If you can indeed measure the focal length to 0.5 or 1% then it's worth it as the labelled focal length can be off by 4% or so.
Correct in principle (I dd mention the front nodal point), but in practice this method does work to amateur accuracy. The reason is that the accuracy of this measurement is limited by how well you can measure:
1) the lens to subject distance, since you don't know the exact location of the front nodal point,
2) the extension, because this is a relatively small distance and I assume that the user is an amateur with say calipers or a micrometer, measuring the extension on a camera with a homemade fixture, not some kind of incredibly rigid and aligned optical bench.
If you set it up so the lens to subject distance is several times larger than the focal length, the error from not knowing the front nodal point is a small fraction of the subject distance, so it doesn't dominate the error budget.
I will give a worked out example, sorry for the tedium. Suppose you have a lens of focal length f=100mm (but you don't know this), and you focus it at a subject distance of d=1 meter measured. The extension from the lens equation is e=11.11 mm, measured. Most of us would probably only measure that to say the nearest 0.1 mm, an accuracy of 1% on e.
The lens equation 1/f = 1/d + 1/(f+e) can be rearranged into a quadratic: f^2 + e*f - e*d = 0. You can solve this with a calculator, or analytically:
f = (sqrt(e^2 +4*e*d) - e) / 2.
Suppose that the front nodal point is 10 mm different than where we thought it was. That means we would measure d=990mm or 1010mm, You can try plugging these numbers into the formula. You'll find that the inferred focal length would be 99.5mm or 100.5mm. A 1% error on subject distance leads to an 0.5% error on focal length (due to the prefactors in the formula and the square root).
It turns out that a 1% error on measuring the extension also leads to about an 0.5% error on the focal length. IMO, measuring the extension to 1% is difficult for an amateur without fabricating a jig.
It's possible to measure focal length much more accurately (I have a process lens that is marked with true focal length to 0.01 mm), but for that I think you'll need an optical bench and techniques that are beyond the scope of this discussion.
You have to add exposure based on image ratio or magnification. At infinity no correction is needed. At ¼ lifesize you need an additional ½ stop exposure, at ½ lifesize 1 stop, at life size 2 stops, at twice lifesize 4 stops. You can adjust aperture or shutter speed or both to set the required compensation.That is what got me thinking about it - you have to adjust for longer bellows, basically because you are changing your effective aperture, as far as I can tell.
You forgot the all-important inter-nodal distance.If you focus on any object so that actual size = size on film plane while the lens internal focus mechanism is left at Infinity (that is, 1:1 magnification is achieved at the film plane) the subject to focal plane distance is 4 * FL
I agree, although it is a bit challenging if you are working with a 135 film camera.That is why Axel's proposal (#27) is the better one.
Why? Your focusing screen is 1 x 1 ½”. Makes it very easy to measure it against a ruler!I agree, although it is a bit challenging if you are working with a 135 film camera.
The aperture is measured as a ratio of the focal length, but the lens elements move forward and backward when focusing. How is the focal length determined? Is it based on the infinity focus, or the nearest focus, or something in between?
Just curious.
You forgot the all-important inter-nodal distance.
The calculators are wrong but the errors are small and have little practical significance.If you use a magnification calculator online, you will find a 100mm lens at 400mm focus distance reproduces 1:1
If you use a magnification calculator online, you will find a 200mm lens at 800mm focus distance reproduces 1:1
...like this one on the Cambridge Color web site https://www.cambridgeincolour.com/tutorials/macro-lenses.htm
Nothing in the calculator considers nodal position!
The calculators are wrong but the errors are small and have little practical significance.
Film-to-subject distance = film-to-rear node distance + internodal distance + front node-to-subject distance. This is true for all lenses at all distances.Perhaps you might illustrate the error magnitude, assuming the mythical 100mm lens, and with two different assumptions about nodal location in the mythical lens, how much error is introduced for each case, for the derived FL, assuming 4 * FL captures 1:1
Film-to-subject distance = film-to-rear node distance + internodal distance + front node-to-subject distance. This is true for all lenses at all distances.
From Schneider documentation, the 100/5.6 Symmar-S' internodal distance is -2.1 mm. The 150/9 G-Claron's is 3.3 mm. The 90/5.6 Super Angulon's is 35.2 mm. Non-zero, as the calculators you trust assume.
Do the calculations.
I am just trying to understand this, as I am not an optical engineer who can readily interpret what you wrote into an example of what might really occur in real life,...
If you can indeed measure the focal length to 0.5 or 1% then it's worth it as the labelled focal length can be off by 4% or so.
I would say that if you wanted an optical engineering sort of text on your bookshelf, this is the one it should be: https://www.amazon.com/gp/aw/d/0070591741/ref=dbs_a_w_dp_0070591741
I've never seen a lens for a 35mm camera without a focal length label. I have large format lenses that aren't labeled. (And isn't this thread is in the large format cameras sub-forum?) My instructions, post #27 should work fine.I agree, although it is a bit challenging if you are working with a 135 film camera.
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