How do i calculate the ratio on an extension tube? I have an 18-55 IS Canon and a 55-250 IS. I'm interested in the Kenko set from B&H with three different tubes.
"An extension tube increases lens magnification by an amount equal to the extension distance divided by the lens focal length. For example, adding a 25 mm extension tube to a 50 mm lens will give a magnification gain of 0.5X. Therefore, if the lens's original magnification was 0.15X, then the new magnification will be 0.15X+0.5X=0.65X. The closest focusing distance will also decrease to ~210 mm."
Use the link below to get to a calculator for your lens' original magnification.
