I am not sure how could I missed out this topic in my old and new blog …let me add this back to my share…
#### from http://en.wikipedia.org/wiki/Extension_tube
An extension tube is an accessory for cameras with interchangeable lenses, used primarily for macro photography. The tube contains no optical elements; its sole purpose is to move the lens farther from the image plane. The farther away the lens is, the closer the focus, the greater the magnification, and also the greater the loss of light (requiring a longer exposure time). Lenses classically focus closer than infinity by moving all optical elements farther from the film or sensor; an extension tube simply imposes this movement.
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other word, it allow us to shoot object closer (so it is bigger) but it do lost infinity focus (mean cannot shoot far) and some light lost ….
the magnification again simple can get by shooting the ruler to see what magnification you have e.g.
if you have crop body 40d, which mean yours sensor size should be 35mm/1.6=21.8mm
at 1:1 your wide of single frame short should occupy by 21.8mm object ……. so add on with ET if your shot is 10.9mm mean u get
21.8/10.9 = 2x magnification 😀
or we can use formula
X/focal length = magnification
Y = X + ET (in mm added)
e.g.
100mm L
X/100 = 1
so X=100
if we add on 12mm + 20mm Kenko ET s0 we will get Y=X + ET = 100+32=132mm
so new magnification should be 132/100 = 1.32x
… there is another thing we may concern is the MFD (minimum focus distance)
1) M = b/a
2) 1/a + 1/b = 1/f
3) MFD = a + b
where
M: Magnification
a: lens-to-subject distance
b: lens-to-film/sensor distance
f: focal length