Re: Buying an Eyepiece
- From: "Neil B." <neil_delver@xxxxxxxxxxxxxxx>
- Date: Thu, 18 Dec 2008 11:01:52 -0500
<pmoscatello@xxxxxxxxxxx> wrote in message
news:8414b67b-c84c-49f1-82ef-2757a29eb9ee@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
I'm very new at this and recently bought a microscope on ebay for $90.
It seems like an ok little microscope for the price. It came with two
eyepieces 10x and 16x, and I would like to add a 5x eyepiece to it.
The person I bought it from says I need a 23mm eyepiece, but I
measured the tube that the eyepiece goes into at 22 1/2 mm and the
bottom of the eyepiece that goes into the tube measures 20 mm.
I'm not sure what to do, could someone please help?
Phil
No one else mentioned that a 5x eyepiece with even a 23 mm OD (so a bit
less ID) will have a very narrow field of view, and 20 mm even worse.
Think: for a telescope or microscope, the image from the objective is
projected to the eyepiece area, and obviously that portion outside the
eyepiece barrel ID is "lost." Then consider what fl. the eyepiece has
and how big that ID circle will look (even w/o a field stop.) So using
standard 250 mm reference distance (not to be confused with projection
distance for objectives), 5x eyepiece means 50 mm focal length. So,
imagine a 22 mm circle to be very generous, then multiply by five to get
110 mm seen at 250 mm. Sounds small, doesn't it? To get field in
degrees: take 2*arctan(55/250) since have two right triangles flush, and
you get a mere just under 25 degree field! That's pretty bad, and 20 mm
OD is even worse.
That's why the longest available 1.25" eyepiece for telescopes is 40 mm
(equivalent to 6.25x microscope ocular). That's about 32 mm OD so we get
about 41° with 30 mm ID - not quite so bad. The main point of 2" OD
eyepieces is to allow good field up to about 60 mm, for low power in big
telescopes. You could do better with "6.3x" ocular which are available,
running up above field to about 31° which doesn't totally suck IMHO.
PS: Microscope objective magnification is based on actual scale
projected to 160 or 170 mm (usually) not the "viewing distance"
standard. Hence multiply by 1.5 to rough convert to "magnifier"
standard. I was puzzled as a boy finding my "5x" objective more powerful
as magnifier than my "5x" hand lens until I read up on how microscopes
work.
.
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