Fabian Wagner -- who was also the cinematographer on lauded 'Thrones' episodes like "Hardhome" and "Battle of the Bastards" -- tells TMZ, he's aware folks are upset that "The Long Night" looked TOO nightly, and he has an explanation.

First, Fabian believes the pixelation and muddy dark colors fans saw on their TVs and mobile devices are due to HBO's compression of the episode ... which leads to poorer visual quality. This is made worse if it's being viewed on a streaming service with a weak connection ... or in a room that's too bright.

As Wagner puts it ... "[GoT] has always been very dark and a very cinematic show" and should be watched in a dark environment. Ideally, this means viewing it like you would a movie -- in a dark theater.

Since that's not realistic for 'Thrones,' Fabian suggests making your viewing room at home as dark as possible, avoid watching on your phone or in places that are lit up ... and adjust your TV settings.

The cinematographer also points out ... the showrunners and director wanted the episode to be dark. Wagner says the battle scenes were intended to be intense, claustrophobic and disorienting -- like they would be in real life -- but not confusing.

Fabian says, "We tried to give the viewers and fans a cool episode to watch," and he believes they did, adding ... "I know it wasn't too dark because I shot it."