Quote:
Originally Posted by giordano
.....
I'll leave it to someone else to calculate the effect of the lens extension on f/stop on DoF!
|
Eh eh, I'm very happy to have caused a deep technical discussion from a innocent question...
And this last statement is intriguing... I know that with significan "bellows extension" (or Viso + rings and so...) one has to take into account an "EV factor" (figures exist on bellows specs...) : this means that the "real f/stop" is closer than the nominal... but DOES this affect DOF ? My guess is "no"... but, as you say, stand to be corrected by someone else...
Reasoning "crudely" on the lens wide open : the "EV factor" becomes accountable at significant extensions for not all the light that passes through the lens systems reaches the film/sensor... the effective angle decreases... a certain amount of lightrays are "lost"... so, one can guess that for some "small" f stops closing the real exposure doesn't change... imagine a f 2,5 lens, very extended by bellows... if i close to 2,8 only, maybe the lightrays blocked by the diaphragm are anyway "unuseful" due to the extension... oh well, let's wait for some real math from someone...