extension tubes/reverse mounting/teleconverters

[Green_Tea]

I realize that with anything except the MP-E there are basically three ways to get past 1:1...

1) reverse mounting another lens
2) extension tubes
3) teleconverters(I know quality is compromised, but I already own one so I am curious)

I am not however clear on how exactly each method affects magnification.
I have done a quick search of the forum and google and I found numerous posts where someone said 'I have lens "x" with extension tube "y" and reverse mounted lens "z", what magnification do i have?' The more knowledgeable members of the community are always able to offer an answer. My question is, how do you know? Is there a formula for each method?

Thank you in advance,
Trevor

[LordV]

1.The Maximum magnification of a lens reversed onto the front of another non macro lens is given by the formula
Magnification = focal length main lens(mm)/ focal length reversed lens
So a 50mm lens reversed on to 100mm lens will give 100/50 = 2:1
For a macro lens you simply add the intrinsic magnification of the lens
so for a 1:1 100mm macro lens with a reversed 50mm the magnification is 1+ 100/50 = 3:1

2. Magnification with extension tubes

For a non macro lens with extenson tubes the magnification is given by the formula
magnification = length of ext tubes (mm)/focal length of lens (mm).
So for a 50mm lens with a set of extension tubes totalling 68mm the max magnification = 68/50 = 1.36:1

For a macro lens again you simply add in the existing magnification, so for a 100mm macro lens with 68mm of extension tubes the max magnification should be 1+ 68/100 = 1.68 :1
However this is not what actually you get because at minimum focus the focal length of a macro lens shortens considerably from that at infinity. The focal length of a macro lens at minimum focus is given by the minimum focus distance/4 which for a 100mm macro lens works out to be around 75mm so the magnification you actually get is 1+ 68/75 = 1.9:1
In practice even this is a slight underestimate so I recommend you actually measure it with the ruler method.

3. A TC as long as it is mounted next to the camera body will just magnify the existing magnification of the lens set by the TC mag factor.
eg for a reversed 50mm onto a 100mm macro lens with a 1.4X TC

Mag = 1.4*(1+ 100/50) = 4.2:1.

4. Lens reversed onto the camera body directly. Never have found a useable formula for this and indeed the results do vary for the same focal length lens from different manufacturers. As an approx guide a 50mm lens reversed onto the camera body will give around 1:1 whilst a 28mm lens will give around 3:1.
A body reversed kit lens makes a neat zoom macro lens going from around 0.5:1 to 3:1

Brian V.

[Green_Tea]

Thank you Brian, that is very helpful.
I have thought of another question... How does each of these methods effect the amount of light which makes it through the lens? The teleconverters I assume is the standard 1.4x=-1stop and 2x=-2stops but how about reverse mounting and tubes?
-Trevor

[LordV]

Have to admit I've never been too bothered about light loss with TTL metering etc.
I'm never sure if different methods of magnification lose different amounts of light or not- I think a full set of ext tubes is quoted as losing 2 stops.
I do know that the apparent aperture which would be linked with light loss goes up as a function of magnification . Apparent aperture = F(1+M) where F is the set aperture and M is the magnification. Unfortunately apparent aperture values seem to affect both diffraction and light loss but not DOF.

Brian v.

[John_B]

Green_Tea,
Here is a visual demonstration of the magnification changes with different setups Macro Variations <-- click to see (http://photography-on-the.net/forum/showthread.php?t=578274)

[Green_Tea]

Thank you both
-Trevor