Binoculars

Binoculars are described by two numbers — 10x42 means ten times magnification and a 42mm objective. Higher magnification narrows the field of view and amplifies hand shake, which is why 8x and 10x dominate and anything above 12x really wants a tripod.

The objective diameter decides low-light performance, and low light is when you actually need binoculars. A 42mm objective is the general-purpose standard; 50mm gathers more light at more weight around your neck all day. Beyond the numbers, glass quality and coatings are what separate a pair you glass with for an hour without eye strain from one you put down after ten minutes.