Magnification Calculator
Work out your reproduction ratio from a lens plus extension tubes or bellows.
Results
- Magnification
- 0.70x (1:1.43)
- Subject width filling the frame
- 5.1 cm
How large the subject appears on the sensor relative to its real-world size.
The real-world width of a subject that would exactly fill the frame edge to edge at this magnification.
How it works
Magnification, or reproduction ratio, describes how large a subject lands on the sensor compared with its real size. At 1:1 a 10mm insect projects a 10mm image, so on full frame it spans a bit over a quarter of the frame width. At 2:1 it fills more than half.
Adding extension between the lens and the body increases magnification, because moving the lens further from the sensor forces it to focus closer. Every millimetre of extension adds magnification equal to that millimetre divided by the focal length, which is why extension tubes do far more for a 50mm lens than for a 200mm one.
The trade-off is light and working distance. Extension costs you effective aperture and pushes the front of the lens uncomfortably close to the subject, which makes lighting the shot harder as magnification climbs.
The formula
m = m_native + (extension / f)
m_native is the lens's own maximum magnification at its closest focus, extension is the total tube or bellows length in mm, and f is the focal length. Subject width that fills the frame is the sensor width divided by m.
Frequently asked questions
- What counts as true macro?
- Traditionally 1:1 or greater, meaning the subject appears on the sensor at life size or larger. Plenty of lenses marketed as macro only reach 1:2, which is why the native magnification figure matters when you add tubes.
- Do extension tubes lose image quality?
- The tubes themselves contain no glass, so they add no aberrations of their own. What they do is push the lens well outside the range it was optically corrected for, and cost you light. Corner sharpness usually suffers before the centre does.
- How much light do I lose?
- The effective aperture becomes roughly the marked f-number multiplied by (1 + m). At 1:1 that is two stops of light gone, so f/8 behaves like f/16 for exposure purposes, and diffraction sets in correspondingly earlier.
Read more about this
- Why cheap lens adapters lose infinity focusOne measurement decides whether a lens adapter can ever work properly, and it is not one that adapter listings tend to mention.
- Why your depth of field calculator disagrees with your photographsThe calculator said the whole scene was sharp. At 100% on screen, half of it clearly is not. Both things are true, and the reason is a number nobody mentions.
- The diffraction limit is not a cliffDiffraction gets talked about as a wall you hit at f/11. It is closer to a slope you have been walking down since f/4.