Can I calculate focal length from aperture size?

[DanK]

I have an unmarked focal length 11x14 lens (11x14 Sterling Improved)

Does someone know the focal length of this lens?

If not...

Can I calculate the focal length of this lens from the lens diameter and aperture diameter?

Has three stops...

I have yet to build a board for it, and I'm not sure I even have the bellows length to check infinity focus from camera....

Thanks in advance,
Dan

[Bob Salomon]

Point the lens out the window. Hold a piece of large white cardboard behind the lens. Move the cardboard torwards or away from the lens until the image projected on the cardboard is sharp. Measure the distance from the lens rear to the cardboard and that is the appx. focal length.

[DanK]

Thanks Bob...I appreciate the response....I will try your method tomorrow AM....


I am still curious as to whether the focal length can be calculated from the lens aperture and lens diameter?

Or is the design of the lens too large a variable?

Thanks again,
Dan

[Peter K]

I am still curious as to whether the focal length can be calculated from the lens aperture and lens diameter?

No, a wide-angle lens with the same focal length like a "normal" lens can have nearly the same diaphragm diameter but the front-lens diameter is as double as big.

[sun of sand]

out a window is better obviously but you can get an estimate with a lightbulb ..or streetlamp or moon

ill estimate to be a normal lens for that size film

[Flauvius]

If you follow these directions, you can determine the focal length of any lens.

(1) Mount your camera with your mounted lens on a tripod,

(2) Tape a sheet of news paper to a "straight" wall,

(3) Measure in millimeters the length of a sheet of newspaper, or anything else that you can easily measure, and be precise.

(4) Focus on the newspaper, and use a lupe to focus as best you can.

(5) Measure the length of the newspaper on the ground glass. For best results, use a seamtress' cloth tape measure.

(6) Divide the length of the newspaper as it appears on your ground glass into the length of the newspaper that you have taped to a wall.

By way of example, the newspaper on the wall is 800mm, the newspaper on the ground glass is 40. So the result would be 20. (Stay with me, as I use this "20" result as part on the next step.)

(7) Add "1" to whatever is the result of your having divided the size of the newspaper on the ground glass into the size of the newspaper that you have taped to a wall.

So, now you have "21", the result of adding steps (6) and (7).

(8) Measure the distance, and be as precise as you can, from the wall to the nodel point of you lens.

(9) Now, divide the result of steps (6) and (7) into the distance from the wall to your lens' nodle point.

(10) Congratulations ! You now have the focual length of your lens.

*** This procedure only applies to lenses that are not tele fotos, or any unconventional lens type.

---------------

*** For the mathematically minded, the forumula for focual length determination is:

f/ = 1/
1/ + (1)
1/m


Robert J. Triffin

[Bob Salomon]

If you follow these directions, you can determine the focal length of any lens.

(1) Mount your camera with your mounted lens on a tripod,

(2) Tape a sheet of news paper to a "straight" wall,

(3) Measure in millimeters the length of a sheet of newspaper, or anything else that you can easily measure, and be precise.

(4) Focus on the newspaper, and use a lupe to focus as best you can.

This assumes that the camera has enough bellows to focus in that range.

[Peter K]

This procedure only applies to lenses that are not tele fotos, or any unconventional lens type.

If one only uses the distances in the subject space together with the image scale, magnification or reduction - e. g. distance between subject and front nodal point - the formula is valuable for all lenses.

focal lenght = subject distance / (image scale +1)

image scale = subject size / image size

With telephoto lenses or reversed telephoto lenses compared with "normal" lenses of the same focal-lenght the distances differs only in the image space.

Have fun
Peter

[goamules]

If you follow these directions, you can determine the focal length of any lens.

(1) Mount your camera with your mounted lens on a tripod,....[9 more steps]

To the OP, or...after step 1, measure from the center of the lens to the ground glass. Unless you like challenging, convoluted ways to determine something.

If so:

I have a lightbulb, is there a way to calculate the wattage without reading what is marked on the bulb?

[Mark Sawyer]

As others have said, the focal length is unrelated to the aperture, but you can figure the f/stop by dividing the focal length by the aperture after measuring both.