- Classically, 'macro' means to image an object so that its image is captured at 1:2 (0.5x) or 1:1 (lifesize) on the film or sensor.
- Classically, non-'macro' lenses had a minimum focus distance (MFD) of about 9*FL
- Along the way, lenses started having 'close focus' capability, and lens manufacturers started to advertise their lens as 'macro' although the lens might not be able to achieve 1:2 with without a supplemental extension ring.
- In modern times the term 'macro' degraded so that now it tends to mean 'to shoot at a closer distance than is normally allowed with a non-macro lens of same FL'...if the 50mm lens focuses to 18", most folks mean 'allow me to focus closer than 18" somehow', and the actual magnification of the object does not matter at all!
- True macro lenses are corrected optically so they perform well even focused at very close distances (even at 5 * FL distances, for example) whereas non-macro lenses might work reasonable they are not optimized for very close subject distance
- The true macro lens is also optimized to have a flat field of focus, so that reproduction of postage stamps can be achieved with everything 'in focus', which might not happen with a lens that has a 'curved field'.
- A longer FL allows you to achieve the same reproduction magnification from a longer distance away from your subject. That can greatly help when shooting skittish bugs, since being to close might scare them away. A 100mm lens gives you about twice the 'working distance' from the bug as you would have with a 50mm lens.
As someone getting their feet wet, you don't truly need to buy a 'macro lens'...simply using an extension tube with a non-macro lens might well satisfy what you wish to achieve, if you do not plan on optimally portraying 10mm x 10mm 2D flat objects at 1:1, you do not need a flat field.
Generally speaking, you need a 100mm extension tube with 100mm lens (focused at infinity) to achieve 1:1, while you need only 50mm extension tube with 50mm lens to also achieve 1:1, so there are other considerations to FL choice in addition to simply 'working distance'. You cannot find 100mm extension tube...you would need to 'stack' tubes (which may not be possible with AF lenses that need electrical contacts in the extension tubes).
Most AF 'macro' lenses actually change FL, so a Canon 100mm macro is actually about 75mm when it is set to achieve 1:1.
Certain techniques USED TO work fine, like the use of a 'reversed' lens, because the lens f/stop could easily be controlled in size, and easily stopped down from focus aperture to shooting aperture. The modern AF lens does this very very clumsily because the lens itself has NO CONTROLS, and the reversed AF lens cannot at all be controlled by the body while it is reversed.
A few additions if I may:
100 mm extension to get to 1:1 with a 100 mm macro lens is true, but that imples startign out from infinity. As many lenses do focus closer than infinity, you generally need less in added extension tubes.
As to Macro lenses that change FL when focusing: that is indeed true for IF lenses, and the 100 mm Canon macros are fairly extreme examples of this effect. The 100L becomes a 72 mm lens at 1:1, the 60 mm EF-S a 50 mm, the EF 180L Macro approximately a 120 to 125 mm or thereabouts.
In effect, they use the same principles as many zoom lenses .
Kind regards, Wim