Extension tube Magnification

Quoted from LordV's macro thread:

Does the orginal MFD of the lens used effect this equation? 70mm on my 24-70 does not give the same result as 70mm on my 70-200. Using the above equation 70mm should always give the same mag.

It makes since to me as with tubes it all about decreasing your working distance but it makes me question that equation. Or is that equation just a guideline?

silvrr wrote in post #9749365
Quoted from LordV's macro thread:


Does the orginal MFD of the lens used effect this equation? 70mm on my 24-70 does not give the same result as 70mm on my 70-200. Using the above equation 70mm should always give the same mag.

It makes since to me as with tubes it all about decreasing your working distance but it makes me question that equation. Or is that equation just a guideline?

Hi the equation is just a guide but is normally fairly accurate with prime lenses.
I think the situation with zooms is a bit different where because of the optics the focal lengths when focused at min focus are rather different (generally lower). I think if you try your two lenses at 70mm on a tripod focused on a subject say 10 feet away the FOVs will be slightly different.

The other thing is that if one is a "semi macro" lens ie it does focus a lot closer giving a significant magnification then that should be used in the equation.

ie Mag= existing mag + (length of tubes/focal length) - normally the existing mag of non- macro lenses is not significant.
Hope that helps.
As I've said a few times it's always better to check the magnification by photographing the mm scale of a ruler.

Brian v.

LordV wrote in post #9750125
Hi the equation is just a guide but is normally fairly accurate with prime lenses.
I think the situation with zooms is a bit different where because of the optics the focal lengths when focused at min focus are rather different (generally lower). I think if you try your two lenses at 70mm on a tripod focused on a subject say 10 feet away the FOVs will be slightly different.

The other thing is that if one is a "semi macro" lens ie it does focus a lot closer giving a significant magnification then that should be used in the equation.

ie Mag= existing mag + (length of tubes/focal length) - normally the existing mag of non- macro lenses is not significant.
Hope that helps.
As I've said a few times it's always better to check the magnification by photographing the mm scale of a ruler.

Brian v.

Thats what I figured but figured I would ask. Thanks.

wish I had even a little math smarts to be able to understand this stuff.