Macro Magnification and Bellows Calculator
Magnification, bellows exposure factor, effective aperture and working distance from the focal length and the extension.
Results
Magnification
0.5000
Bellows exposure factor
2.2500
Extra stops this costs
1.1699
Effective f-number
12.000
Corrected time
0.018000
s
Subject to lens distance
300.00
mm
Width of subject that fits
72.000
mm
Reproduction ratio, 1 to
2.000
Lens to sensor distance
150.00
mm
What this tool does
Moving the lens away from the sensor magnifies the subject and spreads the same light over a larger area, so a macro shot needs more exposure than the meter on the lens barrel suggests. At 1:1 that is two full stops. The effective aperture rises with the same factor, which is why macro work runs into diffraction earlier than anything else.
Formula
m = extension ÷ focal · bellows factor = (1 + m)² · extra stops = 2 log₂(1 + m) · effective N = N (1 + m)
Variables
| Symbol | Meaning | Unit |
|---|---|---|
fl | Focal length as marked | mm |
ex | Extension between lens and sensor | mm |
ap | f-number | — |
sw | Sensor width | mm |
ts | Shutter you had | s |
MG | Magnification | — |
BF | Bellows exposure factor | — |
ST | Extra stops this costs | — |
EA | Effective f-number | — |
TC | Corrected time | s |
WD | Subject to lens distance | mm |
SF | Width of subject that fits | mm |
RR | Reproduction ratio, 1 to | — |
LS | Lens to sensor distance | mm |
Worked example
- Focal length as marked100 mm
- Extension between lens and sensor50 mm
- f-number8
- Sensor width36 mm
- Shutter you had0.008 s
- Magnification0.5000
- Bellows exposure factor2.2500
- Extra stops this costs1.1699
- Effective f-number12.000
- Corrected time0.018000 s
- Subject to lens distance300.00 mm
- Width of subject that fits72.000 mm
- Reproduction ratio, 1 to2.000
- Lens to sensor distance150.00 mm
Limitations
- The result is an estimate based only on the values you type. Real situations often include factors this calculator does not know about.
- The calculation runs at full precision and only the display is rounded. If you copy an intermediate value and retype it, small differences can appear.