1.) The retro-focus design of wide angles puts the glass elements a greater
distance away from the image plane than the focal length. The telephoto
design puts the glass elements closer to the focal plane than the focal
length. The focal length corresponds to the distance a single element lens
(which has the same angle of view) would be from the focal plane.
2.) The focal length of most eyepieces is less than 30mm. You can't get the
film close enough to the eyepiece to get the image in focus. Some of the
microscope "photo eyepieces" have much longer focal lengths. The typical
markings for the Olympus versions are NFK3.3x, NFK2.5x, etc. I'm not sure
how the multiplication factor is determined. Perhaps from the one of the
standard microscope tube lengths 160mm or 180mm. I would bet Mark knows.
Telescopes have very long focal lengths for the primary lens, and short
focal lengths for the eyepiece.
There are a number of people who use two camera lenses connected together to
get extreme macro images. Looks like the best link I had is no longer
working. This isn't as helpful but it works:
<http://www.microscopy-uk.org.uk/mag/indexmag.html?http://www.microscopy-uk.
org.uk/mag/artsep01/vrcoolpix2.html>
For macro work the lens is further from the image plane than the focal
length.
To get a long cheap lens consider getting either the v*vitar macro focusing
2x teleconverter or the Olympus 2x teleconverter. The design of a
teleconverter is different from the design of a normal photographic lens.
-jeff
> -----Original Message-----
> From: orava@xxxxxxxx
>
>
>
> Hello!
>
> I have some questions and I would be happy if you
> could help me with those:
>
>
> 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?
>
>
> 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?
>
>
> Olli (who is in need for a long zuiko and tries to replace it
> with cheaper
> options that are never gonna work ;-)
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