[Brightcolours]

Nope. When you work with magnification, you get the exact resulting FL.

 

I remember now what I did as well. I had a few photographs of tape measures across the entire viewfield, photographed at the above distances. In short, based on magnification and focusing distance (from exif), knowing the sensor width, and knowing the tape length in view in the image, it is then very easy to calculate the resulting FLs.

 

For all intents and purposes, my method is a very practical one, and it works. With magnification you do not need to know the nodes anyway, one can calculate the resulting FL purely based on focusing distance, image size, object size, and magnification.

 

Kind regards, Wim

No, you do not get exact focal length values without. Give the formula you use?

 

Take ANY lens that does not change focal length (any lens that moves the entire lens system to focus will do). When one looks through the view finder and one goes through the focus range, one will notice focal breathing (the subject becomes "larger" when you go towards MFD). FOV changes, while focal length does not.

This makes a simplified method like you use very useless to determine sort of exact focal length (needed to say with any real authority how much focal length is lost). 

 

In order to exactly calculate effective focal length you will need the front and rear nodes of the lens, and those are not that easy to measure (and usually not provided by the lens manufacturers). 

 Interesting articles:

http://www.pierretoscani.com/echo_focal_length.html

http://www.panohelp.com/thinlensformula.html