extension tube formulas

Moderated by: guardian

Visit the FM Store · Image Upload · Buy & Sell
FM Forum Rules
Canon SLRs, primes, and zooms lenses reviews

FM Forums | Canon-mount SLRs | Join Image Upload
end

rdjphoto
Online
Image Upload: Off

p.1 #1 · extension tube formulas

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

Mar 27, 2008 at 06:36 PM

Rampai65
Offline
Image Upload: Off

p.1 #2 · extension tube formulas

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.

Mar 27, 2008 at 06:52 PM

Rainer
Offline
Image Upload: Off

p.1 #3 · extension tube formulas

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

Mar 27, 2008 at 06:59 PM

rdjphoto
Online
Image Upload: Off

p.1 #4 · extension tube formulas

My math fails me

How did you arrive at i=56.25 ?

Mar 27, 2008 at 08:27 PM

Rainer
Offline
Image Upload: Off

p.1 #5 · extension tube formulas

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

Mar 27, 2008 at 08:37 PM

rdjphoto
Online
Image Upload: Off

p.1 #6 · extension tube formulas

Thanks,

Right after I posted the message I figured it out

Mar 27, 2008 at 09:51 PM

Pixel Perfect
Offline
Image Upload: On

p.1 #7 · extension tube formulas

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.

Mar 27, 2008 at 11:57 PM

Dan Lai
Offline
Image Upload: Off

p.1 #8 · extension tube formulas

What is MFD?

Mar 28, 2008 at 12:09 AM

Rainer
Offline
Image Upload: Off

p.1 #9 · extension tube formulas

Dan Lai wrote:
What is MFD?

MFD = minimal focus distance

Mar 28, 2008 at 12:17 AM

Duncan Gibson
Offline
Image Upload: Off

p.1 #10 · extension tube formulas

Minimum Focusing Distance

Mar 28, 2008 at 12:19 AM

Dan Lai
Offline
Image Upload: Off

p.1 #11 · extension tube formulas

Thanks Rainer and Duncan

Mar 28, 2008 at 12:31 AM


FM Forums \| Canon-mount SLRs \| Join Image Upload end