Choose the Right Size Extension Tube
The relationship between your lens’s focal length and the size of an extension tube determines how close you can get to a subject while achieving focus. The closer you can get, the greater the magnification.
By creating more space between the lens and the digital sensor (using a larger extension tube) you enable yourself to get closer to the subject. As you increase the size of the extension tube, you increase the ability to move even closer.
A simple mathematical equation helps you determine what level of magnification you will achieve when using a specific size extension tube on a specific lens:
Additional magnification = Extension tube’s length in millimeters divided by the focal length of a lens
A 50mm extension tube combined with a 50mm lens (50 divided by 50) gives you a 1:1 ratio. Likewise, a 25mm extension tube combined with a 50mm lens (25 divided by 50) can offer a 1:2 (or half size) representation of your subject.
A 1:2 ratio is great for close-up photography and although it’s not technically macro photography, it looks very similar. Macro photography is ideal for subjects that are very small, which have intricate details. However, for some subjects and scenarios you may prefer less magnification in order to show more of the environment.
The figure shows that a 1:1 ratio clearly reveals more detail in the nickel. Notice how it just barely fits into the frame. Subjects larger than the size of a nickel will be cropped into when photographed at a 1:1 ratio.
When photographed at a 1:2 magnification ratio, the nickel fits in the frame with much more breathing room. Consider the size of your subject and how much of its surrounding environment you want to include in your frame when choosing how much magnification you need.
100mm, 1 sec, f/8, 100 50mm + 25mm ext. tube, 0.5 sec, f/8, 100
As a photographer, you need to determine what magnification levels work best in which situations depending on your subject and your intended message. Extension tubes give you access to many levels of magnification. For instance, a 25mm extension tube affects a 50mm lens much differently than it affects a 35mm lens or a 100mm lens.
If you have multiple extension tubes, you can stack them in order to gain even higher levels of magnification. Some brands offer sets of extension tubes at various sizes for discounted rates. Remember to ensure that the brand and models you are purchasing are compatible with your particular lenses.