You have your choice of a bellows attachment, extention tubes or a close up filter as far as getting the close up distance needed for macro shots. The close up filters will offer alot of variation in image quality depending on the quality of the filter chosen. Bellows attachments can get pretty pricy. Extension tubes offer good IQ, a fairly reasonable price, and are simple to use. Be aware though depending on the tubes chosen you may be limited to manual focusing. The length of the extension tube, or tube combination is what changes the magnification ratio thus increasing or decreasing subject size, I cant find it offhand but there is a formula for determining magnification ratio tube length gives with specific lens focal lengths. I'm not sure about the combination of tubes and your 24-70 so someone else will have to chime in on that one. Additional flash may be needed as usually on camera flash doesnt work well with a very close subject to lens distance, a solid tripod is a must and alot of patience.
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