View Full Version : Lens Focal Lenght:

25-Aug-2008, 18:10
Awhile ago , I posted a question dealing with how to figure out the focal lenght of a brass lens I had purchased, which no mfg name or markings of any kind to tell what it was. At first I thought it was 2-1/2 focal lenght, which turned out wrong , which is nothing new for me![LOL]

So after reading all your comments on how to figure out the Lens focal lenght , I just thru up my hands and gave ,To much math for me!

there has to be an easier way to figure that out : So, I drawen a diagram of my set up and hope with your help I can get a good idea of what lenght it is !
I will say this for this unknowen lens MFG Its real sharp to my eye on the ground glass.

erie patsellis
25-Aug-2008, 18:56
try this, first focus at infinity (tree or power lines, far, far away), measure the distance between standards. then focus on a ruler or tape measure, making 1"=1", measure to the same places again. Subtract the first from the last, that's your focal length.

25-Aug-2008, 19:53
erie patsellis: What is meant by [measure the distance between standards] ? there is a lot of terminolgy I lack knowledge of ! Thanks

Jim Galli
25-Aug-2008, 20:15
You're probably making this too hard. Focus on something far, like the moon. Measure from the Iris to the film plane (ground glass) That's it. For your purposes you don't need to get out the calculus book and worry over nodal points etc. You can calculate f stops just about as easily. Lets say you decide it's an 11 inch lens. The f stops are all the usual f numbers divided into 11. So when your aperture is open to 1 inch on an 11 inch lens, you're at f11 :cool:

Jim Graves
25-Aug-2008, 22:57
I find a ceiling light, put a white piece of paper on the floor directly underneath it, take the lens off the camera, hold the lens over the paper, focus the light on the paper, and measure the distance between the paper and the iris (aperture.) I then subtract about 10% of that number (because the light is only 8' away and I want the focal length at infinity) and get a reasonable approximation of the focal length. For example, when I measure my 210mm lens this way, I get 234mm ... I subtract 10%, or 23 mm, and get 211mm.

26-Aug-2008, 04:12
Stick your camera out the window or in the garden and focus on the furthest object.

Measure from the back of the camera to where the lens is stuck on.

Front Standard = where the lens is stuck on
Rear Standard = where the ground glass is

26-Aug-2008, 06:10
erie patsellis: What is meant by [measure the distance between standards] ? there is a lot of terminolgy I lack knowledge of ! Thanks

The attraction of the procedure provided by eirie patsellis is that it really doesn't matter what you measure. For the "front standard", you may use any convenient piece of the camera that moves with the lens as you focus. And for the rear standard, you can use any convenient part that stays in fixed relationship with the ground glass. The focal length is the amount that any such measurement lengthens as you focus from infinity to 1:1.

The reason this works is because the distance from ground glass to rear nodal point is 1 focal length at infinity, but is 2 focal lengths at 1:1. So even if you cannot know exactly where the rear nodal point is, you know that it moves out by 1 focal length as you change focus from infinity to 1:1.

Good Luck - Alan

26-Aug-2008, 08:41
(1/(9x12)+5) + 1/12 = 1/focal length

I get about 10.8 inches with convenient rounding.

26-Aug-2008, 10:04
I thank you all very Much , and I mean it ! I have enough information to go with now and will report back with my findings and foto of the lens, But in the mean time if you wish to see the lens go to this thread [ measuring focal lenght ] and I am very glad you folks have the Resolute to put up with my questions and requestions: Lauren