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

[wiltw]

Initially, I wasn't suggesting anything, but I was curious as to why the system we know was settled on. At the time, lenses were designated with focal lengths in millimeters, centimeters, and inches. Why mm, and why not cm or inches? And with (at the time) there being 3 different designations (or more), why didn't they just abandon the FL, and not go with AoV. That has been answered earlier.

But now that I've been thinking about it, IMO, it would make more sense to use AoV (for the format the lens was intended to be used on), and it would be easier for a layman to have an understanding of what each lens will give them in their final image (as supported by the statement above).

But I'm happy to continue with the old (current) system, although I liken it to using cubits to determine fuel economy when all (standard) distance measurements are in miles or kilometers - 'My car gets 88,000 cubits per gallon!' (that's 25mpg if you're wondering.

Everything changes over time. Some will be accepting of a change, others will fight it tooth and nail. Such is life....

FL is one of the only parameters which is not 'dependent upon the format size it is used on'...it is solely determinant on the rear node location of the design, and the distance of that lens' rear node to the focal plane.

Some folks have been talking about 'Angle of View'...unfortunately, there is the not-associated-to-format Angle of View, and then there is the AOV which is DEPENDENT upon the size of the format it is covering! The first simply states that at the FL distance, the lens will produce an IMAGE CIRCLE of a certain size; the second states, when that FL is used with a frame of certain dimensions, that frame will capture a certain angular distance.

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

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

• Notice that it has an 96 degree Angle of View measurement, which is the one which is associated with the Image Circle diameter. Just as the Image Circle is what limits the EF-S lens from use on FF bodies, this lens can cover a 5" x 7" film frame, but not with an 8" x 10" film frame.
• Notice also, there are TWO Angle of View measurements, the 96 degree one for 'wide open aperture' and the 101 degree one at f/22...this illustrates the fact that the Image Circle diameter does get larger at smaller apertures! Hypothetically, while a lens might not satisfactorily cover 5x7 format at f/5.6, it might be able to do so at f/22. The 5x7 film diagonal is about 217mm, so while the wide open Image Circle is too small at 201mm diameter, it increases to 259mm diameter at f/22...that probably would work!

Now let us take the same 90mm lens and use it on different format sizes...

• If we mounted a 90mm FL on 135 format, at a subject distance of 9' that subject would fill 100% of the 24mm frame height of the 135 format frame, and the subject would be 24mm on the focal plane and the image fills 27 degree AOV.
• If we mounted a 90mm FL on 645 format, at a subject distance of 9' that subject would fill 56% of the 43mm frame height of the 645 format frame, and the subject would be 24mm on the focal plane and the image fills 45 degree AOV.
• If we mounted a 90mm FL on 4x5 format, at a subject distance of 9' that subject would fill 26% of the 94mm frame height of the 4x5 format frame, and the subject would be 24mm on the focal plane and the image fills 84 degree AOV.
• If we mounted a 90mm FL on 5x7 format, at a subject distance of 9' that subject would fill 19% of the 125mm frame height of the 5x7 format frame, and the subject would be 24mm on the focal plane and the image fills 101 degree AOV.

So the ONLY thing which 'never changes' is the FL!
And notice that that 9' subject is the same size on all of the formats, in terms of its size on the film plane. That is because the FL is 90mm for all uses, rendering the 9' subject to be 24mm tall on the film plane, regardless of film format.

 

[Dan Fromm]

Valiant effort, wiltw. It will be ignored. The OP knows its own mind and that's good enough for it.

 

[Sirius Glass]

But I'm happy to continue with the old (current) system, although I liken it to using cubits to determine fuel economy when all (standard) distance measurements are in miles or kilometers - 'My car gets 88,000 cubits per gallon!' (that's 25mpg if you're wondering.

I am partial to furlongs per fortnight as a velocity measure.

 

[cliveh]

I think the OP raises a very valid question. Why can't this be measured as a geometric equation which could encompass the whole?

 

[Dan Fromm]

Clive, I already gave the relationship between focal length as conventionally, defined angle covered and circle covered.

The OP asked the wrong question. It should have asked how to choose the right focal length, conventionally defined, for a shot.

 

[lxdude]

Awesome!

So basically what I infer from your post is that essentially the rest of the world understands AoV/FoV, but only photographers understand and can relate to focal length. We're an odd bunch....

The difference is that we are not using our lenses for viewing. We are using them to make images. They don't even have to consider things like format or aspect ratio.

If AoV/FoV is desired, ask the manufacturers to specify it on their lenses along with focal length. Maybe they will do it. They could probably even find room on the lens to specify it for different formats where applicable, as with FF/APS sizes. If people want it, it would be easy enough for them to provide it.

