens, without the accompanying format as a frame of reference, would not be very informative.

 

[Bill Burk]

Of course it helps for buying vintage lenses, to be able to spot what you need.

This may require you to convert CM to MM and Inches to MM as you come to them.

The f/stop and focal length together are somewhat reliable at giving the idea how much glass there is (with some examples such as 35mm f/2 Super Takumar for 35mm Pentax is a huge piece of glass compared to a 35mm f/2 Summicron for 35mm Leica).

And another fair advantage to working in MM... You can tell what Shutter Speed is safe to hand-hold. This is fairly reliable across formats. A 135mm focal length lens on 4x5 still needs to be handheld at approximately 1/125 second (the closest matching shutter speed)... Just the same as a 135mm telephoto on a 35mm camera should be safe at 1/125 second. The fact that this rule of thumb holds up across formats is likely a complete coincidence, but hey... it's an advantage of working in MM

 

[Ken Nadvornick]

Focal length is a property of the lens itself. It is one of the most important properties of the lens and it is appropriate that lens be marked and characterized by it.

[...]

Usually it is best to label and characterize items by features that are inherent, rather than by assumptions on how they will be used.

Kirk, this reply perfectly answers your basic question from your own post #11...



Ken

 

[Mark Crabtree]

...

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.

I don't know if anybody ever answered this question; I apologize if I missed it here.

Focal length and aperture are a description of the product (a lens). Angle of view for the final image (as pointed out here) relates to camera size (or cropping). Cameras came in many sizes, and lenses could be used on most any of them, so the angle of view wasn't even known to the maker of the lens. This changed with cameras with dedicated lenses, but only for those cameras so still not a universal like focal length. With people regularly adapting lenses to different formats today, the situation is moving back toward what it was in the old full plate, half plate, etc days.

 

[Sirius Glass]

focal length is the distance between lens and film plane that would be focused at infinity. It is also interesting to note that a Schneider s 300mm is actually a 298.something ,something. I.e. effective focal length. what is actual and what is label are two different things. a 50mm lens can vary from 43mm (effective focal length) to something like 55mm depending on what the manufacturer makes) labeling such and such, but not the "real" focal length. all 35mm lenses in 135 format are not exactly 35mm from lens to film plane . they all vary,

If you think that is bad, there are six, count them six, focusing 135mm scales for Crown and Speed Graphics.

 

[frank]

Thank you Chris, et al.