How to calculate Focal Length of a Petzval Lens

[DanK]

I've searched and found posts on focal length, but none really dealing with Petzvals...

My question is... Where do you measure from on the barrel of the lens, if it doesn't have an aperture or waterhouse slot?

Or where is the nodal point generally believed to be with this design?

(I usually hold the lens near the rear wall of the shop and measure from wall to aperture when image is in focus)

Thanks in advance,
Dan

[Louis Pacilla]

Hi Dan

Focus lens to infinity. Measure from center of lens barrel ( w/ or w/out stops)to the film plain & that's the focal length.

Hope this is what your looking for.

[DanK]

Hi Dan

Focus lens to infinity. Measure from center of lens barrel ( w/ or w/out stops)to the film plain & that's the focal length.

Hope this is what your looking for.


Thanks Louis....I wasn't sure if that was the case because of the design...sounds simple enough, and sure appreciate the reply...

Thanks again,
Dan

[Ole Tjugen]

Another approach is to not measure the distance, but the size of the projected image.

I hold my lens up in front of a piece of cardboard at the end of a corridor, with a window in the other end. Then marking off the width of the image of the window, I can compare that to the same width marked with lenses of known focal length - I used a casket set of seven cells from 150 to 750mm as "calibration".

This way you can also measure the non-symmetric lenses, lite telephoto and retrofocus - and yes, they do exist!

[Mark Sawyer]

The most accurate way is to focus carefully at infinity, then focus at one-to-one, and measure the distance the standards travelled.

[Steven Tribe]

"Measure from center of lens barrel". This will only be an approximation.
Mark and Ole have the right ideas!

[eddie]

for us "approximators" the best is to measure to the water house stop slot or the iris. they are not in the center but slight off center towards the flange side of teh lens.


The most accurate way is to focus carefully at infinity, then focus at one-to-one, and measure the distance the standards travelled.

interesting idea but now you have to try and describe how to find 1:1 on a lens with an unknown FL.....

[cowanw]

Put an inch worm;) on a plate and then move the camera and focus until it is an inch on the GG
Regards
Bill

[c.d.ewen]

Bah! You guys are doing it the easy way. Go take a landscape picture with a distant horizon. Pick out an object to the far left and right. Measure the angle between them and then measure how far apart they are on the resulting negative (scan and use PS). Now go back and try and remember your trigonometry.

You guys do carry a sextant in your kit, don't you?

Keep searching, and you'll find an old post by Emmanuel Bigler (or was it Biggler?) on measuring the angle by attaching a stick to the camera.

Charley

[domaz]

interesting idea but now you have to try and describe how to find 1:1 on a lens with an unknown FL.....

Put the lens on the camera focus on something close. When the image measures the same size in the ground glass as in real life then you are at 1:1.

[DanK]

Thanks all for the replies...

I guess the best bet would be focus on a ruler at close distance....and measure it on the GG to find 1:1...

Thanks again,
Dan

[Mark Sawyer]

I just use two rulers, one as the subject, one to measure at the ground glass. I usually run it corner-to-corner, as measuring a longer distance is more accurate, but with a Petzval, I wouldn't push it because of the curved field, which kinda throws a monkey-wrench in the whole focal length thing, though at 1:1 the curve shouldn't be too bad from a lens that covers at infinity, as you're only using the center part of the image circle.

It's also easiest to use a camera with front and rear focusing so you don't have to keep shuffling the tripod or subject ruler an eighth-of-an-inch this way or that. Oh, and make sure the subject ruler is parallel to the plane of the ground glass, and zero your swings and tilts.

Details, details... :)

BTW, the aperture placement moves around a bit on Petzvals, as it's position balances out the best compromise of field curvature and various aberrations. As different lenses use different glass with different refractive indexes, different apertures, different spacings, etc., the optimum location does move around a little bit at the designer's discretion. Another "degree of freedom" in the design...

[Maris Rusis]

I experiment with a variety of home-made lenses and focal length measurement is a regular chore.

The first step is to tape a clear plastic ruler to a bright window and another one to the ground-glass of the camera. By focussing the camera and by moving it back and forth it is possible to find an arrangement where the image of the ruler on the window is the same size as the physical ruler on the ground glass. The match can be done with an accuracy better than 1 millimetre.

Once this configuration is achieved the distance from the ruler on the window to the ground-glass of the camera is 4 times the focal length. Measuring this distance to within 1 millimetre will give you the focal length accurate to 0.25mm. Very precise indeed!

Now refocus on infinity and measure forward from the surface of the ground-glass a distance equal to the focal length determined in step two. Make a discreet mark on the barrel of the lens at this distance. That's the place to measure from for all subsequent lens calculations.

[jb7]

The Petzval I measured using that method turned out to have its focal length measured precisely to its Waterhouse stop-

[Jim Galli]

Nobody's right. Here's how to do it scientifically. Take a photograph of the full moon with your petzval properly focused. Develop the film. Carefully measure the moon on the film. If it measures 3.84mm, you have a 384mm lens. :cool:

[jb7]

Damn-
My 36" AM lens is really only 32.49"...

