Focal Length Confusionism

[Ari]

I may have blacked out and not fully regained my senses, but something is puzzling me.
When I put an old Paragon 12" lens on the camera and focus at infinity, the distance from lens board to film plane is 10".
Similarly, on a newly-acquired Wollensak Vesta 14", the bellows are at 10" when I'm focused at infinity.
Only a 12" Cooke lens lines up with 12" bellows, my other lenses use shorter bellows than the FL indicates.
Am I missing something?
Thanks

[Dan Fromm]

Probably not.

Flange-to-film distance given focal length depends on the lens design. For example, my 101/4.5 Ektar, a tessar type, has a larger f-to-f than my 105/3.7 Ektar, a Heliar type. To add to the fun, lenses in barrel have f-to-f distances that are only loosely related to focal length. For example, I have a 150/6.3 CZJ Tessar in a deep mount whose mounting surface is nearly at the front of the barrel.

For lenses of normal construction -- non-tele, non-retrofocus -- diaphragm-to-film distance is usually approximately focal length, but not always. 6/4 double Gauss types are often somewhat telephoto, i.e., sit a bit closer to the film than the focal length would suggest. This is why many 1950s and early '60s "normal" lenses for 35 mm SLRs have 58 mm focal lengths. Designing a 50 mm 6/4 double Gauss type with adequate performance and enough back focus to work in an SLR took more time than was available ...

[Mark Sawyer]

...the distance from lens board to film plane is 10".

That might be the problem. Measure from the aperture's location, not the lens board's.

[Drew Bedo]

Ah yes . . .The Nodes.

[ic-racer]

Focal length is from rear nodal point to image at infinity. f' in the image below.
The issue you are having is that the rear nodal point is not usually marked on large format lens barrels, shutters or lens boards.
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[Alan Gales]

Ah yes . . .The Nodes.

Isn't there a node thread in the Image Sharing and Discussion section? :)

[Ari]

That might be the problem. Measure from the aperture's location, not the lens board's.

Yes, of course you're right. Thanks, Mark.

[Mark Sawyer]

Focal length is from rear nodal point to image at infinity. f' in the image below.
The issue you are having is that the rear nodal point is not usually marked on large format lens barrels, shutters or lens boards.
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Focal length is from the rear node to the focal plane at ANY distance. Infinity is just a standard distance to measure at, hence the bellows extension factor for close ups; you've changed the focal length of the lens by focusing closer. Quite right that the rear nodal point is seldom marked, but it's generally pretty close to where the aperture is (exactly where the aperture goes is one of the "design freedoms"), so close enough for all practical purposes.

[Dan Fromm]

Son-of-a-gun! The OP asked why flange-to-film distances of lenses with similar focal lengths aren't much the same and was distracted into accepting explanations based on invisible points.

[Ari]

I wasn't distracted, I was just...hey, what's that shiny thing there?!

[ic-racer]

Focal length is from the rear node to the focal plane at ANY distance. Infinity is just a standard distance to measure at, hence the bellows extension factor for close ups; you've changed the focal length of the lens by focusing closer. Quite right that the rear nodal point is seldom marked, but it's generally pretty close to where the aperture is (exactly where the aperture goes is one of the "design freedoms"), so close enough for all practical purposes.

In the diagram there is an image at distance S, this is not the focal length. The focal length, or the distance f', only applies for images focused at infinity. Other images that are not focused at infinity are not at the focal length. This is shown in the diagram.

[ic-racer]

Son-of-a-gun! The OP asked why flange-to-film distances of lenses with similar focal lengths aren't much the same and was distracted into accepting explanations based on invisible points.

I think the OP does not know how to determine the focal length of his lens, therefore, was confused. Flange-to-film distances were not mentioned by the OP. You are not confusing him more with that?

[Mark Sawyer]

Son-of-a-gun! The OP asked why flange-to-film distances of lenses with similar focal lengths aren't much the same and was distracted into accepting explanations based on invisible points.

Invisible points exist. They may not be practically necessary, (why I didn't bring them up), but when someone brings them up, it's good to understand a little about what's going on.

