Incorrect Processing Back | Up | Next

The most common mistake made during image processing is clipping. Clipping means that data is thrown away. This data is cut, or "clipped" off, either end of the brightness range.

Black point clipping means that faint detail on the black end of the brightness range is lost. White point clipping means that highlight detail is lost.

Clipping is most often caused by incorrect adjustments of the contrast of the image. Black point clipping is usually caused when the sky background of the image is incorrectly adjusted so that it is black. White point clipping usually occurs when attempting to increase the visibility of faint detail.

Correctly Adjusted    |    Black Point Clipped    |    White Point Clipped
Hold your mouse cursor over the text descriptions below the image to see how clipping affects the highlight and shadow detail in the image.

In the mouse-over examples above, we can see the effects of black point clipping and white point clipping.

Note how faint detail in the outer spiral arms of galaxy M106 is lost when the sky background is set to black. Also note how highlight detail in the core of the galaxy is lost when the contrast is increased.

An examination of the histogram shows clipping. In the image with the black sky background, the mountain of the histogram, which represents mostly the sky background brightness, is pushed all the way up against the left-hand side of the wall of the histogram box.

Compare this histogram to the correctly adjusted image. In the correctly adjusted image, the mountain of the histogram has some space between the left wall and where the mountain starts to rise up.

In the white-point clipped image, notice how there is a thin spike of pixels crawling up the right wall of the histogram box. This is the clipped detail in the core of the galaxy.

Once shadow or highlight detail is clipped, this data cannot ever be recovered.

  • Solution:
    • Careful monitoring of the histogram during image processing can help prevent clipping. Do not use gross contrast and brightness adjustments. Use controls such as levels and curves which give more control over which part of the brightness range in the image is adjusted. Curves in particular can be very effective because the image can be adjusted without clipping the black or white points.

    • Use of a highlight mask can be helpful in preventing loss of detail in the highlight areas of the image when the visibility of the faint shadow detail is enhanced by various methods such as increasing the contrast. Highlight masking is an advanced processing technique that is beyond the scope of this book, but that I cover in detail in my book on advanced astrophotography and image processing A Guide to Astrophotography with Digital SLR Cameras.

Problem: Incorrect Processing - The Bottom Line

Incorrect image processing, usually involving increasing the contrast of an image, or adjusting the sky background to be black, can cause loss of data through clipping.

Careful attention to highlight and shadow detail and the histogram during processing can prevent this loss.

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