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I regularly take photos on a fixed background under natural light.

I use a grey-card for white balance which works well enough. I take a test shot with the card, white balance it and save that as a preset which I use for the white balance of all images in that session.

However, I'm struggling to have consistent brightness to the final image, especially for images taken during different sessions.

I would like to be able to take a spot (or small area) from the background and be able to tell Darktable to set that area to a certain exposure value.

Does anyone know if that's possible?

  • You know that you can select spots for white balance? – sebix Aug 8 '17 at 20:13
  • @sebix - yes, white balance isn't the problem. After I WB I end up with images with consistent colour, but varying brightness, especially when comparing between sessions. – garethTheRed Aug 8 '17 at 20:58
  • Natural light varies in intensity. Just as you use a tool to insure the color is properly balanced, perhaps you should use a tool to measure the intensity of the light as well as its color? – Michael C Aug 9 '17 at 3:53
  • @MichaelClark That is the plan - if I only knew which tool or technique to use. – garethTheRed Aug 9 '17 at 6:20
  • It's called a light meter. Your camera has one. You can also buy external light meters. You can meter the reflected light (put the meter where the camera will be and point it at the subject) or you can meter the incident light (put the meter where the subject will be and point it at the light falling on the subject). – Michael C Aug 9 '17 at 9:04
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The closest thing I've seen in darktable is the automatic mode in the exposure module, see darktable manual.

Automatic adjustment is only available for raw images. A typical use case is deflickering of time-lapse photographs. You apply an automatic exposure correction with the same set of parameters to all images of the series – differences in lighting get compensated so that the final video sequence does not show any flickering.

But I would have thought the answer would be to shoot in aperture priority mode and let the camera's metering deal with small changes in ambient light by adjusting the exposure time. If the background takes up a enough of the image the camera's matrix / evaluative / ESP metering should be reasonably consistent, though for real accuracy a spot metered reading from the grey card for each image would be best.

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