Desired, not required but great to have handy if you can afford them.
MIIRA fabulous tools for measuring angles and proportions. Note they also have an app that is great!
They also have an inexpensive app for your phone that can be used to measure angles on a picture of your subject.
The View Catcher is a mid gray 3.5×3.5" ABS plastic tool with a small circular opening that helps you to see "True" color. It opens vertically or horizontally to help you find the right composition. It allows you to view your subjects without the distraction of surrounding elements. With it you'll be able to create stronger compositions, choose accurate color values and provide a clear vision of warm and cool color choices. Keep one in your painting gear. The View Catcher is simple to use, easy to learn and pocket size.
Glasses with red lenses will help beginners to see value regardless of hue or chroma.
Preferred Brand: Any
Picture Perfect Viewfinder (3 in 1) - helping artists create better paintings with less guesswork