Hi Adrian
Some useful concepts:
- A “true” macro lens is one that provides a 1:1 reproduction ratio. That is, when a 10 mm long insect is photographed at the closest focus distance of the lens, the image on the sensor will also be 10 mm long.
- Closest focus distance (CFD) is generally measured from the plane of the sensor to the insect when positioned at closest focus.
- Minimum working distance (MWD) generally means from the front surface of the (front) lens element to the insect when positioned at closest focus.
As a general rule, the longer the focal length, the greater the working distance, this can be useful for skittish insects or insects hiding in a shrub.
With my Canon 60mm macro lens I had a CFD of approx. 20cms whereas with my Canon 100mm macro lens it is approx 31cms which means that I don’t have to poke the lens barrel into the shrub. The 60mm is much lighter than the 100mm.
The Canons also have internal focusing. That is, the lens remains at a fixed length throughout the focus range from close up to infinity. Other brand macro lenses may extend at the macro end so you will need to decide if this is important.
Cheers
Dennis