I only shoot 35mm film, and for standard viewing of my self-processed negatives I use a sharp, vignetting-free Schneider 4x loupe with my lightbox. However, I find it essential to also use my Schneider 10x loupe to truly determine how sharp my shots are before I attempt to print them. I say this because some times I've printed a negative after viewing it with my 4x only and have seen on the print that it is not sharp afterall. Thus, my 10x confirms via previewing that that which I choose to print is worth printing in terms of acceptable sharpness. I recommend that you bring a negative or a slide to your chosen retailer and try the various loupes they have in stock, because quality varies from manufacturer to manufacturer.