# Determining Focal length

Discussion in 'Miscellaneous Equipment' started by EASmithV, May 21, 2009.

1. ### EASmithVSubscriber

Messages:
1,931
Joined:
Aug 22, 2008
Location:
Maryland
Shooter:
Multi Format
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?

2. ### EASmithVSubscriber

Messages:
1,931
Joined:
Aug 22, 2008
Location:
Maryland
Shooter:
Multi Format
Does it have to be focused to infinity to get the measurement?

3. ### Mahler_oneMember

Messages:
1,153
Joined:
Oct 26, 2002
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.

Messages:
21,004
Joined:
Jun 21, 2003
Location:
local
Shooter:
Multi Format

Messages:
1,153
Joined:
Oct 26, 2002
6. ### EASmithVSubscriber

Messages:
1,931
Joined:
Aug 22, 2008
Location:
Maryland
Shooter:
Multi Format
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...

7. ### ic-racerMember

Messages:
7,735
Joined:
Feb 25, 2007
Location:
Midwest USA
Shooter:
Multi Format
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.