Burned out highlights

[Marpel]

Trying to understand what is happening here.

I am processing a few images which are macro shots of water drops on flowers. On two so far, due to not paying real close attention during the photography, a bright spot in the drop (light from a bright window) shows as blown out. Because I usually try to get the exposure of the image as close to the right end of the histogram, without over exposure, I regularly keep the "Show Clipped Highlights" button depressed and in both images the red warning is visible.

The following occurs with both images, however, the numbers are not the same, but providing them for one of the images to explain my confusion.

Image without any processing shows the DR in Levels and Color as: 30.2 - 86.7.

Disregarding the clipped highlight (as it was quite a small area), I decided to move the Levels and Color to: 35.7 - 90.6. I wanted to brighten the exposure a bit while keeping some headroom at the highlight end for the next step.

I then ran the resultant image through Composite > Soft Light.

Strangely (to me at least), the "Show Clipped Highlights" no longer showed red. The Dr was now 29.0 - 97.3.

My question is, why would the image initially show the Highlight warning and, even though in the next two steps the exposure (at the highlight end) is increased, the final image shows no clipped highlights? Seems counterintuitive.

Marv

[jsachs]

If you want to precisely control dynamic range, I recommend using curves instead of the dynamic range sliders in Levels and Color.

As I tried to explain before, dynamic range is computed by ignoring a small percentage of the lightest and darkest pixels. In this case, presumably the specular reflections. When you reduce the dynamic range, any pixels brighter than the upper limit are set to the upper limit, so this reduced the highlights so they were no long clipped.

If you were using curves and histograms instead (e.g. by using Adjust instead of Levels and Color), you would have been able to see a bump in the histogram corresponding to the reflections and then decide what level you want to assign to them as well as to the rest of the image.

Histogram.jpg (28.39 KiB) Viewed 2114 times

The histogram and curve above illustrate the situation. The small spike at the right end of the histogram represents the specular highlights in the image. This does not include enough pixels for the Levels and Color dynamic range to detect the specular highlights as the top end of the dynamic range. The detected dynamic range is represented by the innermost two of the four black markers at the bottom of the histogram. When you adjust the dynamic range, you are applying a curve to the image that looks like the red curve. The two markers on the y-axis represent the new dynamic range. Since the upper limit is less than pure white, all the pixels in the image, including the specular highlights that were ignored, are limited in brightness to the upper limit. Using curves, it is easy to adjust the central part of the histogram while leaving the specular highlights alone, but the dynamic range sliders in Levels and Color do not give you enough control over the tone curve to accomplish this.

Histogram1.jpg (24.63 KiB) Viewed 2114 times

Using the histogram display as show above, you can keep the specular highlights pure white while you stretch the rest of the image to increase its dynamic range.

Levels and Color is intended as a simple transformation with a few simple sliders that give you crude control over the image. If you want precise control, you need to use Adjust instead which is much more powerful, but of necessity, more complicated.

[Marpel]

I think I have got it now. Thanks for the explanation.

Marv