Determining Focal length

[EASmithV]

Well, I put a magnifying glass lens onto a piece of black mat board and stuck it on my speed graphic. I like the DOF it gives, as well as the edge distortions. However I want to determine the F-stop.

I know that F-stop is the focal length divided by the diameter of the lens. However, I am having some terminology difficulties.

I know this is probably a dumb question, but here it goes...

Is the focal length the distance from the front standard to the film plane when focused at infinity?

 

[EASmithV]

Does it have to be focused to infinity to get the measurement?

 

[Mahler_one]

Go to Jason Brunners site, and watch his wonderful video on how to determine the focal length of any lens. Also, as I recall, there is a good discussion of focal length in general. Jason is really unique, and is very generous with his instructional videos.

 

[removed account4]

(there was a url link here which no longer exists)

 

[Mahler_one]

Ah....I was just about to furnish the link as well...

http://www.youtube.com/watch?v=CIFNdfjem18&feature=user

 

[EASmithV]

Thanks. That clarifies a lot. So it does need to be focused to infinity.

Actually, I regularly use one of his bellows factor calculator thingies... Bacon...
LOL don't need the stopwatch...

 

[ic-racer]

Do you know the power of the magnifying glass? You can multiply the 'magnifying power' times 5 to approximate the diopters. So if it is a 2x magnifier, then it is about 10 diopters. Diopter is 1 over focal length in meters. So 10 diopters = 1/10 meter = 100mm focal length.