Here is the end to the topic on magnification: Of course magnification in general means the increase in angle subtended when viewing an object. But when one writes "magnification" on a loupe or store magnifier, I now remember, thanks to this list that jogged my memory of old optics classes, that the term refers to an object at distance 250mm, thus Roland is correct that a 25 mm eyepiece is in effect a 10 power loupe. A 5 mm occular would be 50 power. Glenn Shaw ----- Original Message ----- From: Matt BenDaniel Sent: Tuesday, March 05, 2002 11:39 AM To: astro-photo@seds.org Subject: Re: [APML] Magnification At 01:10 PM 3/5/02 -0500, Michael A. Covington wrote:
>I think the equivalent focal length of the eye is about 50mm, and is it
>true that a 5 mm eyepiece therefore is a loupe or magnifier of 10 (50
>divided by 5) power? That's what I remember fom my kindergarden optics
>class taken in the paleolithic age.
>
>glenn shaw
>
>
>But that's not what matters. What matters is the size of the image on the
>retina. Normal viewing distance is normally taken to be 250 mm, so 250/F
>is the "power" of a magnifier. Focal length 5 mm = 50 power.
Visually, magnification is defined as the apparent size of an object (e.g.
in arc minutes) divided by the object's actual size. If M42 is judged to be
1 degree wide, and it just fills an eyepiece (with 50 degree FOV), the
magnification is 50x.
Photographically, magnification is specified in arc seconds per centimeter
(e.g. on the print).
Photographically, another very rough convention is that a 50mm f.l. lens is
taken to be 1x (as Michael infers above). So, 2000mm f.l. at prime focus
yields 40x.
Matt BenDaniel
matt@starmatt.com
http://starmatt.com
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