These are an option on both DSLRs and in electronic viewfinders. You can use the grid to make sure horizons are level and buildings are vertical – some grids confirm to the ‘rule of thirds’ to help you get a satisfying composition.
The percentage of the scene shown by the viewfinder. In better DSLRs you see 100% of the scene that will be captured, but in cheaper models it might only be 95-97%. That small difference can lead to objects showing at the edge of the frame that you hadn’t realised were there.
The viewfinder in a digital SLR is optical because it’s created by an image formed by the lens on a glass ‘focussing screen’. The direct vision viewfinders on some compact cameras are optical because you’re seeing the world through a set of lenses and not via a digital display.
OLED stands for ‘organic light emitting diode’. It’s a more advanced display tech than regular LCDs with wider viewing angles, faster response, better brightness and reduced power consumption. The OLED electronic viewfinder is a selling point in the Fujifilm X-T1, for example.
A tiny blind in the viewfinder eyepiece that stops light entering and upsetting the exposure (normally the eyepiece is covered by your eye). It can be useful for long exposures or other shots where you’ve stepped away from the camera. Some cameras come with a small viewfinder cap fixed to the shoulder strap.
Essentially, this is a tiny LCD display seen through a magnifying eyepiece. They’re used on some bridge cameras and high-end compact cameras, and on many mirrorless cameras. They replace the optical viewing system you get with a DSLR.
A small knob or lever next to the viewfinder which you use to adjust the focus of the eyepiece to match your own vision. The information in the viewfinder should appear sharp without you having to strain to bring it into focus.