How do you find that 3 diopter is enough magnification, I checked out some 3 and 5 diopter magnifiers at the store today and when viewing 0603 imperial I really didn't see any benefit to using the magnifier, it just wasn't enough magnification to justify the hassle of having to look through the device. 3 diopter is only 1.75X and 5 diopter is only 2.25X.
Also from looking through the magnifiers at the store the smallest size that I found that was acceptable was 5 inches in diameter, anything less and it's just too small to look through with both eyes.
I think what would be closer to ideal would be a 6 inch or greater diameter lens with at least 8 diopter (3X) magnification.
The glass lens is 6" tall, and 7.25" wide. Definitely, you can look with both eyes.
I've used these even when working with SMD 0805/0603. I usually wear reading glasses 1.0x, or 1.5x but with these lenses, I can get away not wearing my reading glasses to do fine closeup work. Of course, wearing the glasses + the magnifier lens increases magnification. For me that's enough magnification. If I need more than that, I have my camera/TV setup.
As I said, the magnifying lamp is not a microscope. If you want a Mantis, get a Mantis stereo scope. For high magnification inspection of SMD soldering (checking for shorts between SOIC pins), I use my TV + camera. The problem with the TV screen/USB scope and the like is you don't have any depth perception. It's hard to work/solder where you're looking at a TV screen and your hands/soldering iron are elsewhere. It's like a disconnect on where your hands are, and where you're looking at. So probably a stereo microscope is what you really want/need.
My opinion is you need both kinds of tools. A stereo microscope isn't good for general work (tedious, eyestrain, limited field of view), and a magnifying lamp may not have enough power to for very small SMD work.
As someone already mention, you can wear 1.5x, 2x reading glasses along with the magnifying lamp and it is a comfortable setup, you can work for hours looking into the glass with it.