Exposure With The Histogram

[Dennis G]

K3CCD TOOLS AND WXASTRO CAPTURE PROVIDES A HISTOGRAM AID FOR EXPOSURE, BUT HOW DOES ONE INTERPRET THIS FEATURE?
Attached processed capture, which I consider to be reasonably exposed without a histogram and the second attached is a Photoshop histogram of this capture, which doesn't tell me a great lot other than the right hand side represents the high lights and the left the darks. One reads of clipping. What exactly does this represent on the histogram. Most captures of the waxing moon's terminator, will be light on the left grading to black on the right. What does one aim for using these diagrams to set the exposure? I apologize for the poor presentation of the graph but photographing it was the only way I could attach it.

[Ford Prefect (James)]

Dennis,

A histogram is a standard digicam tool that tells you about the distribution of light and dark in your image. As you go from left to right across the histogram, it is telling you the number of pixels at each level of brightness, from black to white.

Looking at the histo you've supplied, it tells you that most of the pixels in the image range from pure black to medium brightness (ie mid gray on a grayscale image), then tapering off quickly as you go to the right. What this tells you is that there are lot of dark and low-medium brightness pixels in your image, and virtually no very bright pixels.

The ideal photographic exposure in normal (eg portrait, landscape) is generally thought of as having a peak in the middle of the histogram, and tapering off to either side - i.e. balanced. If the histogram is skewed to the left (dark), the photo will be thought of as dark and underexposed. Skewed to the right is what you'd think of as overexposed, burnt out, where the brightest part of the picture has turned white.

So, your photo would traditionally be considered a bit underexposed. However, that's not really surprising since you're imaging the night sky! Some of the astrophotographers can tell you what is an "ideal" histogram for astro purposes.

[Dennis G]

Setting the exposure to capture images with interesting detail along the moon’s terminator such as the attached requires – in my humble opinion - a compromise between over exposed crater rims that catch the sunlight and the totally black shadows within the lit areas. The histogram for the attached capture shows a series of peaks and valleys, which I assume one would expect, seeing that it is far from a gradual change between the light and dark areas. If the exposure slider is moved to lighten the image all the highlights will become over exposed with the loss of detail. This begs the question: would this picture be greatly improved without loss of detail if the exposure had been set using a histogram?

[Matty P (Matt)]

Originally Posted by Dennis G

Setting the exposure to capture images with interesting detail along the moon’s terminator such as the attached requires – in my humble opinion - a compromise between over exposed crater rims that catch the sunlight and the totally black shadows within the lit areas. The histogram for the attached capture shows a series of peaks and valleys, which I assume one would expect, seeing that it is far from a gradual change between the light and dark areas. If the exposure slider is moved to lighten the image all the highlights will become over exposed with the loss of detail. This begs the question: would this picture be greatly improved without loss of detail if the exposure had been set using a histogram?

Hi Dennis,

From looking at your image. It looks like you have correctly exposed the histogram. You should use the histogram to make sure you aren't over or under exposing the image.

Keep up the great work Dennis. Great moon shot.