Hi Alan
My favourite combo for macro at near 1:1 is the Zeiss 50mm f1.4 with a 2x Macro converter (not the common 2x converter). Teleplus and Vivitar make identical 7 element ones, which allow you to go from infinity to 1:1. The effect is the same as using a 100mm f2.8 Macro, so is similar to the Tamron, Yashica etc.
I have owned the Zeiss 60mm 1:1 macro, Yashica 100mm Macro and two of the Tamron 90mm Macro models and sold them all. In terms of cost/quality/ease of use, I kept going back to the Zeiss 50mm f1.4 + 2x Macro converter, even after (because of) careful bench tests.
Unless you are very particular about the setting up of the shot, on tripod, ideal lighting and critical about the settings needed to cover depth of field (hardly likely with a baby as the subject!), you are not going to see the slight benefit of a prime Zeiss Macro lens. You won’t be disappointed in the combo I mentioned.
I have recommended this to quite a few entomologists and those that took it up have given me excellent feedback. In one magazine test of 7 element ordinary 2x converters, the Teleplus (Vivitar is identical) was include and came out top, even when used as an ordinary converter, not as a macro converter! But the Mutar was not included in the test! I have shots available...but of course they loose impact when compressed for emailing!
Full frame 35mm image scanned at 2700dpi, resized to 500 pixels wide
Extract from above at full screen image size
Even at new prices, the 2x macro converter will save you good money over a second-hand Tamron, and if you find a converter second-hand it will be unbeatable in terms of value vs. quality. Note that it can only be used in AE mode even if your 50mm is MM. I don’t advise the matchstick, plastic insert work around for fooling cameras into MM modes!
If you do not need to go to near 1:1 then the other solutions mentioned will be fine.
Cheers, Bob.