Optifine

[ Optifine ]

For those who ask about optifine. The project called "Optifabric" does exist to help bridge the compatibility, however, it is quite invasive due to optifine's design, and is hard to get it working with more than 50 mods. As an alternative, there is a list of great mods that replace and supersede optifine. Below I am putting some excerpts from the optifabric page. https://www.curseforge.com/minecraft/mc-mods/optifabric

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THERE ARE NO SHADERS FOR FABRIC. The closest you will get is Canvas, however it and Sodium can sometimes cause certain mods to render incorrectly, so use at your own risk.

Sodium https://www.curseforge.com/minecraft/mc-mods/sodium - [Client Side]
Sodium is a free and open source fabric mod made by JellySquid that drastically increases the performance of Minecraft. Sodium utilizes modern rendering techniques along with a range of other optimizations. More more information can be found on the download page linked.

Canvas https://www.curseforge.com/minecraft/mc-mods/canvas-renderer - [Client Side]
Canvas focuses the use of shaders to improve the visual appearance of the game along with providing performance improvements. (Incompatible with Sodium)

Ok Zoomer - Ok Zoomer is in the Aof3 modpack a great replacement to Optifine's much loved zoom feature. They provide more options than optifine's zoom functionality.  

LambDynamicLights https://www.curseforge.com/minecraft/mc-mods/lambdynamiclights - [Client Side]
LambDynamicLights adds dynamic lights to Minecraft similar to Optifine's.

Lithium and Phosphor - this is already in the aof3 modpack [Can be Client or Server Side]
Lithium provides a great improvement to server performance while Phosphor targets lighting performance. Both mods can be installed on the client or the server and are also made by Jelly Squid.