Each exchangeable lens for a camera has their closest focal distance.
The distance for almost all of them is 25cm – 50cm minimum. Moreover when it's possible to approach a subject, it's possible to do close-up photography.
That's because the size of the subject projected onto an image sensor has become larger.
The shortest focal distance can be shortened further by installing a Close-Up filter to the lens.
If you are able to approach subject close enough, you can photograph it.
The point to choose appropriate magnification of the close-up lens is the closest focus distance of the lens.
When the closest focus distance of the lens is more than 1m, you will use close-up lens +1 and +2.
When it is 30cm - 50cm, you will use close-up lens +3 and +4.
As for the combination of a macro-lens (originally the closest focus distance is short) and close-up lens, the effect is little.
When installing the +10 close-up filter, the closest focus distance will be 10cm, so please be careful about this working distance.
Calculation of the closest focus distance when close-up filters are installed:
There are types from +1 multiplication to +10 multiplication, and the closest focal distance of each lens with the close-up filter attached in the ∞ (infinity) setting can be calculated from the formula below.
+1 lens: the closest focus distance in the ∞ (infinity) ≒ 1m
+2 lens: the closest focus distance in the ∞ (infinity) ≒ 50cm
+3 lens: the closest focus distance in the ∞ (infinity) ≒ 33cm
+4 lens: the closest focus distance in the ∞ (infinity) ≒ 25cm
+10 lens: the closest focus distance in the ∞ (infinity) ≒ 10cm
This calculation is an approximation. May vary depending on lens.
The closest focal distance is the distance to the subject from the sensor position. It isn't the distance from the top of the lens.
(＊1) Be careful about the full length of the original lens.