DavieR wrote:How can I work out the magnification range when using Raynox lenses on a 100mm 1:1 macro?
Bob S wrote:Adding a 25 diopter lens to a 100mm 1:1 macro lens ought to give you about 2.5X when the main lens is set for infinity, and about 5X at close focus.
The 2.5X at infinity focus should be very close to exact. That configuration gives just the ratio of the focal lengths: 100 mm on the rear vs 40 mm (1000/25) on the front.
What happens at close focus is a lot harder to predict and depends on details of the lenses that aren't published. I'm suspecting a lot less than 5X.
As a quick test, I tried just now sticking a 50 mm lens (+20 diopters) in front of my Sigma 105 mm macro.
At infinity focus, the pair gives 1.98X versus a theoretical 2.1X.
But with the Sigma focused at 1:1, magnification increases only to 2.75X. That's versus a theoretical 5.2X based on the simplest "thin lens model" of a 105 and a 50 crammed tight together at 210 mm away from the sensor. Obviously simple theory does not work well here.
I've spent nearly the entire evening searching the forum and other places and I can't find the answer I'm looking for (possibly not asking the right question!). When I do find something, I get lost in the maths
I share your frustration, but there's no getting around the fact that lens combos are complicated and hard to predict --- as illustrated above.
Nonetheless, both theory and experiment suggest that you can reach your 3X target, or very close to it, using the stronger Raynox. As Bob says, only a real test will tell for sure.
--Rik
