Quote:
Originally Posted by jaapv
Yes, lens extension will influence DOF. The light loss you mention is caused by the lengthening of the focal length of the lens. The aperture is a mathematical function of the effective diameter of the lens and the focal length, so you are reducing the "real" aperture, effectively influencing DOF. The relation between the influence of the focal length/magnification shift by the extension and the shrinking of the aperture I leave to somebody else...  |
The secret key can be into ghammer's formula :
2*c*N*(m+1)/(m^2-(c*N/f)^2)
N/f = 1/D where D is the nominal Diameter of the lens (front lens in std. lens designs)
How one has to treat N and f into an "extension environment" ? Does f must be set at the "lenghtened" value due to extension ? Does N has to be still treated as the nominal Fstop set by user ? Or... do the two values result into an INVARIANT ? ("real" f increases with extension, so as "real" N....the ratio...)