[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

 

Thank you for the links, I love reading about optics.

 

As to the calculations: I used single lens formula equations, and to be very honest, all I am interested in is effective FL, not theoretical FL based on node calculations. This simply because it does not matter for the end result. A given magnification at a giving shooting distance with known object size and known size of that object in the (sensor) image makes it possible to calculate the effective FL This takes the FoV into account automatically as well.

 

To be very honest, all any extra node calculations provide is potential bragging rights for the manfacturer, as this generally overstates the effective FL, certainly for telelenses. In principle any lens can be replaced by a specific single lens FL, at any distance and magnification, which essentially provides the end result for the photographer. That is to me what it all is about.

 

As to focus breathing in your statement above: in that case the only way to prevent focus breathing is actually to change magnification ver the focus range in such a way that FoV stays exactly the same, no matter where the lens is focused. That is only possible with an IF-type of lens, whether prime or zoom. This is easier to achieve with a zoom lens, in principle, because focusing and zooming can be combined in these lenses. However, it makes a lens with absolutely no focus breathing effectively a zoom lens under all circumstances, as the FoV has to stay the same at all times. The lens therefore has to shorten FL with decreasign focusing distance.

 

From a photographic POV, for me, especially when shooting macro and portraits, this is unacceptable, as it obviously changes magnification and perspective at close focus drastically.

 

HTH, kind regards, Wim