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

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

(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....

2. 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.

3. 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.

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

5. @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/n...es-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.

6. Originally Posted by frank
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.

7. Originally Posted by frank
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.

8. Originally Posted by 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....
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.

9. 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.

10. Ok, to re-iterate:

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

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