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| 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.