Here are my thoughts on macro:
1 - Do you shoot or plan to shoot a LOT. At least enough to cost justify a new lens?
If not, like me, get an older manual focus macro lens for a LOT less $$$.
2 - But, if you want to do focus stacking, if your camera supports it, you NEED to use a modern AF macro lens, where the camera can control the AF of the lens, to do the focus stacking.
3 - Working distance.
This was mentioned above. This is the distance between the front of your lens and the subject.
A longer focal length lens will give you more working distance.
It can be to keep your distance from an insect, or to give you more room to work your lights.
For me it was lights, having 6 inches of working space is easier to arrange lights, than having only 2 inches.
But like any lens, focal length also affect perspective, for a given subject image size.
4 - How much magnification do you want/need?
Different than #3, I don't mean how close you get to the subject, but the size of subject vs. size on the image of the subject on the sensor.
Do you want 1:1 or 3:1 or ?
This depends on the size of the subject.
- My small model is 3 inches long, which is longer than the sensor, so I cannot do a 1:1. The image on the sensor is smaller than the subject. A 4:1 or even 5:1 is probably more realistic.
- A dime is easily a 1:1 shot. The dime will easily fit on the sensor.
- A small bug is the other end, where the image on the sensor is LARGER than the subject. A 1:4 or 4x magnification is possible.
- - This is the world of "micro" photography, or greater than 1:1.
Just because a lens can do 1:1 magnification, does not mean YOU can do it. On my model, a 1:1 magnification would get less than 1/3 of the model. The subject size will limit the magnification.
On the other hand, if the lens is limited to 3:1, you will NEVER get a 1:1 image. The largest the image will be is 1/3 real life.
5 - Lighting
Close up and macro photography requires different approach to lighting, because you don't have the space and room.
When my lens is only 4 inches (or less) from the subject, I do not have the space for my large lights. I have to use SMALL lights, that I can physically position to light the subject.