Macro Extension and Bellows Exposure Calculator
Estimate magnification, working aperture and exposure factor for a symmetric thin lens extended from infinity focus.
YOUR CALCULATION
Result
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Calculation steps
Work through one step at a time or reveal the complete calculation. The steps use your current inputs.
How to calculate
Use the thin-lens extension relation m=extension/focal length. For unit pupil magnification the working f-number is N(1+m); exposure factor is its squared ratio to N.
m=e/f; working N=N(1+m); exposure factor=(1+m)²; stops=2log2(1+m)
Worked example
A 50 mm lens with 50 mm total extension has ideal magnification 1×. A marked f/8 acts as f/16 with exposure factor 4, or 2 stops. A 36 mm sensor covers 36 mm object width.
Assumptions and input details
Symmetric thin lens with pupil magnification 1, no internal focusing or extra optical elements. Total extension is relative to infinity focus, not necessarily the length printed on an extension tube. Object/image distances are from ideal principal planes, not the front of a real lens. Camera TTL or effective-aperture reporting may already account for light loss.
References: Edmund Optics: working f-number and magnification
Unit definitions
Unit selection is independent of language. Each selector contains only units for its physical quantity. Calculations use defined base units internally.
Choose from 23 length definitions, including explicitly labelled regional measures. Area conversions square the length factor; geometric volumes cube it.
Regional measures are not interchangeable merely because their names look similar. This is a documented selection, not a claim to include every historical unit.
NIST measurement tables · Japanese traditional units · Mainland China and Hong Kong unit distinctions
Using this calculator
Can I enter my own problem?
Yes. Replace the example values with your own inputs. The result, calculation steps, tables and applicable diagrams update from those inputs. Blank or invalid values clear the result; example values are never silently substituted for missing inputs.
Are the results exact?
Results use floating-point arithmetic and displayed rounding. Read the formula, assumptions and any stated data precision before interpreting the result.
Is my input uploaded?
The calculation runs in your browser. No account, file upload or external AI service is required. Inputs are not saved between page reloads.
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