Extension Tube Magnification Calculator

Adding an extension tube to your lens will increase its magnification. Enter your lens details and extension tube details into the magnification calculator below to find out the new magnification:

Extension Tube Theory

An extension tube is simply a hollow tube containing no optics. By placing an extension tube between your lens and your camera, you decrease your camera’s Minimum Focusing Distance (MFD). This enables your camera to focus on objects which are closer to the camera, or in other words, to move your camera closer to your subject. This has the effect of increasing the magnification. As we all know, 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. As a simple example, if you have a 100mm lens and a 50mm extension tube, the additional magnification is 50/100 = 0.5

We can see from this that the smaller your camera lens is the greater the effect of the extension tube is. If you have a 50mm lens and a 50mm extension tube, the additional magnification would be 50/50 = 1

It is important to note that this is additional magnification. Each lens has a certain magnification at its Minimum Focusing Distance which is published by the lens manufacturer. The very definition of a macro lens is a lens which has a magnification greater than or equal to 1

So wrapping it all up, if we have a 100mm lens with a magnification of 1, and we add a 50mm extension tube, we end up with a magnification of 1 + (50/100) = 1.5


Adding extension tubes to your lens is a great way to achieve a higher magnification, but they do have their drawbacks. When you shoot closer to your subject, the depth of field decreases, meaning that you may have to increase the f-stop to get the desired depth of field. Increasing the f-stop means that you need will a longer exposure time or additional lighting in order to get the right exposure.