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Checking Magnification - An alternative trick

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#1 KennyJ

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Posted 03 June 2004 - 05:02 PM

I've just been playing around with a few pairs of binoculars INDOORS and I've realised that one way of checking ACTUAL magnification can be carried out in a similar way to that of the tried and tested method.

In case anyone wonders to which method I am referring here , it is that of comparing squares / circles of card , cut to specific sizes representative of the differences between 1x and whatever x magnification is being tested , attached to a wall from the closest focussing distance of any given binocular.

This "indoor" method follows the same procedure whilst eliminating the inconvenience associated with checking true magnification values of binoculars with very long "short -focuses" , such as those of most "giant" binoculars , where the cards would need to be placed perhaps as much as 60 or more feet away in some cases.

I've discovered that when looking through any binocular BACKWARDS -- i.e through one objective lens , with ONE eye, whilst looking at an adjacent object naked eye with the other eye at the same time , the object viewed through the reversed binocular ( which can be placed typically only 3 or 4 feet from the binocular ) the artificially "reduced" image dimensions will be reduced by a factor exactly inversely proportional to the ACTUAL magnification.

Thus , when viewed through a 10x binocular BACKWARDS , a card measuring 10cm x 10cm will APPEAR to look like a card 1cm x 1cm , and so forth.

Hardly revolutionary for sure -- but perhaps a useful little trick to be aware of -- and one which can be quite fun to try on a damp dark night.

Regards , Kenny.

#2 sftonkin

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Posted 04 June 2004 - 01:45 AM

Very interesting. I haven't done any sums or experiments on this yet, but I suspect that it will prove to at least partially counteract a "characteristic" of indoor measuring of magnification, as a consequence of the object distance being a greater multiple of the focal length of the "new" objective lens (i.e. the eyepiece).

One thing to be aware of when doing any indoor measurement of magnification is that magnification varies within the same instrument for different object distances. This is particularly noticeable with roof prism binocs that have internal focus (by moving a lens).


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