View Full Version : Measuring magnification

12-04-2008, 05:55 AM
How to Measure magnification
How can I see how strong a magnifying glass is?

I bought this magnifying glass http://www.liangdianup.com/images/tools/magnify/htmlpage/189901s.jpg and I want

to know how to test it to see how strong it is. I hear a lot of people talk about

magnifying and how strong the magnification is, but I would like to know the true

magnification of my magnifying glass. I have a few of them and some seem stronger then

others. How can I rate these? How can I pin a correct number on mine? How do the companies

that make these come up with these numbers.

12-04-2008, 06:08 AM
They on average either double or quadruple the image size.


12-08-2008, 08:01 PM
The power of a magnifying glass is measured in diopters (inverse of focal length). Focus the sun's light to a point and measure the focal length. Diopters = 1/focal length.

Magnification depends on the distance between the lens and the object. Therefore it is inconvenient to rate lenses in terms of magnification because it varies. That is why diopters are used.

Magnification at a certain distance = focal length/(focal length - distance between object and lens).

The greater the diopter strength (the shorter the focal length) the greater the magnification.

Papa Bear
01-06-2013, 05:47 PM
That is a good explanation and thank you for your post, I think this thread should be in the Underwater Photo section it is very good explanation of how a Dome Port works!