Checking exposure with Photoshop's Blur Average?

Jul 23, 2005
Sometimes, when I've been working on a specific image (e.g. high key) and move on to another image, I just don't see it anymore. The image that you've been working on is sort of burned into your mind and affects your next edits. In order to get away from it and to have a more absolute reference, I like to see some sort of average exposure.

Some people use a good image that prints well as a reference point, but I was thinking about a more numerical approach to evaluate exposure of images in Photoshop in order to support perceptual desicions.

We've all heard about the 18% reflectance concept, that our cameras are calibrated to set exposure in such a way that a scene renders 18% reflectance. This 18% is a linear value, not a perceptual one.
In a perceptual sense this 18% is somewhere around middle gray, or to be more precise, 0.18^0.4545=46% (0.4545 = function of lightness sensation, the inverse of a gamma of 2.2).When you take a look at the Lab (perceptual model) luminance value of 50%, this 50% translates into aRGB 118 of (which is very close to 46% of 256 = 117.4).

I know that this 18% reflectance is being disputed by people like Thom Hogan et al, who argue that cameras are calibrated to ANSI gray (based on luminance instead of reflection), which transalates into a reflection of something like 12-14%. He seems to have a point as Kodak states that its 18% gray card should be overexposed with 0.5 stop (to get 18% reflectance). Anyway, 12-14% tranlsates to a perceptual value of 38-41%

Well now, my point is that when you run the Filter > BLur > Average, Photoshop calculates an average color and you can then use the Lab luminance value of this color to check exposure.
For example make a gradient from red (255,0,0) to white (255,255,255) and run the Average filter. Sample the average color and it'll read out: 255,127,127. An average color.
Now take a photograph, run the Blur Average filter, sample the average color and read out the Lab luminance value. If the Lab luminance value is around 50% the exposure is based upon approximately 18% reflectance (the average luminance of the image is 50%).

Of course this is not a general rule (think e.g. of beach/snow scenes).

Any thoughts?

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