🎨 Color and Lens Correction

Post-processing the photos is an important step towards creating a solid documentation. The two most important steps here are White Balance and Lens Distortion. Depending on your gear, you might even do a Colour Profile or adapt the Exposure. Every situation is different and with various possibilities for lighting conditions, the correct colouring of your objects gets distorted. A White Balance can fix that, as long as you have provided a colour chart or grey card with your photos. Also, we want to compensate for lens distortions, as each setup can produce lightly distorted photos.

Screenshot from Adobe Bridge, where we can select the photos we want to post-process with Camera Raw.

But first things first. If you are working with the Adobe Suite, the easiest way of opening up all images of a dataset in the Camera RAW software is via Adobe Bridge. So simply open that software and navigate to the folder where you have stored your RAW images. If you found your folder, select all images by either pressing CTRL+A or clicking the first image and then holding SHIFT and clicking the last image. With all the images selected, right-click on them and select Open in Camera Raw.... This will open the whole dataset in the Camera Raw application, where you will be able to post-process the images as described below.

Screenshot from Camera Raw where I have chosen a good image (the first one) and selected the other ones as well..

When open in Camera Raw, you need to make sure that you first select an image on the left pane, where you can clearly see the colour chart or grey card in the photo. This is important, because some of the following steps involve clicking on the colour chart or grey card. When you have selected the image, you should see it in the center part of the Camera Raw window. If you are happy, click again in the left pane on the Select All-Button. This is necessary, because whatever we do with this one image, we want to do the exact same to all the other images of the dataset. In the following chapters, I will explain certain steps you can do to post-process the images before you continue using them for Reflectance Transformation Imaging or Stucture from Motion.

White Balance

The most simple way of colour-correcting your images is by doing a White Balance. For doing this, we simply need a calibrated grey card or a corresponding grey field on a colour chart. This grey is generally middle grey, or 18% grey. We need that White Balance, because depending on the lights you use or the environment provides, we might end up with wrong colours. In archaeological documentation, this is of course not very good. We therefore want to do at least a White Balance. If you have a grey card, White Balance is easy. When doing this with a Colour Checker, you need to know which patch corresponds to 18% grey. In my case, I usually use the X-Rite ColorChecker Classic and the 18% grey is the fourth patch from the white one in the bottom row. To do a White Balance with Camera Raw, you need to select the White Balance Tool from the top row or hit the I-key. Now, you click on your grey patch or card and you will see some changes happening.

Colour Profile

Screenshot from ColorChecker Camera Calibration 2.2.0..

If you want to be really exact, you can also use a so-called colour profile. This is very specific to the colour checker you have used and I can only describe the procedure for the X-Rite ColorChecker Classic, but it should fairly be the same for other products. First, you need to find the software needed to calibrate your camera. In my case, I downloaded the ColorChecker Camera Calibration 2.2.0 from X-Rite. You should know, that the profile is not only specific to the colour checker you use, but also to the lighting conditions. So if you change the lighting conditions, you also need a new profile. Fortunately, the process is rather easy. You start the software and drop a DNG-file from your dataset into the software, that clearly shows the whole colour checker. The software automatically detects the colour checker in the image and each coloured patch in it. If not, you can adapt manually. When done, simply hit the Create Profile-button. It should automatically offer the Camera Raw profiles folder. In Camera Raw, you can now select the profile under the Camera Calibration tab on the right. There is a dropdown called Camera Profile. You should find your profile there.

Exposure

The Exposure slider in Camera Raw can halp you a bit to lighten dark images up and dimming bright images down. Do you see the funky curve in the top right corner? This gives you an impression on how good the exposure of the image is. When the curve has its peak way over to the left, the image is underexposed. If the peak is way over to the right, it is overexposed. Try to find a setting, where you have a good distribution with the peak in the center. Remember, that you are dealing with a whole dataset here and should do all the changes to the whole set. Especially in Reflectance Transformation Imaging, there will be photos that are darker than others. If you calibrate your whole dataset on a dark photo, you will have some overexposed images in the end. Try to find a photo that is somewhat in the middle already and then adjust the exposure.

Lens Correction

The Lens Correction is also a important part of post-processing the images. Each camera and each lens have different specifications and do slightly distort the images. This does normally not really matter, but as we want to use the images for further computing, we want to make sure they are as undistorted as possible. Fortunately, the Lens Correction is easy and straigt forward, especially when you are working with a newer camera. To do the Lens Correction, you first have to switch the working tab on the right pane to the tab called Lens Corrections. Here, you simply click Enable Lens Profile Corrections and choose the manufacturer of your lens in the Make-dropdown-menu. If your camera is known, the rest is filled out automatically based on the metadata of your RAW images. So have chosen Nikon and Camera Raw immediately knew that I used a NIKKOR 50mm lens. If Camera Raw does not fill out these fields for you, you have to do it yourself.

Export

The last step in this process is to export the post-processed RAW images into JPG-files to compute further. Both, Reflectance Transformation Imaging and Stucture from Motion need JPG-files to proceed. We therefore need to export them first. To do so, you click on the Save Image-button in the lower left corner. In the following dialogue, you choose a destination folder by clicking on the Select Folder-button. We usually place the export folder in the same folder as the rest of the dataset but call the export folder jpeg-exports. This has no reason really, except that in older days, this was the name of the export folder we had to use for Reflectance Transformation Imaging. We need to do this no more, but some things stick.