Slide holding for Scanning - Ohnar and Other Options

Halina3000 wrote:

I take your point that the enlarger lens could also have its effective 'centre' further back than the MD prime... but it's hard to know these things.

Not further back but more central, mainly because the lens doesn't have to make room from a mirror. Shorter focal length SLR lenses will be retrofocus (AKA reverse telephoto) forming an image further behind the lens than their focal length suggests. Enlarger lenses are often symmetrical designs (symmetry helps reduce aberrations) so their effective centre is often at the physical centre.

With a 50mm MD prime focused at infinity the rear measurement should be from about 5mm in front of the mount (quite far back), with a 35mm lens it would be from about 10mm behind the mount (further back than any of the lens), and with a 135mm nearer 90mm in front of the mount (towards the front of the lens).
 All based on MD having a registration of about 45mm which is a typical figure for SLRs (I think MD is actually slightly shorter, but a few mm won't change the point)
Focal length - registration will give the distance from the flange in each case.

Most lenses of this era are block focusing so move all the elements away from the mount to focus - the point to measure from will move with the elements.

I can't come up with a simple way to find the entrance plane without experiment but there's no reason to expect it to be at the front of the lens. - That distance is of use as it gives us the working distance, but doesn't quite relate to the optics. (The difference of course being negligible in most everyday photography, but significant in macro)

Internally focusing lenses & I suspect most zooms will behave differently, with the reference point moving as the lens is adjusted.