7. What is a close-up filter ?
It is an add-on lenses, works more like a magnifying glass which you can screw it onto the end of your lens on the filter threads. Such filter allows you to move much closer to a subject than you normally can, permitting macro photography at the cost of some image sharpness. Cheap closeup filters have only one glass element and thus are very vulnerable to chromatic aberration. Better closeup filters have two elements.
The focusing distance is mainly depend on the diopter value of the close-up filter and not the focal length of the lens. See below focusing distance calculation:
+2 close-up filter will give about 1000/2 = 0.5m
+4 close-up filter will give about 1000/4 = 0.25m
+5 close-up filter will give about 1000/5 = 0.2m
+10 close-up filter will give about 1000/10 = 0.1m
- 100mm with +2 close-up filter
- 200mm with +2 close-up filter
- 300mm with +2 close-up filter
- 400mm with +2 close-up filter
All above setups will have same or similar focusing distances but the magnification increases when focal length increases.