The monitor itself applies a gamma correction of 2.2 (let's assume the monitor uses the sRGB color space). This is called the display gamma. So if a program sends [128 128 128] the monitor doesn't display 50% but 22% brightness. Images are usually encoded in sRGB so they can be sent to the monitor unmodified, to encode an image that way from linear image data the encoder must apply the inverse gamma correction of .45455.
Photoshop (and other image editors) usually ignore all of this and don't do any gamma corrections.
I made another post about this a while ago. When editing pictures you don't see much artifacts because of this but sometimes it will cause problems (e.g. if you blur a small highlight the result will be too dark).
Octane always works in linear space. If a renderer uses anything else the rendered images will look completely wrong.
Any images coming in have to be converted back to linear space. Octane asks for the gamma correction applied to the image data after loading it (the inverse of the encoder gamma, usually equal to the display gamma, 2.2. Some programs ask the encoder gamma instead, which is .45455).
And before writing out a PNG image or displaying an image it will apply a gamma correction of .45455 to compensate for the display gamma. In the tone mapping settings if you disable the film response curve you tell Octane the display gamma of your monitor, 2.2.
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Roeland