Magnification question - EF-S 18-135 STM lens with Canon 25mm Extension Tube II

[Noonievut]

If the minimum focus distance with this lense is 15.4 inches and I were to add the 25mm extension tube, does that mean that I can go closer to the subject, or is the 15.4 inches still minimum but the extension tube simply makes the subject look larger?  And, how much larger?

I've read some guidelines but I'm still not getting it

I'm looking to use one lens for nature photography and would love to add a lightweight macro option (using a T5 camera).

Thanks

[Skirball]

Yes, extension tubes reduce the minimum focus distance of the lens.  I can't tell you exactly how much, as it's a complicated forumula for a zoom lens based on the focal length, magnification factor, and MFD.  You can probably find some calculators online. 

[jrhoffman75]

http://www.cambridgeincolour.com/tutorials/macro-extension-tubes-closeup.htm

[Skirball]

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@jrhoffman75 wrote:

http://www.cambridgeincolour.com/tutorials/macro-extension-tubes-closeup.htm

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I saw that calculator earlier, but I didn't understand how they could show the new MFD (or in this case, CFD) without knowing the previous, based on which lens you use.  Is the CFD the same for all (non) lenses?  I guess I always assumed that the MFD for any specific lens depended on the internal elements of the lens.