Re: The length of the focal length?

Bob G wrote:

On Oct 5, 11:18 am, Richard J Kinch <ki...@xxxxxxxxxxx> wrote:

Bob G writes:

That doesn't explain the OP's "discrepancies".

It does explain, because it is the fundamental optic theory underlying why
a lens system's physical length may be short than its focal length.


Back focal length is measured from the rearmost element, focal length
is measured from the nodal point and is given by the manufacturer,
effective focal length is the focal length at a given focusing
distance, as given by 1/F effective = 1/u + 1/v. You can see
that F effective equals F when u (focusing distance) is at infinity.

No. In optics, the "effective focal length" is the distance from the
principal point to the focal point. Object distance has no bearing. The
words "effective" and "focal length" may be used in photographic
discussions as you have cited, but that is not the "effective focal
length" of optical theory.


Object distance not only has bearing, it is impossible to ignore if
you grant the Principal Law of Optics.

When you speak of the focal point you must perforce be speaking of an
image cast by the lens on that point. That image must be of an object
somewhere in the physical world, and if so, it must be at some
distance from the lens, which is the definition of object distance. No
lens can form an image of nothing.

Focal length is the distance from the nodal point of a lens focused at
infinity to the focal plane. Effective focal length is the distance v
from the nodal point to the focal plane when the lens is a distance u
from the object focused upon. When u approaches inifnity, v (the
effective FL) approches the FL.

Interestingly, a lens cannot be focused on an object closer than its
focal length: When u approaches F the effective FL approaches
infinity.

I tried an extension tube on a super wide angle lens and infinity was somewhere inside the lens :-)

--
Paul Furman Photography
http://edgehill.net
Bay Natives Nursery
http://www.baynatives.com
.

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