Extension tubes and working distance

[PatrickO]

I want to get a bit more magnification with a Tamron 90mm macro lens.

If I use a 36mm tube, I believe it gives me about 1.4X.

My question is: what difference does it make to working distance?

TIA

 

[Snapsh0t]

The obvious trite answer is that it will be smaller. Just how much, I don't know as I've never worried about it but it should be in proportion to the increase in magnification..

 

[sk66]

It increases the focus distance by ~ 1/3 the length of the extension tube, and it increases magnification equal to extension ÷ FL.
The Tamron 90mm Macro is 1:1 at 300mm MFD. With a 36mm extension tube the MFD is now 312mm and the magnification is increased 36÷90 = 0.4. However focus/minimum focus distance is subject to sensor and you asked about working distance (between objective and subject)... that is reduced by the other 2/3 of the extension.

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