[Mark Sawyer]

In the diagram there is an image at distance S, this is not the focal length. The focal length, or the distance f', only applies for images focused at infinity. Other images that are not focused at infinity are not at the focal length. This is shown in the diagram.

The diagram is for a lens focused at infinity. Personally, I don't always photograph at infinity. Then the lens is focused at a different focal length, because by definition, it's focused at a different length.

But I think we're arguing mathematical semantics.

[Dan Fromm]

I think the OP does not know how to determine the focal length of his lens, therefore, was confused. Flange-to-film distances were not mentioned by the OP. You are not confusing him more with that?

In the first post in this discussion, the OP wrote:


When I put an old Paragon 12" lens on the camera and focus at infinity, the distance from lens board to film plane is 10".
Similarly, on a newly-acquired Wollensak Vesta 14", the bellows are at 10" when I'm focused at infinity.
Only a 12" Cooke lens lines up with 12" bellows, my other lenses use shorter bellows than the FL indicates.

If that isn't flange-to-film distance, what is it?

[Ari]

Most of the lenses I tried have apertures at or very close to the lens board plane. That's why I used the term lens board.
The only one that's different is the Vesta, whose aperture is about 2.5 inches in front of the lens board. And there's my mistake, I continued measuring from the lens board with the Vesta.

None of this is bad or even undesirable. On the contrary, especially in the case of the Vesta.
My 2D doesn't have an extension bed yet, so for now I'm using the 17" bed. I have lots more bellows to use than I thought I would.

[Pere Casals]

Just an easy rule. To know the rear nodal point location we look at the difference between the Flange Focal Distance and the real Focal Lengh. This tells the exact Rear Node position from the flange.

The real Focal Lengh may vary a bit from the mm Commercial designation, datasheet tells the real one.

[Nodda Duma]

TL;DR: Flange focal length does not equal effective focal length.


Addendum: all these cryptic definitions were purposely invented to keep lens designers employed. 50% of my career involves explaining optical terms to non-optics folks.

[Tin Can]

:cool:

LOL

[Dan Fromm]

The real Focal Lengh may vary a bit from the mm Commercial designation, datasheet tells the real one.

Interesting if true. Unfortunately not true.

I just looked at a couple of H'blad data sheets for early and late 38/4.5 Biogons. Both show the same focal length, 38.6 mm.

Years ago I bought 10 A.G.I. F.135 cameras. Each held 2 38/4.5 Biogons. The F.135 has no focusing mechanism, the lenses were focused to infinity by shims, thickness marked in 0.01 mm, that went behind the back of the shutter and the body main casting. The lenses were marked with measured focal length and FFD. Measured focal lengths ranged from 38.3 mm to 38.8 mm.

[Pere Casals]

Interesting if true. Unfortunately not true.

I just looked at a couple of H'blad data sheets for early and late 38/4.5 Biogons. Both show the same focal length, 38.6 mm.

Years ago I bought 10 A.G.I. F.135 cameras. Each held 2 38/4.5 Biogons. The F.135 has no focusing mechanism, the lenses were focused to infinity by shims, thickness marked in 0.01 mm, that went behind the back of the shutter and the body main casting. The lenses were marked with measured focal length and FFD. Measured focal lengths ranged from 38.3 mm to 38.8 mm.


Dan, hasselblad lenses are another war, they are IF, Internal Focus lenses displace an inner element with the help of the focus ring. In LF we play with the Unit Focus way, so the focal is the well stablished value stated in the datasheet.

If we go to 35mm format, the Nikon 50mm f/1.4 is IF, the real focal varies depending on focus ring setting, instead the 50mm f/1.8 is Unit Focus, sporting focus breathing, aparent focal varies but the real focal not, it works like a regular LF lens.

[Dan Fromm]

Dan, hasselblad lenses are another war, they are IF, Internal Focus lenses displace an inner element with the help of the focus ring. In LF we play with the Unit Focus way, so the focal is the well stablished value stated in the datasheet.

If we go to 35mm format, the Nikon 50mm f/1.4 is IF, the real focal varies depending on focus ring setting, instead the 50mm f/1.8 is Unit Focus, sporting focus breathing, aparent focal varies but the real focal not, it works like a regular LF lens.

