What's the f stop and focal length?

[Mainecoonmaniac]

I salvaged a data projector lens from the trash. I mounted it on Sinar lens board and checked out the image on the ground glass. The contrast is pretty good. How do I figure the focal length and aperture? Here's a pic.

 

[Soeren]

Focal lenght is determined by pointing it against something very far away and measure the distance form lens (should be nodal point) to the sharp image on wall or similar. That gives you a rough estimation. Measure the opening to determin the f stop.

 

[dasBlute]

if you know the focal length and can somehow measure the diameter of the opening...

"f/stop" = (focal length)/(aperture diameter)
thus for an f/2 lens the aperture opening is 1/2 the focal length

or say that aperture is 20mm, if the focal length is say, 120mm, it'd be an f/6 lens.

 

[Ian Grant]

That maths doesn't work for Telephoto or Zoom lenses.

Ian

 

[Soeren]

Whoops, no missed that part......
Compare the angle of view to known focal lenghts?

 

[juan]

For focal length, mount it on a camera, focus on something far away, tree branches are good, then measure from the lens board to the ground glass for a close enough approximation.

 

[wiltw]

For focal length, mount it on a camera, focus on something far away, tree branches are good, then measure from the lens board to the ground glass for a close enough approximation.

That only works for optical designs which are neither 'retrofocus' in design ((short FL whose rear node to focal plane > its true FL)) nor 'telephoto' in design (long FL whose rear node to focal plane < its true FL)

To OP, here is some information which will help... Dead Link Removed

 

[RalphLambrecht]

For focal length, mount it on a camera, focus on something far away, tree branches are good, then measure from the lens board to the ground glass for a close enough approximation.

I never understood the meaning of 'close enough'.

 

[Mainecoonmaniac]

Thanks for all the responses. The focal of the lens is not on the lens. But I have a feeling that I'm going to have to experiment to calculate the approximate f stop for the lens. The lens is pretty unusual. This is my first fixed f stop zoom lens. I don't want or expect quality. Some funky bokeh would be nice.

 

[MattKing]

As it is a zoom projection lens, the f/stop might change as you change the focal length.

Seems like a great lens to use with one of those Sinar ground glass meters .

 

[Mainecoonmaniac]

As it is a zoom projection lens, the f/stop might change as you change the focal length.

Seems like a great lens to use with one of those Sinar ground glass meters .

I've lusted after one for a while, but you need the meter, probe and the special back to for the probe. It's opens up a very expensive can of worms

 

[tih]

First of all, since it's a zoom, the specs are going to change as the zoom control is adjusted, of course. I suggest testing at both ends.

You need to find the optical center of the lens. This is the point, along the central axis of the lens, around which you can tilt the lens without the projection shifting. Being a zoom, the optical center is probably going to move as you change the focal length, so be prepared for that. Get the lens to project a focused image of something far away onto a suitable surface. Then hold it by two fingers on opposite sides, along a line perpendicular to its axis, and swivel it slightly around this line. If you're at the optical center, the projected image will not move - although it will exhibit focus changes and vignetting.

Focal length: Once you have found this "swivel axis", you have the optical center on it, in the middle of the lens. (For a telephoto lens, the optical center may be out in front of the front element. For a retrofocus wide angle, it may be behind the rear element. For your typical large format lens, it'll be aligned with the lens board.) To find the focal length, you measure the distance from the optical center to the projected image plane, when the image of a sufficiently faraway object (e.g. the horizon) is in focus.

Speed: hold the lens in front of a bright surface, and look at it from some distance away from the front element. You'll see an apparent opening through the lens, where you see the bright surface. Now place a ruler so its measuring edge crosses the front of the lens, dividing it in half. Looking at the apparent opening from a distance, you'll see that the ruler can now show you how wide the apparent opening is, i.e. its diameter. The speed of the lens, as what people tend to call an "f-stop", is the ratio of the focal length to this diameter. A lens with a maximum aperture of f/2 is a lens where the apparent opening is half of the focal length. An f/4 lens has an apparent opening (OK, it's actually called the "entrance pupil") that's 1/4 of the focal length.

And now you know why it's "f/2", and not "f2".

 

[Arklatexian]

I never understood the meaning of 'close enough'.

I think "close enough" is only part of the quote. The total quote is: "close enough for government work". Now as to what that means, I defer to someone (anyone) else. I do have a feeling that the term might be world-wide in scope......Regards!

 

[TheFlyingCamera]

either that or close enough for horseshoes, hand grenades and thermonuclear weapons.

 

[Dan Fromm]

Since no one has mentioned these,


http://www.largeformatphotography.i...61880-Petzval-nodal-point&p=584863#post584863


http://www.largeformatphotography.info/forum/archive/index.php/t-16692.html