Photoshop - Curves works the other way around on Mac

Hi,

I teach Photoshop. One of my students has the following problem:

when he opens the adjustment layer Curves, the curves works the other way around.

When he drags the curve down, the photo gets lighter, when he pulls the curve up, the photo gets darker.

Also the histogram shows the highlights on the left and the shadows on the right. Input en output have changed places.

When he opens another adjustment layer like Levels, the problem does not occur, nor on other adjustment layers. The problem only occurs with the adjustment layer Curves.

i'm unable to help him, I tryed and searched online but I can't find a solution.

kind regards,

He works on a MAC.

Ann

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