Expert: Tom Whiting - 9/15/2010

Question
If there is an object 1760 yards away, - how close would it appear to be if viewed at full zoom by 15x, 12x, 10x 8x lenses. Is there a table to compute apparent nearness for different power zooms. I have always assumed that 2x halves the distance, 4x quarters the distance and so on, but I read that a 10x zoom makes the object appear 10 times nearer. Is that correct? The two aren't compatible.  Thanking you in anticipation.  Paul W Hunt

Answer
Hi Paul,
I don't know about your "zoom" or what that means.  But you are correct about the power divided into the distance. (Actually a simple, basic definition of Power).  The moon at 238,000 miles when viewed at 10 power (whether you "zoom" to that power, or not) is being seen as from 23,800 miles, and at 100 power at 2380 miles distant.
AND at 1000 power, 238 miles. But of course, at the extremes, other factors come into play because you're also magnifying all the scope's optical defects, atmospheric distortion, etc. So you are OK to a point, but eventually resolution suffers and the image becomes fuzzier and fuzzier with added magnification.  In a well ground and figured scope of very high quality, you can expect a maximum useful magnification of about 50 power per inch of aperture (front lens in a refractor, or mirror diameter in a reflector telescope).  So say a 6 inch scope should be good to a maximum of 300 power.  Any more than that and you risk getting what is called "empty magnification"... the magnified optical defects over-ride and over-power... the image you are magnifying, so you rapidly lose resolution (clarity of image).
Solution?  Get a bigger and better telescope!   


So yes, 2 power halves the distance and 4 power 1/4th the distance,
and 10 power will 1/10 the distance.  (It's just the basic definition
of power, or more properly, magnification; that's all).  No tables needed, just simple division, that's all.
Hope this helps,
Clear Skies,
Tom Whiting
Erie, PA  USA
PS... Sure, they are compatible.  Ten power means 10 times nearer which in turn MEANS seeing it at 1/10 the actual distance to the object.  No incompatibility there.  Means the same thing to me.
Unless our American language is different from English language! ;-)