The length of the focal length?

Just for fun, I measured a 200mm lens from the front objective, to the camera
film plane, and it was 200mm... nice... but I tried a 300mm lens and it was only
230mm??

What gives?

Thanks!

On Oct 4, 7:40 pm, wrote:
Just for fun, I measured a 200mm lens from the front objective, to the camera
film plane, and it was 200mm... nice... but I tried a 300mm lens and it was only
230mm??

What gives?

Thanks!


Telephoto designed lenses focus an image at a distance shorter than
their focal length. That's their main virtue.

Long lenses (such as your 200mm) for medium and small format cameras
(including digital cameras) are typically of telephoto design.

I'm surprised at the results you got with the 200mm. I suspect it's a
large format lens adapted to a medium format camera.

Can you give us the types and brands you were measuring?

What gives?

Two properties of a lens: effective focal length versus back focal length.

Not the same thing.

wrote:

Just for fun, I measured a 200mm lens from the front objective, to the camera
film plane, and it was 200mm... nice... but I tried a 300mm lens and it was only
230mm??

What gives?

Pinhole 'lenses' will follow that rule... which makes it pretty much
impossible to do a wide angle pinhole shot on an SLR with a swinging
mirror that can't belocked away while shooting (I tried with tin foil).

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

Richard J Kinch wrote:
What gives?

Two properties of a lens: effective focal length versus back focal
length.

Not the same thing.

Quite. My undergrad optics textbook (1st published 1957) defines the ratio
of the two (though it uses "equivalent focal length") as the "telephoto
magnification".

Mike.
--
If reply address = connectfee, add an r because it is free not fee.

On Oct 4, 10:34 pm, Richard J Kinch wrote:
What gives?

Two properties of a lens: effective focal length versus back focal length.

Not the same thing.


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

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.

On Oct 5, 1:12 am, Paul Furman wrote:
wrote:
Just for fun, I measured a 200mm lens from the front objective, to the camera
film plane, and it was 200mm... nice... but I tried a 300mm lens and it was only
230mm??

What gives?

Pinhole 'lenses' will follow that rule... which makes it pretty much
impossible to do a wide angle pinhole shot on an SLR with a swinging
mirror that can't belocked away while shooting (I tried with tin foil).


The focal length of a pinhole lens is equal to the distance from the
pinhole to the film (or digital sensor, these days) plane.

Paul is right, that WA pinhole photography is impossible with a fixed
(as against a view) camera. I believe the distance between the film
plane and the front mount on a typical SLR is of the order of 50mm, so
that a pinhole lens is perforce 50mm or longer.

On Oct 4, 8:40 pm, wrote:
Just for fun, I measured a 200mm lens from the front objective, to the camera
film plane, and it was 200mm... nice... but I tried a 300mm lens and it was only
230mm??

What gives?

Thanks!

They all use negative lenses in the optical train to effect a longer
focal length that the total length would imply. A tele-converter is
in nearly every telephoto lens.

In article ,
writes

Just for fun, I measured a 200mm lens from the front objective, to the camera
film plane, and it was 200mm... nice... but I tried a 300mm lens and it
was only
230mm??

What gives?

Your 300mm is definitely a telephoto lens, your 200mm is just a long
focal length lens - the two are not the same, no matter how popular that
misconception is.

http://en.wikipedia.org/wiki/Telephoto

You can also get inverted telephoto lenses where the distance from the
rear element to the focal plane is longer than the focal length. Almost
every wide angle lens for use on an SLR camera is an inverted telephoto
lens.
--
Kennedy
Yes, Socrates himself is particularly missed;
A lovely little thinker, but a bugger when he's ****ed.
Python Philosophers (replace 'nospam' with 'kennedym' when replying)

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.