DOF will be the same at the same magnification whatever the focal length of the lens - the working distance for 1:1 magnification will obviously be greater for the longer focal length lenses. The one drawback of the tamron 180 is its minimum apeture is f32, whereas the sigma 105 is f45, so when working at 1:1 there is a bit less wriggle room for focussing. (Mind you, differential focussing is generally something I find quite useful in macro composition)
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