Measuring magnification

[Sarah]

They on average either double or quadruple the image size.

http://en.wikipedia.org/wiki/Magnification

[Nemo]

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]

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!