Papi, you went wrong and now you've gone farther wrong. The 38/4.5 Biogon is a unit focusing lens.

[Mark Sawyer]

Just an easy rule. To know the rear nodal point location we look at the difference between the Flange Focal Distance and the real Focal Lengh. This tells the exact Rear Node position from the flange.

I don't see how that can be accurate. Location of the flange is completely arbitrary, wherever the shutter or lens barrel manufacturer decides to put it. It's a mounting decision, not part of any optical formula.

Early Dagors had the flange at the very rear of the barrel. When they started putting them in shutters, the flange moved to between the elements because that's where it was on the shutter. It didn't move the nodal point.

[Pere Casals]

I don't see how that can be accurate. Location of the flange is completely arbitrary, wherever the shutter or lens barrel manufacturer decides to put it. It's a mounting decision, not part of any optical formula.

Early Dagors had the flange at the very rear of the barrel. When they started putting them in shutters, the flange moved to between the elements because that's where it was on the shutter. It didn't move the nodal point.

Mark, it is completely accurate, optic Focal distance determines exactly the film to Rear Node distance. The FFD determines exactly the lens placement (focus at infinite). From the two distances you know the Rear Node placement in the lens...

Look this example, Sironar-S 300mm The FFL is 277mm, so the Rear Node is placed 23mm far from flange, no doubt:

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Papi, you went wrong and now you've gone farther wrong. The 38/4.5 Biogon is a unit focusing lens.

Sorry, I'm not a connoiseur of the hassies... only used the planar 80 and the sonnar 150

[Mark Sawyer]

How in the heck does where the manufacturer places the mounting flange on the outside of a lens change where the rear node is inside the lens? It's completely independent of the optics.

[grat]

How in the heck does where the manufacturer places the mounting flange on the outside of a lens change where the rear node is inside the lens? It's completely independent of the optics.

Semantically, there is a relationship in that fixed-mount lenses have to take into account the back focus distance from the flange to the image plane of the camera, so the total focal length has to incorporate that distance, or you can't focus to infinity.

That's less important with LF lenses, since the flange to image plane distance is variable (but it does explain why ultra-wide lenses may need a recessed lens board).

So the flange position may be important (and therefore not "completely independent"), and even dictate the design of the optical path, but it doesn't determine the location of either nodal point, no.

[Pere Casals]

How in the heck does where the manufacturer places the mounting flange on the outside of a lens change where the rear node is inside the lens? It's completely independent of the optics.

Mark, this is not plain geometry, this is substracting. For the Sironar S the flange is 277mm far from film and the Rear Node is 300mm far, 300-277 = 23mm, the rear node is 23mm far from flange inside the lens, close to the iris.

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[cowanw]

What Pere is saying is that the rear node is one focal length from the film plane when focused at infinity. I think. Which is OK if you know the focal length already.
Here is an interesting quote from 2014.
https://www.largeformatphotography.info/forum/archive/index.php/t-114173.html
Compare also the tone of the posts in each thread,eh.
"Here is how to determine the focal length and size of the entrance pupil of any lens.
First, determine the focal length by autocollimating. This requires a mirror capable of covering the front of the lens. It can be done using a view camera. A first surface mirror is ideal but a standard makeup or shaving mirror will do. Just be sure to use the plane side, not the magnifying side. Place the mirror over the lens. Distance does not matter but the mirror should be parallel. Place a small light, a pencil flashlight is ideal, against the ground glass, near but quite at the center. The mirror will reflect an image of the light back to the ground glass. For easier focusing you can draw a small cross on the ground side with a pencil (will come off again). Focus the image as sharply as you can. This focuses the lens _exactly_ at infinity. Mark this distance at some convenient point on the
camera. Now, set the camera up for an exact 1:1 image of some object. A small ruler is handy since you can tape a similar one to the ground glass to match it. When adjusted for 1:1 the diference between the focus point and the infinity focus point will be _exactly_ the focal length of the lens. To measure the _effective_ size of the stop set up a inhole source at the exact infinity focal plane (which you determined in the first measurement). Place a sheet of some
translucent material over the front of the lens, and record the diameter of the circle of light. This is the effective size of the aperture. Divide this into the focal length to get the f number. The size of the effective aperture may not be much different than the physical aperture in many lenses but the difference will be significant for some. This method takes into account the magnification of the stop by the lens. If you make a test stop of known size you can determine the difference between the physical size and effective size and use that to calculate the size needed by the actual stops.

