Related Links:
Macro Photography Tips
From: "Adrian Seah" adrianse@mbox4.singnet.com.sg
To: hasselblad@kelvin.net
Subject: Exposure Factor With Extension Tube
Date: Fri, 20 Mar 1998
The basic calculation for exposure compensation is
Exposure Increase factor (EIF) = (1 + m)
m = magnification desired
F = the focal length of the lens
d = length of the extension tube
m = d/f
Example
8mm Extension Tube and 80mm lens
---------------------------------------------------
Using the formulae above m = d/f = 8/80 = 0.1
Therefore the magnification = 1: 10
The EIF =3D (1 + m ) = ( 1 + 0.1 ) = 1.21
16mm Extension Tube With 80mm lens
---------------------------------------------------------
m = d/f
= 16 / 80
= 0.2 (or 1 : 5 )
EIF = (1 + m )
= (1 + 0.2 )
= 1.44 or approx
= 1.5
Below is the conversion of EIF to f-stop value
EXPOSURE FACTOR f-stop Value
1.5 0.5
2.0 1.0
2.5 1.25
3.0 1.5
3.5 1.75
4.0 2.0
5.0 2.25
6.0 2.5
8.0 3.0
10.0 3.25
The basic calculation for exposure compensation is
Exposure Increase factor (EIF)= (1+m)
m = magnification desired
F = focal length of the lens
d = the length of the extension tube
m = d/f
Example: 8mm Extension tube and 80mm lens
---------------------------------------------------
Using the formulae above
m =d/f = 8/80 = 0.1
Therefore the magnification = 1:10
The EIF = (1 + M) = (1 + 0.1) = 1.21
16mm Extension Tube With 80mm lens
---------------------------------------------------------
m = d/f = 16 / 80 = 0.2 ( or 1 : 5 )
EIF = (1 + m) = (1 + 0.2) = 1.44 or approx. 1.5
Below is the conversion of EIF to f-stop value
EXPOSURE FACTOR f-stop Value
1.5 0.5
2.0 1.0
2.5 1.25
3.0 1.5
3.5 1.75
4.0 2.0
5.0 2.25
6.0 2.5
8.0 3.0
10.0 3.25
End of Page