(800mm)

[c.d.ewen]

Nobody's right. Here's how to do it scientifically. Take a photograph of the full moon with your petzval properly focused. Develop the film. Carefully measure the moon on the film. If it measures 3.84mm, you have a 384mm lens. :cool:

Gee, Jim, should we shoot the moon as it rises or when it straight overhead.....;)

OK, another confession. during times of waxing/waning moons, darkness or inclement weather, I've been known to measure focal lengths thusly:

I have an old process lens sitting at the top of a column, with a backlit transparent target mounted exactly one FL below it. Above that column is a section of Horseman monorail mounted so that it can pivot around a center point (tripod mount). Mounted, separately, above that is another section of Horseman monorail, with another standard. This standard holds a piece of GG. I focus the projected target, then swing the lower monorail to see if the target moves back and forth. If it does, I move the lower standard up or down (and refocus) until the target doesn't move anymore. I call the distance from the pivot point to the GG the FL.

Go ahead, tell me I'm wrong. I'm used to it. I've been married a real long time.

Charley

[goamules]

Nobody's right. Here's how to do it scientifically. Take a photograph of the full moon with your petzval properly focused. Develop the film. Carefully measure the moon on the film. If it measures 3.84mm, you have a 384mm lens. :cool:

Jim, I tried your moon technique the other night, but it suddenly turned darker. Does that mean my aperture was on "auto-mode" It's just an old Petzval, how do it know? And does a lens "auto-stopping" down when shooting the moon affect your formula?

[DanK]

I just realized the ruler trick at 1:1, didn't get me any closer than where I was initially....

Where to measure from on the lens body....

So....until I have a little more time to try Maris's suggestion....I'm gonna approximate from center....

Although, if it wasn't so cloudy outside right now, I'd sure try Jim's 'Moon' method :D

Thanks again all,
Dan

[Jim Galli]

Jim, I tried your moon technique the other night, but it suddenly turned darker. Does that mean my aperture was on "auto-mode" It's just an old Petzval, how do it know? And does a lens "auto-stopping" down when shooting the moon affect your formula?

Maybe it doesn't work in Arizona! :eek:

[dsphotog]

And then......
What's the secret calculation to find the f number?

[Mark Sawyer]

I just realized the ruler trick at 1:1, didn't get me any closer than where I was initially....

Where to measure from on the lens body....

So....until I have a little more time to try Maris's suggestion....I'm gonna approximate from center....

Although, if it wasn't so cloudy outside right now, I'd sure try Jim's 'Moon' method :D

Thanks again all,
Dan

Approximation from the center is way more than close enough for figuring exposure. If you're sloppy, you may miss by an eighth of an f/stop...

But if you want to find the correct place to measure on the lens body, first you find the focal length. Then you focus on something at infinity and measure the focal length distance from the ground glass to a spot on the lens barrel. That's the spot.

To find the f/stop value of the lens, measure the diameter of the smallest part of the barrel opening (aperture) through the front element. Divide the focal length by that diameter. That's your f/stop at infinity.

For calculating your f/stop when focusing at closer distances, just measure the distance from the ground glass to that spot on the lens (that's your new focal length), and divide it by the aperture diameter. There's your f/stop for that bellows extension. That's all the f/stop is, the ratio of focal length to aperture.

[dsphotog]

Thanks,
That's the info I needed!

[cowanw]

I just realized the ruler trick at 1:1, didn't get me any closer than where I was initially....

Where to measure from on the lens body....

So....until I have a little more time to try Maris's suggestion....I'm gonna approximate from center....

Although, if it wasn't so cloudy outside right now, I'd sure try Jim's 'Moon' method :D

Thanks again all,
Dan

Start focused at infinity;make a mark on the rail where the GGis
Focus at 1:1;make a mark on the rail where the GG is. Measure the distance between the two marks. That is your focal length.
Focus on infinity, and measure your new focal length from the GG to the lens; make a mark on the lens,
You're done
Regards

Bill

missed the next page before responding

[aduncanson]

Once this configuration is achieved the distance from the ruler on the window to the ground-glass of the camera is 4 times the focal length. Measuring this distance to within 1 millimetre will give you the focal length accurate to 0.25mm. Very precise indeed!



Almost, but not quite. The subject to image distance at 1:1 is 4 times the focal length PLUS the (generally unknown) principal point separation (PPS - the distance from the front nodal pint to the rear nodal point.) Schneider publishes (https://www.schneideroptics.com/info/vintage_lens_data/large_format_lenses/symmar/data/5,6-360mm.html) the PPS for some designs. For Symmars the PPS is less than 2% of the focal length, but for telephotos it can be as much as 20%. Perhaps somebody knows what would be a typical PPS for a Petzval lens.

This is the reason for focusing at 1:1 and also at infinity and taking the focal length to be the change between the two focusing positions.