While you're at it, ask them if they can do something about aperture values. People get really confused about how 5.6 is twice as much as 8, which is twice as much as a 11, etc. Maybe they could come up with a better way to do that. Though I suppose with everything being AE these days, maybe it no longer matters...

 

[Nodda Duma]

I think the OP raises a very valid question. Why can't this be measured a a geometric equation which could encompass the whole?

That equation is available in any basic optical textbook.


Stick with focal length for interchangeable lenses. It's easier to print one number on the side of the lens barrel rather than a whole chart if you were to specify field of view.


Edit: lxdude beat me to it

 

[Nodda Duma]

The difference is that we are not using our lenses for viewing. We are using them to make images. They don't even have to consider things like format or aspect ratio.

I respectfully and wholeheartedly disagree. Photographers are not the only group who use rectangular imagers. Aspect ratio is always an important consideration for non-direct view optics.

 

[lxdude]

One thing about the OP's question: I think that the angle of view as usually specified, i.e., diagonal, would be of limited usefulness. If the point is to make it easier to visualize, then horizontal field of view is in most cases more useful. The exception would be, I think, those cameras with a vertically oriented format, like the Bronica RF645. So maybe it would be a matter of specifying the AoV on the long dimension.

 

[Ken Nadvornick]

While you're at it, ask them if they can do something about aperture values. People get really confused about how 5.6 is twice as much as 8, which is twice as much as a 11, etc. Maybe they could come up with a better way to do that.

U.S. (Uniform System) stops???



Ken

 

[John Koehrer]

A 50mm lens is always the same FL.
The give the same image on any format film. Some formats just won't be covered completely and others will have wasted coverage.
Coverage will change not AoV. The fact is that coverage changes not AoV.

The assumption that the average American can understand the geometrical differences stated in AoV is a chuckle when so many
people can't do simple arithmetic..
When's the last time you went into a store, made a $12.37 purchase, gave the clerk $20.37 and WITHOUT using a register or calculator; couldn't figure out they owed you $8.00?

BTW using AoV how do determine the f stop? Now it's a simple ratio. Also there are thousands who can't deal with that too, all they see is 4/5.6/8/11/16 on the dial and know that somehow(magic?) if you one of these, you need to change a shutter speed.

 

[NedL]

U.S. (Uniform System) stops???

I thought the same thing... I kind of like my old cameras with US system. I understand why we use the values we do for f-stops, but there's something convenient and intuitive about doubling the stop doubling the needed exposure. Every once in a while I mess up, say by jumping from f/16 to f/32 and forgetting that there's an f/22 in between! Or try measuring your light for weston emulsion speeds and convert to US system stops, all in a hurry because the light is failing!

 

[ciniframe]

I dunno, when you start using a new format you just learn what you need or want to know
When discussing or using different formats I just kind of automatically switch gears and think in that format. I see in my imagination a frame hanging in space in front of me for what ever camera/lens combo I'm using. What's so hard about that?

 

[frank]

Let's all start using mass instead of volume to measure our photographic chemicals.

 

[Sirius Glass]

Let's all start using mass instead of volume to measure our photographic chemicals.

U. S. Standard, Imperial or metric - mks or cgs?

 

[alanrockwood]

Describing lenses by angle of view would be useful, but at the same time it would also complicate things considerably. The same 90mm lens would have a different angle of view on various medium format cameras, e.g. 4.5x6, 6x6, 6x7, 6x8, etc., not to mention yet other possibilities, such as adapting the same lens to a 35mm camera, a half-frame camera, a crop frame d*g*t*l camera, etc. Each lens would have to have a table written on the side, or perhaps sold with a loose piece of paper with the table, which the owner would promptly lose.

 

[Ken Nadvornick]

Let's all start using mass instead of volume to measure our photographic chemicals.

As the common method of measuring mass is weight, doesn't that mean we must then also compile a table of differing weights dependent upon which planet the measurement takes place?

Or would it be better to measure the gravitational anomaly created by that mass? Perhaps by utilizing the already relativistically-corrected GPS satellite network to measure the time dilation thereby induced?

:confused:

Ken

 

[Sirius Glass]

As the common method of measuring mass is weight, doesn't that mean we must then also compile a table of differing weights dependent upon which planet the measurement takes place?

Or would it be better to measure the gravitational anomaly created by that mass? Perhaps by utilizing the already relativistically-corrected GPS satellite network to measure the time dilation thereby induced?

:confused:

Ken

Which gravity map model?

 

[Ken Nadvornick]

Which gravity map model?