Since you now know the focal length you can determine the locations of the principle points by focusing the lens exactly on infinity and measuring back one focal length toward the lens from the focal plane. That is the location of the principle point for that end of the lens. When the lens is in its "normal" position this is the second or rear principle point. To get the first or front principle point turn the lens around and refocus it. Knowing the principle points is sometimes useful. The internal structure of lenses which are not too complicated can often be determined by shining a small light into each end of the lens and counting the reflections. Easier if you can examine the cells separately. Glass/air surfaces, even when coated, are bright, cemented surfaces are dim. Sometimes its possible to determine the power, or rather the sign, of the surface this way, but it can be tricky.
My guess is that this is probably a Petzval portrait lens.

---
Richard Knoppow
Los Angeles, CA, USA"

[Mark Sawyer]

Mark, this is not plain geometry, this is substracting. For the Sironar S the flange is 277mm far from film and the Rear Node is 300mm far, 300-277 = 23mm, the rear node is 23mm far from flange inside the lens, close to the iris.

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The flange location is an arbitrarily-placed measuring point. If you removed the 300mm Sironar S from the shutter and put it in a properly spaced barrel with the flange at the very rear instead of in the center shutter, it would still have a 300mm focal length, and the nodes would still be in the same place relative to the optical system.

That's why your chart specifies with a footnote, "With Copal shutter for scale 1:infinity". A different shutter/barrel with the flange in a different place would change the flange length, even though all optical properties of the lens remain the same.

Have we confused you enough yet, Ari? :)

[cowanw]

As the flange location changes, both the film to Flange distance and the flange to node distance change by the same amount; so Pere's statement is true as it is stated; which was
At infinity, the real Focal Length minus Flange to film distance = Rear Node position to flange distance.
Put another way: focal length = film to flange + flange to node, at infinity.
Moving the flange changes the numerals on the right side of this equation, up and down by the same amount, but does not change the addition.
What is unclear is how this observation, which requires you know the focal length, helps in this thread, where the focal length is the
unknown (or known unknown?).

[Pere Casals]

The flange location is an arbitrarily-placed measuring point. If you removed the 300mm Sironar S from the shutter and put it in a properly spaced barrel with the flange at the very rear instead of in the center shutter, it would still have a 300mm focal length, and the nodes would still be in the same place relative to the optical system.

Mark, yes, while modern shutters are normalized for the critical dimensions allowing to mount the same cells in a compur or in a copal, it happens that the rear cell flange to the shutter flange may vary slightly.

For example for the copal 0 size that dimension is 5.7mm (see highlighted in red) while in the compur the dimension is 6.1mm, so difference is 0.4mm

Saying it clear, from the manufacturer's datasheet you can locate the Rear Node exactly, but if (for example) you replace a factory mounted Copal by a Compur then the FFD and flange to Rear Node distance is to vary by 0.4mm, in the 0 size lenses. Well, not much !!!



For Copal 0 it is 5.7mm.

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For Compur 0 it is the 6.1mm value (the 4.5mm value is for 00 size)

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[Mark Sawyer]

When you change the location of the flange, all you're changing is the location of the flange. You're not changing the focal length, or the location of the nodes, or any other optical characteristic. All you're changing is where the flange is.

[Dan Fromm]

When you change the location of the flange, all you're changing is the location of the flange. You're not changing the focal length, or the location of the nodes, or any other optical characteristic. All you're changing is where the flange is.

You can drag a dead horse to water but ...

[Mark Sawyer]

You can drag a dead horse to water but ...

...if you can get it to do the backstroke, you've really got something!

Yeah...

[Pere Casals]

When you change the location of the flange, all you're changing is the location of the flange. You're not changing the focal length, or the location of the nodes, or any other optical characteristic. All you're changing is where the flange is.

Of course Focal lenght won't change, but FLD will change, in particular by 0.4mm with the size 0 (copal vs compur)