Why do I get a black screen when I enable shaders with Optimum Realism
A black screen the instant a shader loads is almost always a GPU driver or an Iris/OptiFine ↔ shader version mismatch — not the pack. Update your graphics driver and match the shader to your loader version and it renders.
Update your graphics driver
A black or frozen screen the moment a shader loads is the classic out-of-date GPU driver symptom. Grab the latest from your vendor and reboot before testing again.
Match the shader to your loader version
Iris and the shader both target a specific Minecraft version. An Iris build for one version running a shader exported for another loads to a black screen. Re-download the shader build that matches your Minecraft + Iris/OptiFine version.
Remove conflicting rendering mods
Some rendering mods (Optifabric, old Canvas, "shader" forks) collide with Iris. Keep only Iris, Sodium and Indium, confirm it loads, then add other mods back one at a time to find the culprit.
Force the dedicated GPU
On a dual-GPU laptop, Minecraft sometimes launches on the integrated chip, which black-screens heavier shaders. In the NVIDIA/AMD control panel, set Java (javaw.exe) to use the High-performance GPU.
Drop to a lighter shader to confirm
Load Complementary Reimagined or BSL. If those work and a cinematic shader does not, your GPU is hitting a memory or feature limit on the heavier shader — lower its shadow resolution and render distance, or stay on the lighter one.
Did this help?