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First, I am a wildlife ecologist who does some very amateurish photography on the side. I would like to start using digital photography techniques to collect field data.

The Problem: I need to take pictures of a certain colored object outside. Let's call it a red ball or a red cylinder. It will be partly obscured by plants in most cases and photos will be taken during different times of the day. I need to be able to control for the variation in shading and lighting that may occur so that the reflectance of the object returns the same value in each photo.

Options: Can I use a sun-shade, flash, and/or restrict my photo times to certain parts of the day to help with this? is this as simple as metering off a standard 18% gray card every few photos (or say every 30 minutes)? Could this be accomplished in post-photo editing software? It doesn't have to be perfect. It just needs to be repeatable.

Thanks, Matt B.

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    Will the object ever be so obscured that you can't simply spot meter off of it every time? – junkyardsparkle Mar 31 '16 at 1:00
  • "I need to be able to control for the variation in shading and lighting that may occur so that the reflectance of the object returns the same value in each photo." - do you mean that the object should look the same i.e. have no shadows on it? – Euri Pinhollow Mar 31 '16 at 8:10
  • Unfortunately, yes. – Matt Apr 1 '16 at 13:37
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is this as simple as metering off a standard 18% gray card every few photos (or say every 30 minutes)? Could this be accomplished in post-photo editing software?

The issue here is white balance, and there are basically two options: you can take steps to make sure that the light is always the same color, or you can adjust the white balance to compensate for the changing color of the light. The first option is what @dpollitt mentioned: provide your own light using strobes so that the light always has the same color. If you provide enough light, you'll completely overpower the ambient light and the subject should have consistent color.

The second option is to compensate for the changes. Set your camera to save your photos in RAW format. This will prevent the camera from interpreting the data and giving you what it thinks the is the correct white balance, and you can then adjust the white balance yourself later. That's where the gray card comes in -- place the card just outside the frame, and every so often either move it into the frame or reframe to include the card and take a photo. Including the gray card every 30 minutes is probably fine in the middle of the day, but in the early morning and late evening the sky changes much more quickly than that, so include the card more often near dawn and dusk.

When you import the photos into whatever photo management program you're using, you can go through them and find all the gray card shots. Every photo manager has an option to adjust color temperature based on a neutral gray tone in the photo, so pick the card. After that, you should be able to apply the same adjustments in bulk to the photos following that one, up to the next gray card shot.

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    If you are shooting the gray card every 30 minutes, a better practice would be to use a particular gray card derived setting for all of the photos taken from 15 minutes before through 15 minutes after each gray card shot. That way the gray card shot is centered in each sequence instead of on the hard edge of each sequence. No shot will be taken more than 15 minutes one side or the other from the gray card shot used to process it. By your methodology half the shots would be taken longer than 15 minutes after the gray card shot used to process it, and some might be as long as 29 minutes. – Michael C Mar 31 '16 at 5:55
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    @MichaelClark Agree, but I didn't want to complicate things. When you get right down to it, it would make the most sense to have the card (or any neutral object) included in every shot. – Caleb Mar 31 '16 at 6:15
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You could kill the ambient light and light the scene only with a flash unit or strobe. See: What does it mean to "kill the ambient"?

  • I like the idea; however, I doubt I could kill the ambient light sufficiently during the day in the field with my particular sampling/photo-taking protocols. Would taking photos at night and using a flash circumvent the need for continually shooting gray cards and post-photo processing? – Matt Apr 1 '16 at 13:48
  • Oh you could during the day. Use strobes(off camera). – dpollitt Apr 1 '16 at 13:48
  • Let's say the field sites would prohibit excessive camera gear. Would night-time photos be an option with white-balance set to 'flash' setting? – Matt Apr 1 '16 at 16:33
  • I don't see why nighttime would be a problem. It would be easier as far as I can see. You still would have some ambient if the moon is around but that would be easy to overcome. – dpollitt Apr 1 '16 at 16:34
  • Awesome! That will work wonderfully... except for the late hours! – Matt Apr 1 '16 at 16:40

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