Histogram guide for astrophotography

Reading it

• Left is dark, right is bright

  The horizontal axis runs from pure black on the left to pure white on the right. The height at each point shows how many pixels have that brightness.

• The big hump is the sky

  In a deep-sky photo most of the frame is dark background, so most pixels pile up in one tall hump. Where that hump sits is the thing to watch.

• Stars are the thin tail

  Stars and bright nebulae are a small fraction of the pixels. They show up as a low tail trailing off to the right.

• Walls mean clipping

  A pile-up against either edge means pixels have hit pure black or pure white. Detail there is gone and can't be recovered.

Good and bad histograms

Six typical shapes, with what each one means and what to change.

• Underexposed

  Fix it

  The sky hump is jammed against the left wall. The faint detail is buried in the noise, or cut off entirely.

  What to do: Use a longer exposure or a higher ISO or gain, until the hump lifts clear of the left edge.

• Well exposed

  Good

  A clear hump a quarter to a third of the way across, with a gap at the left and a gentle tail from the stars.

  What to do: Nothing to fix. Keep this exposure and collect lots of frames.

• Overexposed

  Fix it

  The hump has been pushed up toward the right edge, and bright parts pile up against it. Nebula cores and stars lose their color.

  What to do: Shorten the exposure, lower the ISO or gain, or add a light-pollution filter.

• Light-polluted sky

  Watch out

  A tall, narrow hump in the middle. The sky glow is eating the range, and faint targets sit on top of it.

  What to do: Go to a darker site, or use shorter exposures with a filter. Sub-exposures this bright will be flattened when you stack and stretch.

• Color cast

  Watch out

  The red, green and blue humps don't line up, so the background looks tinted. Here blue is on the right, so the sky looks blue.

  What to do: That's usually fine in the raw frame. Line the channels up in processing with a background neutralization step, or a white balance.

• Blacks crushed in editing

  Fix it

  A stretched image where the black point was pushed too far. Faint nebulosity at the dim end of the sky has been clipped to pure black.

  What to do: Pull the black point back so the hump sits just off the left edge. The sky should stay a dark gray, not black.

Good habits

• Your camera shows the preview, not the raw data

  The histogram on the back of a camera is usually computed from the JPEG preview, which has contrast applied. Your raw file has more room than the screen suggests, so a slightly dark-looking histogram is often fine.

• A few saturated stars are normal

  The brightest stars will always hit the right edge. Don't chase them. Worry when bright nebula cores or large areas clip.

• Protect the bright parts of the target

  Orion's core, the Andromeda galaxy's center and similar spots clip quickly. If they matter to you, shoot a second, shorter set of frames and blend them in processing.

• More frames beat longer frames

  Once the sky hump is clear of the left edge, you gain more by stacking many frames than by pushing a single frame brighter.

• Check in color, one channel at a time

  If your camera shows red, green and blue separately, glance at all three. A hump that touches the left edge in one color only is the first sign of clipped blacks.

• Judge it again after stacking

  A stacked image looks very dark until it's stretched. In processing, aim to leave a small gap between the hump and the left edge, so the sky stays dark gray rather than black.

Questions

Where should the histogram peak be for astrophotography?

A common rule of thumb is to put the main hump roughly a quarter to a third of the way in from the left, clear of the left edge. Under very dark skies you can sit a little lower, and in light-polluted skies the hump will sit further right whatever you do.

Should the histogram touch the left side?

Not if you can help it. A hump that touches or climbs the left edge means the faintest detail is being clipped to black. Raising the exposure slightly moves it away.

Why does my histogram look so narrow?

Light pollution and moonlight add a bright, even glow, which makes the sky nearly one brightness. A narrow hump is normal in a bright sky. A filter or a darker site helps.

Does a good histogram mean a good photo?

No. It tells you the exposure is sensible. Focus, tracking, framing and noise still decide how the photo turns out.

What about raw files and the histogram?

Because the preview histogram comes from the JPEG, it can look brighter than the raw data really is. If you shoot raw, some people deliberately expose a touch brighter than they would with JPEG, as long as nothing important clips. Test with your own camera.