The classic Watchmaker's Loupe Nr. 3*

*Everybody has different Preferences
For their favorite magnification. I prefer rather large magnification and thus will accept a smaller aperture. But everybody has a different preference. Obviously the range of loupes commonly used is from Number 3 to 8. I sometimes even use a Number 1 loupe, e.g. when oiling cap jewels.

Number of Loupe Magnification
1 10 x
1.5 6.6 x
2 5 x
2.5 4 x
3 3.3 x
4 2.5 x
5 2 x
6 1.6 x
7 1.4 x
8 1.25 x

Today I use a whole variety of loupes with headwire and I also have a 20 x and 40 x stereoscope in use. That stereoscope makes working a lot easier. Oiling cap jewels becomes really easy because of the three dimensional view of the magnified parts. Of course an experienced watchmaker will have no difficulties doing this with the simple loupe in the picture.
I have seen that stereo magnification glasses seem to gain in popularity as well. Here is a link to one for sale in the "Poor Man's" Watch Corner.