Extension Tube Magnification Calculator for Close-Up Photography

This calculator estimates how much magnification the added extension contributes from the tube length and the lens focal length. You can also enter the lens's native close-focus magnification to estimate the combined reproduction ratio. The result is a practical planning figure for comparing tube and lens combinations before you pack equipment or arrange a close-up setup.

Enter focal length and total extension

Use the focal length marked on the lens, in millimetres, and add together the lengths of every extension tube in the stack. A 12 mm tube plus a 20 mm tube should therefore be entered as 32 mm of extension. For a zoom lens, enter the focal length selected for the photograph, not merely the shortest or longest value printed on the barrel. The calculator divides extension by focal length, so both measurements must use millimetres. If the lens specification gives a native maximum magnification, you may enter that dimensionless value as well: 0.25 means one-quarter life size, commonly written 1:4. Leave it at zero when you want only the idealized magnification added by the tube from an infinity-focus starting point. The result reports the added magnification, an estimated total, and a reproduction ratio. This makes it easy to compare, for example, why the same tube produces a stronger close-up effect on a shorter lens than on a longer lens.

Understand the estimate and its formula

The calculation uses the standard thin-lens approximation: added magnification equals extension length divided by focal length. A 25 mm tube on a 50 mm lens therefore adds 0.5× magnification. When native magnification is supplied, the calculator adds that figure to the extension contribution to provide an estimated total. This simple relationship is most directly applicable to a lens focused near infinity and modeled as a thin lens. Real photographic lenses are compound optical systems. Internal focusing can change effective focal length at close distances, and the physical extension relevant to the optical calculation may not exactly match the tube label once the lens itself changes focus. For those reasons, treat the answer as a planning estimate rather than a laboratory measurement. It is especially useful for ranking combinations and anticipating framing. For exact reproduction, photograph a ruler or calibrated target at the intended focus setting and compare the captured subject width with the sensor width.

Plan a workable close-up setup

More extension increases magnification, but it also changes the practical behavior of the camera system. The lens loses infinity focus, the working distance can become short, and effective exposure at the subject may fall as magnification rises. Autofocus may slow or hunt, particularly with low-contrast subjects, so manual focus and moving the camera gently toward or away from the subject are often more predictable. Start with the smallest tube that reaches the framing you need, then check whether there is enough room for lighting and whether the subject can remain still. Longer focal lengths generally preserve more working distance but receive less added magnification from the same extension, while shorter focal lengths gain more magnification and may place the front of the lens uncomfortably close to the subject. The estimate does not predict depth of field, exposure compensation, image quality, or working distance. Those depend on aperture, lens design, focus setting, sensor format, and the exact tube-and-lens combination.