p.1 #2 · Help with using extension tubes for macros
Extension tubes 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 90 mm lens will give a magnification gain of 0.277X. Now, if the lens's original magnification was 0.25X, then the new magnification will be 0.277X+0.25X=0.52X. I don't know what the 90 TSE native magnification is so you will have to use the principle above to figure it out.
p.1 #3 · Help with using extension tubes for macros
According to canon museum , the max mag of the 90tse is .29 with a mfd of .5m (always remember the mfd is from the sensor plane and not the front of the lens)
So looking at the Above formular you mag will be .56 with the 25mm on
p.1 #5 · Help with using extension tubes for macros
You know, the simplest thing to do is to just take photographs of a ruler. At 1:1, the markings on the ruler will correspond to the dimensions of the sensor, so for a 24x36 mm sensor, the image will show 36mm of the ruler along the longer dimension. Empirical measurement generally wins against theoretical calculation.
The bonus is that you will be able to determine with reasonable precision the resulting magnifications of all 8 combinations of the lens + tubes (including the case with no tubes). You may even wish to measure the minimum magnifications when using tubes--simply turn the lens to infinity focus (one way to do this precisely is to manually focus on a star at night), and then with various tubes attached, adjust the subject distance until the ruler is in sharp focus. You might need a fairly long ruler (or be able to measure an object with a tape measure), but you only have to do this once--afterward, you will know what ranges of magnifications each configuration will yield. And if you are really interested, you can use a tripod and measure the working distance at each min/max. It's a good exercise for anyone interested in macro photography.
