Are you talking about looking for errors/varieties? Just inspecting? Photographing? I use between 10-15X while on the go, 10-20X when searching for varieties.
Since you asked about grading, 3x-5x is ideal. Too high magnification can make even the best coins look rough. If for varieties, 10x-16x should be fine for all but the minor. Regardless of what you decide, do your eyes a great favor and buy only QUALITY glass. Eschenbach is excellent, as is Zeiss.
I like the Bausch and Lomb loupes. I actually inherited one from my grandfather who passed away a few years after i was born. It is labelled as an 8 times but they made it custom for him because he was a big inventor in the Rochester area and he was going blind; so for a normal person it's really a 10 times. I really like the amount of magnification that has and the quality of their lenses are excellent.
I use one of the disposable B&L aspheric 5x loupes for grading and general inspection. I also have a 10x Zeiss that only gets used for in-depth inspection.
My favorite glass for grading is a Nikon 50mm f2.8 EL-Nikkor. For variety searching I prefer a Bausch & Lomb 1x Stereo Microscope with 15x eyepieces.
5X for general veiwing, nothing to 5X for grading, 5X for die varieties on older coins, 10X for doubled dies or RPM etc.
No kidding about the quality glass. I use a cheap 16x and it about makes me go cross eyes and gives me nice headache when I use it alot in a short period of time.
Here are some previous threads that discuss proper magnification & coin grading. http://www.cointalk.com/threads/advice-for-a-new-collector.229637/#post-1729901 http://www.cointalk.com/threads/loupe.205215/#post-1427308 http://www.cointalk.com/threads/what-power-magnifying-glass-is-the-best.201280/#post-1380506 This is my first post since the CT Website software was installed. It will be nice to see if I receive notifications via Email for this thread. I have not received any notifications by email since the software upgrade.
i have a H.E. Harris 16x that i like, but i also have a plastic one (not sure of the magnification) but it has a light in it too, which REALLY helps
For general grading I use a Bausch & Lomb eye loupe that attaches to my glasses. This type of loupe comes in 4x and 7x or with double lenses of 4x & 7x or 3x & 5x. I have a 30 year old one that has both a 5x and 10x lens but it's a bit heavy - and no longer made. They are called Classic Metal Eyeglass Loupes or just Eyeglass loupes. Here's a picture from the B&L website
