I know I read this somewhere here before, but I was wanting to know the formulas regarding using extension tubes.

For example for a given lens adding an extension tube of 25mm how do you calculate how much closer can you focus.

Also how would you calculate your magnification factor eg. how close do we get to true macro 1:1

Thanks

For a lens set at infinity you would need focal length equivalent of extension to make it give you 1:1. That means a 50mm Lens would need 50mm of extension at Infinity focus to achieve 1:1 magnification. This formula also comes with some other caveats, like the lens has to be symmetric and all that I believe.

Best thing is to shoot a scale at the MFD and see whats the max magnification you can get.

You will need the thin lens formula, which is

1/o + 1/i = 1/f

where o and i are object and image distance, respectively, and f is the focal length. As an example, let's look at the 50 f/1.8. The closest focusing distance of this lens is 0.45 m = 450mm. We now get the image distance from

1/450mm + 1/i = 1/50mm,

i = 56.25mm. The magnification factor is i / o = 56.25mm / 450mm = 0.125. Now add the extension tube. i' = i + 25mm = 81.25mm. We find o' from

1/o' + 1/81.25mm = 1/50mm,

o' = 130mm, the new closest focusing distance. And the new magnification factor is i'/o' = 81.25mm / 130mm = 0.625.

The slight differences from the magnification factors given in Canon's Lens Works are due to these lenses being not exactly "thin" lenses.

Rainer

My math fails me

How did you arrive at i=56.25 ?

rdjphoto wrote:
My math fails me

How did you arrive at i=56.25 ?

Let's do it step by step (ignoring the "mm"):

1/450 + 1/i = 1/50

0.002222 + 1/i = 0.02

1/i = 0.02 - 0.002222 = 0.017778

i = 1 / 0.017778 = 56.25

I hope this helps,

Rainer

Thanks,

Right after I posted the message I figured it out

If you don't need to know the new MFD, then the extra magnifcation m' the tubes gives is simply

m' = ext/FL = 25/50 = 0.5

which is added to the inherent magnification of the 50mm, so

m" = m' + 0.125 = 0.625.

What is MFD?

Dan Lai wrote:
What is MFD?

MFD = minimal focus distance

Minimum Focusing Distance

Thanks Rainer and Duncan