calculating macro magnification ratios and zoom length using extension tubes???

[xdjoynerx]

i just purchased a 3 step extension tube. with all 3, its 75mm

i have a tamron 28-80 lens.

how can i calculate what my macro magnification ratio will be, and how long it will be extending my zoom?

[ScottE]

Life is too short to calculate magnification ratios, and what are you going to use them for anyway? The formula is complex and only works with the lens focused to infinity. Focusing the lens to any other distance introduces an unknown amount of extension that is difficult include in your calculation.

The simple solution is to set up your camera, extension tubes and lens and take a picture of a ruler (showing mm not inches). The sensor on an D60, 10D, 20D or 300D is 15 x 22.5 mm. If your picture of a rule shows 22.5 mm length you have 1x magnification. If it has more or less than that you can quickly calculate magnification by dividing the length you can see by 22.5.

More useful is to not the size of image you can frame with each focal length/extension tube combination. That way when you see something you want to photograph you will have a good idea of which extension tube to use.

Good luck with your macro photography.

[Jon]

Check this thread (http://photography-on-the.net/forum/showthread.php?t=51952) for both the formuals and the caveats. The formulas apply regardless of where the lens is focussed; you just/ need to calculate the lens extension at any given focussing setting. The quickie application of the formula is that lens extension you add will equal focal length of the lens for a life-size (1:1) image (lens to film plane distance = 2x focal length), and working distance (which is total distance between lens and subject) will be the same. So you'll be able to get pretty close to life-size at the 80 mm. end of things (depending on how close the lens focusses on its own).