Correct gamma when using Image Adjustments Exposure?

This is a question on the gamma adjustment available in Image > Adjustments > Exposure on Photoshop CS2 for Windows.
The Gamma parameter in Image > Adjustments > Exposure seems to calculate something other than a simple gamma function (i.e. new_pixel = old_pixel ^ gamma) for the shadows.
The online help says:
"Gamma Adjusts the image gamma, using a simple power function. Negative values are mirrored around zero (that is, they remain negative but still get adjusted as if they were positive)."
Now if I create a simple 8bit greyscale gradient, then apply that function with a gamma value of 1.5 and then plot the result, I get some funny curve in the darks. If I do the same with the grey (middle) slider in the Levels dialog, I get a nice exponential function as expected after a gamma correction.
Can anyone shed some light on what is happening in Image > Adjustments > Exposure > Gamma?
Thanks a lot in advance
Martin

I cannot repro that. Here's what I do:
1. Start a new document, 256x256 pixels, sRGB color space
2. Add two vertical guides to the left and the right border, so you can snap to it
3. Make a black-to-white gradient with dithering on from left to right, snapping to the guides
4. Duplicate the background layer twice, producing layer 2 and 3
5. With layer 2 active, open Image > Adjustments > Exposure and set Gamma=1.5, click OK
6. With layer 3 active, hit Ctrl+L for Levels and set the middle (grey) input levels value to 1/1.5 = 0.6667 and hit OK
7. Now turn the visibility of layer 3 on and off --> you will see a difference in the blacks, in pixel columns 60 and below; you could also use the color sampler tool to verify the pixel values are different. For example, the pixel at x=40, y=165 has RGB=1/1/1 with the exposure tool, and RGB=4/4/4 with the Levels tool.
Note: At least on my system, the difference is MUCH more visible when I use the color profile generated by an Xrite eyeone that calibrates my system to D65, gamma 2.2, 120 cd/m^2. Without the profile, the difference is barely visible.
Martin

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