Macro magnification and extension calculator
Extension buys magnification at the cost of light. At life size you lose two full stops before anything else happens.
Magnification
0.5000
- Effective aperture
- 12.000
- Light lost
- 1.170 stops
- Exposure factor
- 2.250
- Subject width across the frame
- 72.00
- Lens to subject distance
- 150.0
- Approximate depth of field
- 4.176
Show the working
- Lens focal length = 50
- Extension = 25
- Lens own magnification = 0
- Set aperture = 8
- Sensor width = 36
- Magnification = 50 > 0 ? 25 / 50 + 0 : 0 = 0.5
- Effective aperture = 8 * (1 + 0.5) = 12
- Light lost = 2 * log2(1 + 0.5) = 1.169 93
- Subject width across the frame = 0.5 > 0 ? 36 / 0.5 : 0 = 72
- Lens to subject distance = 0.5 > 0 ? 50 * (1 + 1/0.5) : 0 = 150
- Exposure factor = (1 + 0.5)^2 = 2.25
- Approximate depth of field = 0.5 > 0 ? 2 * 12 * 0.029 * (1 + 0.5) / 0.5^2 : 0 = 4.176
Values are shown to six significant figures here; the arithmetic itself runs on exact fractions and is only rounded where a number is printed.
Scenarios & local history
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The formula
- magnification from extension = extension / focal length
- effective aperture = set aperture x (1 + magnification)
- light loss in stops = 2 log2(1 + magnification)
At 1:1 the effective aperture is double the marked one, costing exactly two stops. Cameras metering through the lens handle this automatically.
Depth of field at macro distances is measured in millimetres, and diffraction sets in early because the effective aperture is so small.