Extension tube magnification.

bclaff wrote:

TomBarth wrote:

The basic calculation for magnification with extension tubes is to take the total length of the tubes installed on the camera (mm) and divide that by the focal length of the lens (mm). This gives you the magnification resulting from the extension tubes. Then add to that the lens' native maximum magnification (MM) value. The MM of the lens is part of the published specification of every lens. For instance, the Canon 50mm f/1.8 has an MM of 0.15 (very poor). But with a 36mm extension tube the result is 0.15 + 36/50 = 0.87 (not quite 1:1 but not bad)

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The fundamentals are true but most lenses drop focal length as you focus closer than infinity so if the 50mm lens drops to 35mm at closest focus it would be higher 0.15 + 35mm/35mm = 1.15

That's why some calculators, like mine, ask for additional information like closest focus distance. Closest focus distance allows a better estimate of focal length at closest focus.

Thanks for the link to a nice calculator that you made.  I made a calculator like that, too but I can't seem to get the extra information from the lens manufacturers.  I actually made a test stand for my camera and lenses to measure everything to determine the difference myself.  I did that for my 35mm lens and then added extension tubes, focused, measured the exact magnification, and compared the results to my calculator.  Geez, that took too me just too long.

I wondered about trying to explain that when asked about the magnification but since others likely can't go through all that I thought that the rule of thumb for gain at infinity was good enough.

Tom