Originally posted by Nass Just curious, what mm value would you theoretically need to set SR at for say 50 reversed onto 100 (2:1)? I appreciate it might not be effective but even breathing gives me a wobbly image so this must be worth at least testing - anyone have an idea/equation?

Here's the simple math of what's going on - two equations are involved

*(thin lens approximation)*
image.distance = focal.length(1+m)

*m is magnification* **Lens extension**
When the image.distance = focal length, m=0. Now say you increase the image distance to focal.length + extension:

focal.length + extension = focal.length(1+m.extended) or:

**m.extended = extension/focal.length** **Lens stacking:**
Another way to change magnification is to change focal length but not image distance.When lens f' is stacked onto lens f a new lens is formed with a new focal length f"

f" = f'f/(f+f' -d)

* where d is the spacing between the two lenses, usually assumed to be zero.*
say an original lens f is focused at infinity so its image.distance= f then we stack on another lens f' to make a new lens f"

image.distance = f = f"(1+m.stacked) or after some algebra:

**m.stacked = (focal.length.original-d)/focal.length.stacked** **Combined extension and stacking:**
If the original lens is extended and a second lens is stacked, macro properties change like this;

(1+m.total) = (1+m.extended)(1+m.stacked)

image.distance = focal.length.original(1+m.extension)

f-stop.effective = f-stop.original(1+m.extension)

*SR setting*