Convertible lenses and focus shift

[Tom Stanworth]

Did I read somewhere that when AA took moonrise he advanced teh camera's focus by x amount to correct for focus at f32 with the single 23" element. Why is this and would it apply to all such lenses, including the new cooke?

 

[Deckled Edge]

It's a node thing.

Like I usta node what that meant, but I done forgot.

If I 'member correct, like, light converges from the front element at or about the iris/shutter. With a single rear element there is no convergence and the iris, out front, changes what convergence there is--smaller iris=more convergence, effectively changing the focal length, and therefore the focus of the lens. There are some very savvy lens guys in this group that can bring more than junior college optics into this discussion, and I hope they do.

 

[Ed Sukach]

If I 'member correct, like, light converges from the front element at or about the iris/shutter. With a single rear element there is no convergence and the iris, out front, changes what convergence there is--smaller iris=more convergence, effectively changing the focal length, and therefore the focus of the lens. There are some very savvy lens guys in this group that can bring more than junior college optics into this discussion, and I hope they do.

I've been trying to visualize this a' la ray traces ... Ow!!! My head hurts!!

This is all news to me ... I'm eagerly awaiting replies...