Permit me to expand a bit on the subject of "Principal Planes" and their relevance.
This is a Nikon large-format lens of "standard" design, a Nikkor-M 450mm f/9 (see note*):
The Principal Planes are identified on a lens diagram by dots on the optical axis, labeled respectively
H and
H' (H Prime).
The dots identify the front (
H) and rear (
H') nodes, at the intersections of the optical axis and the two planes.
As mentioned earlier, the lens focal length is from the rear node (
H') to the image plane, measured along the optical axis.
Here's a Nikkor-T 500mm f/11 telephoto design for comparison:
The blatantly obvious difference is that the two Principal Planes are waaay forward (to the left) of the front element, not within the lens body as for the Nikkor-M design. This is typical of telephoto lenses.
This lens has a focal length of 500mm, but a physical length of only 129.5mm.
Note also the Flange Focal Distance, called out at lower right. That is the shutter mounting plane.
On the standard lens, the FFL is almost equal to the optical focal length (428.4mm v. 450mm), while on the telephoto they are very different.
I chose to illustrate this post with large-format lens diagrams because they have far more detail than those for fixed-format lenses.
The optical principles involved are the same regardless of the intended format.
Different applications may dictate different optical designs.
- Leigh
*Note:
The diagram is a cut-away view of the lens mounted in a shutter.
The upper half shows the lens elements themselves, while the lower half shows the outer view.
NB: Nowhere in these or any other lens data sheets in my library is there any mention of "mount register".