Macro Magnification & Extension Tubes
Magnification from extension tubes or a close-up (diopter) lens, the field of view it gives, the working distance, and the light lost as an effective aperture.
The magnification tubes or a close-up lens give on your lens, the subject area that fills the frame, the close-up lens’s working distance, and the effective aperture and stops lost to the extension.
Example: A 50 mm lens with 36 mm of extension tubes reaches 0.72×, filling a full-frame sensor with a 50 × 33 mm subject; f/8 on the lens behaves like f/13.8.
Extension over
focal length.
How tubes and close-up lenses create magnification, what the field of view and working distance are, why the light falls, and where thin-lens figures run out.
Tubes
Moving a lens further from the sensor lets it focus closer; with a thin lens focused at infinity, extension e adds e ÷ f of magnification, so short lenses gain more from the same tube. If the lens already focuses closer, its own magnification m₀ adds to it.
Close-up lenses
A close-up (diopter) lens of power D in front of a lens focused at infinity brings the focus to 1000 ÷ D mm and gives a magnification of f·D ÷ 1000 — so a +4 on a 100 mm lens gives 0.4× at 250 mm. Longer lenses gain more from the same diopter, the opposite of tubes.
Field and light
At magnification m the subject area that fills the frame is the sensor size divided by m. Extension also spreads the light: the effective aperture is N(1 + m), a loss of 2·log₂(1 + m) stops — two stops at life size. Through-the-lens metering already accounts for it; flash and hand meters do not.
What the number does not settle
Thin-lens figures with a pupil magnification of 1: internal-focus and zoom lenses lose focal length up close and give less, and the working distance with tubes depends on the lens design. Inputs stay in your browser; the same anonymous usage counts as the rest of the site apply.