Macro Magnification Calculator
Use the thin-lens approximation to estimate magnification with an extension tube. Enter focal length, extension, f-number, and sensor dimensions to see image scale and exposure effects.
A QUICK WALKTHROUGH
How to use this tool
- Enter focal length, extension, f-number, and sensor width and height.
- Review approximate magnification, effective aperture, exposure compensation, working distance, and sensor coverage.
- Treat the result as a planning estimate; real macro lenses and focusing methods can differ.
Approximate thin-lens model
Magnification m is extension ÷ focal length. Effective f-number is nominal f-number × (1 + m), and exposure compensation is 2·log₂(1 + m) stops. Working distance from the lens principal plane is approximated as focal length × (1 + 1/m); this is an estimate, not a manufacturer specification.
Sensor dimensions are user supplied
Enter active sensor width and height from a reliable specification or your measurement. The tool does not invent camera or brand sensor tables.
Applicability limits
This simplified model is for extension tubes and a thin lens focused near infinity. Internal focusing, floating elements, bellows factors, crop modes, teleconverters, and close-focus lens design can change the result. Positive finite values only; practical limits keep inputs bounded.
GOOD TO KNOW
Common questions
Is this the true magnification of my macro lens?
No. It is a thin-lens estimate for extension. Internal focusing and lens design can make the actual value different.
Why does the effective aperture get smaller?
At magnification m, the effective f-number is approximated as nominal f-number × (1 + m), reducing light at the sensor.
What does working distance mean here?
It is an approximate distance from the lens plane to the subject plane. Physical clearance can be shorter and must be measured for a real setup.