orava@xxxxxxxx wrote:
>
> 1) If lens has a focal length of for example 21 mm (like my zuiko 21/3,5) it
> means that the lens collects all rays of light into one point that is located
> 21 mm behind the lens. But is this measured from the surface of the rear
> element or from somewhere else?
It is measured from the rear nodal point which is exactly the focal
length away from the film plane. Depending on the lens that could be
just about anywhere. Since the OM's have a registration distance of
46.00mm (distance from lens mounting flange to film plane) it is clear
that the 21's rear nodal point is outside of the lens. Short focal
length SLR lenses need to be designed this way to avoid being hit by the
mirror. It's also part of the problem of using existing 35mm SLR lenses
for small format digital sensors. To get wide angles of view with a
small sensor you need a really short focal length. That's why Nikon,
Canon, Minolta, etc have come up with modified lens mounts and lenses to
supplement the full frame designs.
All lenses have front and rear nodal points. It's just that a simple,
convex lens has both nodal points in the middle of the lens.
>
>
> 2) And a very off topic question:
> I heard that you could build a telescope using two lenses: one with long and
> on
> with short focal lengths. Then you just line up those lenses so that their
> focal points coincide. I tried this with two of my zuiko lenses and it really
> works as a short telescope. But how could I take photos with that kind of set
> up? The problem is that I can look through that "telescope" and see the image
> sharp at a great range of distances between my eye and the eyepiece (that is
> the shorter ie. aft lens). This is due to my eye that has a built in focusing
> mechanism. But when I tried to connetct that telescope to the camera I
> couldn't
> see a sharp image through my cameras viewfinder. So, how could I connect that
> self made telescope to my OM4Ti THEORETICALLY? (If we consider mathematical
> lens formulas.) What about in REALITY? And is it possible that the image
> fills
> only part of the film (makes a black barrell around a circle shaped image) or
> should it fill the whole image?
A telescope is simply two lenses where the objective (front) lens has a
longer focal length than the eyepiece (rear) lens. Magnification is
determined by dividing the focal length of the objective by the focal
length of the eyepiece. You vary the magnification by using shorter
focal length eyepieces. Even given very good optics there is a
practical usage limit of about 40-50 power per inch of aperture on the
objective lens. Furthermore, in practice, atmospherice turbulence
usually places a much lower limit.
However, when these two lenses are used as a telescope (for an object at
infinity) the two lenses must be separated by their focal lengths.
(Based on the discussion above about nodal points this is easy to
determine for simple lenses but best done experimentally for complex
lenses). To understand what's going on consider that the objective lens
is projecting an image into the space behind it. If the object is at
infinity the image is one focal length behind the rear nodal point. Or,
easier, if it's an OM mount lens as the objective lens then the image is
exactly 46mm behind the lens mounting flange.
The eyepiece lens must then be placed exactly one focal length (its own)
away from the image projected by the objective lens. This is because
the eypiece is simply a magnifying glass which must project its own
image off to infinity. When so arranged there are parallel light rays
emerging from the eyepiece which is exactly what your eye (another lens
in the path) needs to focus the image on your retina.
If we're using another OM lens as an eyepiece lens then the eyepiece
lens should be focused at infinity and have its rear end pointed to the
rear end of the objective lens. Since both come to focus 46mm behind
their mounting flanges, if the two lenses are separated by 92mm they
will make a telescope that's in focus for your eye for objects at
infinity. To focus on closer objects keep the eyepiece lens on infinity
and focus with the focusing collar of the objective lens.
Now, then. All of this is much different than getting this contraption
to work as a telescope for your camera. The arrangment of the lenses
for your eye is emerging parallel rays focused on infinty from the
eyepiece. The camera wants exactly the opposite. It needs to have
those light rays focused to form an image only a short distance behind
the combined lenses.
The easiest solution is a teleconverter which is designed to do exactly
what you're attempting. You could take a 200mm lens and put a 3X
converter on it and have a 600mm lens. You could even combine
teleconverters. You could put a 3X and a 2X together and have a 6X
giving you 1200mm from your 200mm. You probably wouldn't like the
results very much, however, and you'd have a devil of a time seeing to
focus.
The final point is that, contrary to what most people think, the main
function of a telescope is to gather more light than can the unaided
eye. Magnification is secondary and can be detrimental if overdone.
When you magnfiy the image you take a fixed amount of light and spread
it over a larger area and make the image dimmer. Too much magnfication
and you can't see anything in the end.
Hope that helps some,
Chuck Norcutt
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