Why in color space transformation, Gamma 2.2 looks darker than Gamma 2.4

This phenomenon is common and often counter-intuitive. In short, Gamma 2.2 output looks darker because the software is lowering the signal brightness to compensate for a monitor that it expects to be very bright.

The "Inverse" Rule of Color Management

To understand why the image gets darker, you have to look at what the software is trying to do for the display.

• Gamma 2.2 Display: This display curve is naturally bright (the curve rises quickly). To make an image look "correct" (neutral) on this bright screen, the software must darken the video signal slightly.
• Gamma 2.4 Display: This display curve is naturally dark/contrasty (the curve rises slowly). To make the image look "correct" on this dark screen, the software must brighten (lift) the video signal.

The Result: If you simply toggle your Output Color Space between Gamma 2.2 and Gamma 2.4 without changing your physical monitor's setting, the Gamma 2.2 signal will look darker because it contains lower pixel values (less lift) than the Gamma 2.4 signal.

Recommendations

• For Web/YouTube: Set Output to Rec.709 Gamma 2.4 (standard) or Rec.709-A (Mac compatibility). Avoid tagging Gamma 2.2 unless you are certain your viewers are on strict sRGB displays, as it often leads to inconsistent "dark" results on phones/TVs.
• For Grading: Always set your Output Color Space to match your actual physical monitor's calibration (usually Gamma 2.4 for reference monitors).