Re: [APML] Curves

[Jerry Lodriguss <jml@astropix.com>]

>=It is the auto levels that bloat the stars. The article 
>(http://starmatt.com/articles/curves.html) shows exactly why that happens.


Hi Matt,

It's not Auto Levels that bloats the stars. It is the auto clipping (PS 
calls it Auto Range) setting for auto levels that clips highlight and 
shadow detail.

This is because auto clipping is set to a default of 0.50% in 
Photoshop.  This means that 0.5 percent of the brightest pixels are ignored 
when the white point is automatically set.

This is easily changed by opening the levels or curves dialog and holding 
down the alt key, which will change the Auto button into an options button. 
Click on the options button and change the Auto Range black clip to 0.00 
and the white clip to 0.00.

The Auto button in the Curves dialog does exactly the same thing as the 
Auto button in the Levels dialog, it sets the lightest and darkest pixels 
in each individual color channel to the pixel values that are specified as 
black and white.

You specify the pixel values that the lightest and darkest pixels will be 
mapped to when auto levels or auto curves is used by double clicking on the 
black or white point eyedroppers and adjusting the numbers.

For example, you could have auto levels map the brightest pixels in the 
image to 250,250,250 by changing those numbers from the defaults of 
255,255,255 to 250,250,250 under the white eyedropper, and by setting the 
auto clipping for the whites to 0.0.

If the Auto Clipping settings are set to 0.00, using Auto Levels or Auto 
Curves will be exactly the same as setting the black and white points by 
hand and dragging the black point to where the pixels start at the left of 
the histogram in each individual color channel, and likewise for the white 
points.

Auto Contrast (Image > Adjust > Auto Contrast) however, automatically sets 
the black and white points based on the auto-clipping settings in the 
combined RGB color channel which is useful for preserving a color cast 
while increasing contrast if the full dynamic range is underutilized.

Photoshop 7 lets you make these changes to the auto clipping settings and 
watch the effect on the image in real time.

There are two different parameters here that we need to think about: Auto 
Clipping (Auto Range), and the target pixel values.  Normally the white 
point target pixel values are left to 255,255,255, but the black point 
target pixel values can be set to 10,10,10 or even 20,20,20 to leave room 
for shadow detail as you mention in your Curves article.

Auto Clipping is the most critical parameter when using Auto Levels or Auto 
Contrast, and this applies to scanner software as well as Photoshop.

Jerry



Astronomical photography: http://www.astropix.com




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