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The Apparent magnification is the focal length of the Tube/focal length of the eyepeice - so if you double the tube focal length then you double the magnification. Equally you could halve the focal length of the eyepeice to acheive the same result.
Magnification is everything though - If you don't have the aperture to capure enough light then constantly stepping up the magnification (using the smaller focal eyepeice logic) tends to result in fainter and fainter images
A 1500mm Tube Focal length with a 25mm Eyepeice should result in an apparent magnification of 1500/25 = 60x If you double the OTA focal length to 3000mm or halve the EP to 12.5 you get
3000/25 or 1500/12.5 both of which are 120x
Although I have discussed EPs the same rule applies to imaging when using Barlows
Mike
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