[Leica] Re: [Rollei] True focal length

[Jerry Lehrer <jerryleh@pacbell.net>]

Richard

The worst disparity I have ever seen in this regard was in the
Leica 125mm f2.5 Hektor, which was actually less than 120mm.
This was done when reissuing a projector lens as a camera lens,
to qualify at a lower customs import duty.  I dunno!

Jerry

Richard Knoppow wrote:

> ----- Original Message -----
> From: "keller.schaefer" <keller.schaefer@netcologne.de>
> To: <rollei@mejac.palo-alto.ca.us>
> Sent: Friday, May 30, 2003 11:27 PM
> Subject: [Rollei] True focal length
>
> > Has anybody successfully tried to find out the true focal
> length of a lens
> > by measuring object size, object distance and image size
> on the film and
> > calculate from there? At first sight, even if I take into
> account
> > considerable misreading I would still arrive at the true
> value plus or minus
> > one millimeter. Am I missing something? I AM missing an
> Autocollimator,
> > obviously.
> >
> > I have some lenses where I doubt what the engraving says
> is true. I always
> > thought this would be marginal, but now I have a 85 mm
> lens that is actually
> > 'longer' than a 90...
> >
> > Sven
> >
> >
>   Yes, I've done this many tjmes. Not with terriffic
> accuracy vbecause I don't have an optical bench. However,
> you can ger reasonabble results ith a view camea.
>   There are two relations which can be used to measure the
> focal length. One if the extension of the lens between focus
> at infinity and at 1:1 magnification. thi is exactly one
> focal length. At 1:1 magnification the distance between
> image and object is exactly four times focal length. Getting
> exact 1:1 requires some care. Perhaps the best method is to
> draw a circle of a given diameter on a tranparent sheet, or
> on the back of the ground glass. Use a circle of the same
> diameter as the target. Its a little easier to see when the
> image and guide are mismatched than it is with a linear
> target, say a small ruler. Getting a 1:1 image for short
> focal length lenses is more difficult than for lenses in the
> range used on press and view camera.
>   Finding exact infinity focus is easy. The lens is
> autocollimated by using a mirror. While a first surface
> mirror is ideal a shaving or makeup mirror will work,
> provided you make sure to use the plane side. The lens will
> reflect a sharply focused image of a a target at the focal
> plane back to the focal plane. For a view camera its
> sufficient to hold a pencil flashlight with a pinhole over
> it against the ground glass. A better target is a pencil
> mark near the center of the ground glass illuminated with
> the flashlight. One can also use a large white card with a
> pinhole in it illuminated from the rear. The lens is mounted
> so that you can move it around. The mirror placed over its
> front and the lens set for best focus of the reflected image
> of the pinhole as reflected on the card.
>   Actually, any ratio of magnifications can be used and the
> focal length calculated from the relative focus
> displacement. However, infinity and 1:1 are convenient and
> result in the largest, hence easiest to measure,
> displacement.
>   I think you could probably measure a 90mm lens within 1mm
> this way but it would be tedious. I've mostly measured view
> camera lenses.
>   If you need to know the principle points they are easy to
> find once you have the focal length. By definition they are
> one focal length from the image toward the lens. The
> principle point of whichever side faces the image is the
> rear or second principle point when the lens is used in that
> position. When turned around it becomes the front or first
> principle point.
>   Finding the principle points for telephoto and retrofocus
> lenses is an intersting excercise. For some telephoto lenses
> both principle points may be in space in front of the lens!
>   Finding the principle points will allow calculation of the
> combined focal length of lenses since the d distance between
> the lenses is the distance between the rear principle point
> of the front lens and the front principle point of the rear
> lens. If you are working with convertible lenses which don't
> any longer have the charts showing combination FLs this
> makes calculating them easy.
>
> ---
> Richard Knoppow
> Los Angeles, CA, USA
> dickburk@ix.netcom.com

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