After reading this thread in its entirety somehow the data behind this guy seemed most appropriate, don't you think?



Amazing Grace
Credit: NASA




Ken

 

[lxdude]

I respectfully and wholeheartedly disagree. Photographers are not the only group who use rectangular imagers. Aspect ratio is always an important consideration for non-direct view optics.

I was referring to folks such as users of binoculars and telescopes. As the OP was about ordinary people understanding a concept, I was limiting my statement to the same. Ordinary people using optics for non-photographic purposes are using them in the form of binoculars/spotting scopes, rifle scopes, telescopes, etc. (an obvious exception is photographing through a telescope).

 

[Dan Fromm]

Hmm. I know the problem. We don't understand that the advent of digital changed everything. Everything.

We gotta get with the zeitgeist.

 

[Nodda Duma]

I was referring to folks such as users of binoculars and telescopes. As the OP was about ordinary people understanding a concept, I was limiting my statement to the same. Ordinary people using optics for non-photographic purposes are using them in the form of binoculars/spotting scopes, rifle scopes, telescopes, etc. (an obvious exception is photographing through a telescope).

I question your assumption of who this "ordinary person" is. I've never met him or her..each person I've met has been unique with their own extraordinary life story and needs. Maybe you just need to meet more people?? I kid I kid But seriously, talking about an "ordinary person" artificially limits the scope of the discussion to a small slice of imaging optical design. 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. Hence my disagreement.

Imaging optics encompass a great deal more than photography or direct-view optics, and aspect ratio is an important factor in any system with non-round images and not just to present a pleasing frame of the object of interest.

As an example (just one of many I can think of), consider a remote surveillance system, where high resolution video must be transmitted to a central observation post. The design requirement would be to provide as wide a field of view as possible at the highest resolution possible. However, a limit on the total pixel count is the available bandwidth of the video feed.

Aspect ratio is an important consideration in this case, and it has nothing to do with the composition of a photograph. It has everything to do with available bandwidth or frame rate trading off with resolution requirements in one or more directions in the image. A square or round image would waste precious bandwidth. I'd rather have an HD aspect ratio. Vertical image height is needed, but not too much and not quite as important as scanning the horizon. So horizontal field of view (important for the system) is specified, and with the selection of imager we flow that and the aspect ratio down to the lens design requirements as well as the signal compression selection and electrical design requirements. Thus the aspect ratio is very important to the system performance and cost.

That's just one example. Any system that processes images for surveillance or detection of objects of interest (automatic or by an operator) is going to be concerned about aspect ratio for bandwidth reasons. That is not a small market, by the way.

Btw Dan, this argument would hold true before the advent of digital imaging. Think line-scan imagers in satellites, laser collimators (a non-imaging example), reconnaissance cameras going back to the early observation biplanes, etc.

 

[Dan Fromm]

Jason, I take your point.

But panoramic aerial cameras, e.g., rotating prism cameras such as the kb18a/b, are a relatively recent development. Older USAF and other western air forces aerial cameras shot nominal 6x6, 4.5" x 4.5", 9" x 9" and (here we go) 9" x 18". I believe, could be mistaken, that east bloc aerial cameras used similar formats. The 1963 GOI catalog, however, lists some aerial lenses for 30 cm x 30 cm. When wider views were needed multiple cameras, sometimes in a "fan", e.g., Tri-Met, sometimes just in a pod and facing in different directions were used.

With respect to the OP, I think he's urging us to get with the zeitgeist.

 

[MattKing]

Hmm. I know the problem. We don't understand that the advent of digital changed everything. Everything.

We gotta get with the zeitgeist.

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.

 

[Dan Fromm]

Matt, you might be right. But I'm not sure.

When I started with 35 mm photography in 1970 no one talked about focal length equivalents or angles of view. We just chose what seemed like the right lens for the desired composition given how far we stood from the subject. If we guessed wrong we either moved or changed lenses. Most of us had just one lens and had to change position to frame the shot we wanted tightly. When moving wasn't possible, well, we didn't have to print full frame.

That's why I'm puzzled by the original post and the original poster. Perhaps the OP was thinking about how to decide which lens to use to get the shot it wanted and decided that thinking in terms of angles of view made better sense than thinking in terms of focal length. As has been said, given that the lenses to be used cover the format to be used, they're equivalent as long as one thinks in terms of the one true (for the moment) format.

Me, I'm all for suck and try. When I was shooting movies I had a couple of high ratio zoom lenses that usually made the problem moot. To use a 6-70/1.4 Variogon is to love it.

Depth of field confuses nearly everyone. If you doubt this, look at the many surreal discussions of DoF here and in other forums.