David,

The statement made was correct, the upper telescope has an objective
with a focal length corresponding to that of the optical tube length
of a fixed tube length microscope to the second image plane. As with
any telescope, it can be focused to infinity or to any intermediate
distance. However, in forming the image, the overall correction for
spherical aberration will only be correct for the upper focal length
for which the objective is computed. An 'infinite tube length
objective' is a normal objective with correction elements to render
the image plane at infinity, rather than at a fixed distance from the
back focal plane. As with a normal objective, I can use it with
different focal lengths in the upper telescope, which will give me
different magnifications. But the primary aberations will only be
corrected with a telescope of the correct focal length. So the effect
of different telescope focal lengths is the same as that of a
microscope with a draw tube, the correction for spherical aberation
will be degraded either side of the optimum.

Cheers,

Merv

--- In Microscope@yahoogroups.com, "David Sykes" <killspammers@b...>
wrote:
>
> Merv wrote:
>
> >the focal length being that necessary to provide the magnification
> marked on it with a fixed tube length.
>
> Merv, that is not strictly correct correct.
> The focal length for any given magnification is determined by the
> focal-length of the tube-lens.
>
> A x100 infinity-objective with a focal-length of 1.8mm is used with
> a tubelens of 180mm focal-length.
>
> >If I interpose a second telescope, set for the correct tube length
>
> There is no such thing as adjusting a telescope for a particular
> 'tube-length'.
>
> If the objective is focused to infinity, any optical-system also
> focused to infinity will image it.
>
> While playing-around adding C-mounts to the CVS camcorders, I had to
> make a collimator that projected a test-target to infinity.
> I adjusted it by attaching an infinity-focused webcam and altering
the
> lens-target separation until sharp-focus was achieved.
>
> I then pointed my film SLR at it and the microprism and split-image
> confirmed the image was at infinity.
>
>
> > the Olympus final tube length is 180mm
>
> That is the focal-length of the tube-lens, whose position is such as
> to focus the image at the correct point in the eyepiece tube.
>
>
> David Sykes
>