How to determine focal legnth of unknown lens?

[s_p]

I have a model 250 Polaroid camera, circa 1970 I guess. Of all

the consumer grade Polaroid cameras I have seen in thrift stores

and flea markets, it has the nicest viewfinder (Viewfinder is

marked as made by Zeiss Ikon; camera by Polaroid). The shutter

has begun to get a little erratic; I have never been happy with

the auto everything shutter anyway.

<p>I am considering replacing the lens and shutter with an old

lens and shutter from a viewcamera. Many of the old speed

graphic lenses are for sale for less than $75.00. Since I

bought the Polaroid for just a dollar or two, I would be getting

a Polaroid test camera for less than $100.00. My problem is, I

don't know what the focal legnth of the lens of the Polaroid

camera is. I would guess it to be 100 mm or more. Does anyone

know a way of determining this?

Your help is appreciated;

stefan

[david_l._dupuy]

Stefan, I presume you would need to find a lens with the same focal

length as the old lens, in order to be able to focus the camera? How

does it focus?

 

<p>

 

One way to determine the focal length of a lens is to take a

photograph with two objects which are a known distance apart (say,

the two corners of your house), and one must also measure the

distance from the camera to these objects. Then one can measure the

separation of these objects on the print and determine the focal

length. Email me direct if you want to try this and I'll give you

more details (or calculate it for you). David

[tsun_tam]

The Polaroid cameras usually have simple lenses. The quickest way to

determine the approximate focal length of the lens is to focus at

infinity and then measure the distance from the approximate center of

the lens to the film plane. This is your rough approximation of the

focal length of the lens.

 

<p>

 

The precise way requires knowing where the rear nodal point is then

measuring the image distance from it to the film plane. This point is

not easily determined for most lenses (although it is my understanding

that for symmetrical lenses, the focal distance is measure from the

location of the iris blades.)

 

<p>

 

In any case, the focal length is some 'nice looking' number like 100,

127 or something like that. It is unlikely to look like 82 or 129mm.

 

<p>

 

Use a metric ruler having divisions in millimeters!