02-14-2013, 09:07 PM
Nope, I haven't went away yet. I'm still trying to figure out how to calculate the focal length (effective) of a lens when used at the MFD.
This guy says his method can calculate it. He also claims to be using one of the cardinal points of the lens. I did his tests at 20 meters and I already had the numbers at 1 meter (MFD). You can work out the formulas by hand or use his calculator. They amount to the same thing. Trying to get as much accuracy as possible, my numbers are all in millimeters. The page is here;
http://www.panohelp.com/lensfov.html
My measurements and his calculations are in my attachment. This method says the effective focal length of the 70-200 f/4 @ 200mm and MFD is 196.75mm. Go figure?
He gives you some other lenses to play with, where he did the measurements.
This guy says his method can calculate it. He also claims to be using one of the cardinal points of the lens. I did his tests at 20 meters and I already had the numbers at 1 meter (MFD). You can work out the formulas by hand or use his calculator. They amount to the same thing. Trying to get as much accuracy as possible, my numbers are all in millimeters. The page is here;
http://www.panohelp.com/lensfov.html
My measurements and his calculations are in my attachment. This method says the effective focal length of the 70-200 f/4 @ 200mm and MFD is 196.75mm. Go figure?
He gives you some other lenses to play with, where he did the measurements.