Errors under 3 Delta E (dE) are generally invisible to the naked eye.For more, see How to Buy a PC Monitor, plus our Best Gaming Monitors list. For general productivity or entertainment, look for high contrast for high image quality. You should have a minimum of a GTX 1070 Ti or RX Vega 64 for medium settings at 4K or, for high or better settings, an RTX-series or Radeon VII. For gaming, higher refresh rates and Adaptive-Sync (AMD FreeSync or Nvidia G-Sync) are priorities, alongside a beefy graphics card. They're common in this price range and generally cheaper. If you don't need to share the screen, consider 28-inch and 27-inch screens. What size do you need? For a budget monitor, 32 inches is a nice sweet spot, providing ample screen space while still fitting atop a desk.
