Inadequate Exposure Back | Up | Next

Hold your mouse cursor over the image to see a comparison between a 30 second exposure and a 300 second exposure. These images were taken at f/6 at ISO 1600 with a telescope with a focal length of 420mm.

Inadequate exposure is easy to diagnose. Simply look at the histogram. It is easy to determine the correct exposure. If the "mountain" of the histogram is pushed all the way up against the left side of the histogram box, the image is underexposed.

A correctly exposed image's histogram is well separated from the left side of the histogram box, as seen in the mouse-over above of the 300 second image.

Correctly exposing the image gets the faintest detail up out of the noise of the camera.

When the underexposed 30-second image is stretched to try to bring up faint detail, the result is just a whole mess of noise in the image.

Above we can see the result of trying to stretch the inadequately exposed 30-second image to bring up the faint detail in the spiral arms of galaxy M81. The noise in the image overwhelms any faint detail present when the contrast is increased. Even stacking ten 30-second exposures to equal one 300 second exposure would not save this image.

Remember, we do not want the sky to be black in our long-exposure deep-sky images. The faintest detail in an image is just above the sky background, and if the sky background is black, then the faintest detail will be lost in the noise.

  • Solution: Increase the exposure.

Note that increasing the ISO does not increase the exposure. The only way to increase the exposure is to actually increase the time the shutter is open, or to use a larger aperture telescope at the same focal length. A larger aperture gathers more photons, which increases the exposure.

Problem: Inadequate Exposure - The Bottom Line

For a given astrophotography setup and sky and object brightness, there is a minimum exposure required to get the faint detail in an image up out of the camera's noise.

This minimum exposure can be determined by exposing long enough so that the "mountain" of the histogram is separated from the left wall of the histogram box.

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