Adding an extension tube to your lens will increase its magnification. To find out the new magnification enter your lens and extension tube details into the magnification calculator:
The Theory
An extension tube is a hollow tube containing no optics. Its sole purpose is to move your camera lens further away from your camera sensor. This decreases the minimum focussing distance and allows you to bring your camera closer to the subject.
Things that are closer appear to be bigger than things that are further away!
It's a very simple concept which is great for macro photography. The extra magnification is calculated by dividing your extension tube length by the focal length of your camera lens.
Example:
If you have a 100 mm lens and a 50 mm extension tube, the extra magnification is 50/100 = 0.5
It is important to note that this is additional magnification. Each lens has a defined magnification at its Minimum Focusing Distance. To find the magnification of your lens and extension tube combination, you simply add the extra magnification to the lens magnification.
So wrapping it all up - if we have a 100 mm lens with a magnification of 1, and we add a 50mm extension tube, we end up with a final magnification of 1 + (50/100) = 1.5
What do you mean by Magnification?
Lens magnification refers size of the subject relative to the size of the image formed on the sensor. Usually, a scene that is larger than the sensor is being reduced in size by the lens, so that the image of the scene can fit onto the much smaller sensor. A lens that reduces the image by a factor of 4 would be reported as having a magnification of 0.25x or 1:4 (the image on the sensor is 1/4 or 0.25x the size of the subject).
The magnification of a lens is greatest when you are focussing on something close to the lens - hence, the magnification of a lens is usually reported for its minimum focussing distance.
Macro photography is where we use a magnification of at least 1x (or 1:1). At 1x magnification, the image formed on the sensor is the same size as the subject. True macro lenses, by definition, require a magnification of at least 1x. Adding an extension tube allows you to increase this above 1. In such cases you are actually enlarging the image on the sensor to greater than life size.