Determining Focal Length

[dazedgonebye]

I've seen some interesting work with single element lenses, such as magnifying glasses, on home made cameras.
This leads me to what might be a simple question.

How do I determine the focal length of a single element lens?

Thanks,

 

[VictorM.]

I've used simple 'magnifing glass' type lenses on a 4X5 view camera. The focal length almost always was about 150mm.

 

[ferider]

Compare to a camera of same format with a lens that you know. Compare photos taken at the same focus distance.

Best,

Roland.

 

[Al Kaplan]

Magnifying glasses are mostly all single element with both sides ground to the same curvature. Go outside at night and use the magnifier to project an image of a street light several blocks away or the moon on a sheet of paper. Measure the distance from the glass to the paper to get the focal length. You find the f/stop by dividing that by the diameter. You can "stop it down" with a hole cut in some black paper over the front of the glass. A 150mm/6 inch focal length with a 75mm/3inch diameter is f/2.

 

[dazedgonebye]

Magnifying glasses are mostly all single element with both sides ground to the same curvature. Go outside at night and use the magnifier to project an image of a street light several blocks away or the moon on a sheet of paper. Measure the distance from the glass to the paper to get the focal length. You find the f/stop by dividing that by the diameter. You can "stop it down" with a hole cut in some black paper over the front of the glass. A 150mm/6 inch focal length with a 75mm/3inch diameter is f/2.

So, I need a light source distant enough to be a point of light and that's about it.

Ok. Thanks.

 

[Tim Gray]

Yeah, a ceiling light will work too. Just project onto the floor or a table top.

 

[delft]

... Go outside at night and use the magnifier to project an image of a street light several blocks away or the moon on a sheet of paper. Measure the distance from the glass to the paper to get the focal length...

Sound advice, but it's more fun to project an image of the sun on a piece of paper & burn a hole in it, or to set fire to a shoelace. That's what these glasses are made for. Then measure the distance between lens and target.

Greetings,
Dirk