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While knowing that 8x42 means 8x magnification with 42mm objective lenses is a good starting point for choosing binoculars, there are a number of other terms it helps to understand.
- **Objective Lens**: This is the big lens, at the opposite end from from where you look. The size of the objective lens determine how much light reaches your eye. In general, all other things being equal, a larger objective lens gives you a brighter image.
- **Field of View**: This refers to how much you can see. The wider the field of view the more you can see. The standard measurement is how many feet wide the viewing area is at 1000 yards. This can then be translated to degrees. You'll see field of view (also often just, FOV) listed in both degrees and feet, which can make it hard to compare. It's also abstract and can be difficult to wrap your head around, which is why I like the [FOV visualization tool](https://www.allaboutbirds.org/news/choosing-binoculars-field-of-view-and-close-focus/) on Cornell's All About Birds website.
- **Eye Relief**: Eye relief refers to the ideal distance from your eye to the eyepiece glass. The ideal distance varies, but for most binoculars it'll be in the 10mm to 20mm range. In order to see the full view provided, you eye needs to be at the right eye relief spot. Too close and you'll see black shadows, too far and the FOV shrinks. This is primarily an issue for those of us who wear glasses, as some binoculars make it nearly impossible to get a full field of view when wearing glasses, which is why I try to always note how each model is wearing glasses.
- **Roof Prism**: This refers to a type of binocular design, the alternative being Porro prism (see below). Roof prism design puts the objective lenses directly inline with the eyepieces, giving you the H-shape common in binoculars these days. This design makes for a lighter, smaller, more streamlined binocular. Nikon has a [nice visualization](https://imaging.nikon.com/sport-optics/guide/binoculars/technologies/technologies_06/) showing how the different designs bounce light to your eye.
- **Porro Prism**: Porro prism binoculars use offset objective lenses that are not inline with the eyepiece—think the wider, bulkier binoculars you sometimes see. Porro prisms are often sharper (a birding friend of mine has a pair of 1940s Zeiss Porro prism binoculars that have one the sharpest images I've ever seen), but bulkier. Porro often produce a more 3D image with better depth perception, which is why they're still common in hunting and military applications.
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