I have the MF version of the 100mm f/3.5 (same optical layoput as Pentax AF, cosina AF, etc.) and while the focal length is potentially nice as a macro that also can do potrait/telephoto, I dislike the images. They just seem very “flat.”
Anyway from the things you mention—a longer FL will put you too far away, and will give a perspective that will likely not be natural. I suggest a 50mm, or if you are using a cropped sensor, possibly even the 35mm macro (except it does not have aperture ring, so it is too limiting IMO).
Anyway check out how you will set up for slides, as with cropped sensor you likely will want 0.67 magnification, which you can do w/ an older macro with 0.5 max magnifiaction plus extension, or if it goes to 1.00 magnification w/ K-1, again do-able with older macro lens with 0.5 and extension. Actually, while I have not used a newer macro allowing 1:1 w/o adding extension, they may be less convenient for other than macro, as the rotation may be too small for far distances, and you are paying a lot more when adding an extension tube is inexpensive, and AF is of little use at higher magnification.
On the other hand for insects/portraits a 90~100mm FL is nice. Take care w/ the Adaptall-2 tamron 90mm (believe the version before the 72B) as it likely will give central flare spot with digital. The point being a macro lens likely should give you other uses, or one tends not to carry it.
P.S., If you have a reasonably slow 50mm lens (e.g., 50mm f/1.7 or f/1.8/2.0)**, either extension tubes (as Clackers says) or a Vivitar macro 2x TC [but again this 100 mm FL may be too large] will get you fine results, and then you can get a real macro when you know more. This should be about $50.
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** slower means more symmetrical optics which are best for macro
Last edited by dms; 08-20-2019 at 10:43 PM.