David Saffir: Using the SpyderCube with Histograms
Aug 2013 06

Remember the “old-style”, ordinary middle-gray card, the one so often used to help set exposure? As time went on, and digital photography came into its own, that same gray card was sometimes used to set in-camera white balance, or in post-production to neutralize color-cast effects produced by in-scene illuminants.

The problem is that many gray cards look gray, but the reflected light from the card may be affected by materials used its construction. These materials are considered to be “spectrally-biased”, in other words, they may reflect a color-cast from the illuminant in use.

If one uses a different material, one that is “spectrally-neutral”, it will return a result (reflected light) that is accurate regardless of lighting used. The material will have a neutral appearance, with little or no color-cast. The SpyderCUBE, which is discussed below, is constructed of these materials. That is what makes it so useful in setting in-camera white balance, and in neutralizing overall color in post-production.

Here’s a demonstration using an ordinary gray card purchased at my local camera shop.

An image of an ordinary gray card opened in Photoshop, next to an expanded three-channel histogram:

This shot was illuminated with a daylight-balanced, high-quality strobe.

Take note of a few things: first, the card looks “gray”. Next, the spikes in the histogram panels are more or less in the middle, which indicates an approximately correct exposure. Last, note that none of the histogram channels is perfectly aligned – particularly the red channel.

This comes from the construction of the gray card – even though it is illuminated by a high-quality light-source, its materials skew its reflectance qualities a bit toward the red. Not very good if you’re using the card for in-camera white balance, or in post-production with a white/gray eyedropper. Under a more difficult light source, the skew would be even greater.

Here’s an example of the same card illuminated with a tungsten/halogen light source:

The effect of the illuminant used is very noticeable in this case. Also take note of the impact on the histogram. This is the combined effect of using a spectrally non-neutral card with a spectrally biased light source (toward red, as per the histogram).

The SpyderCUBE is made from ABS Cycoloy, a custom-pigmented hybrid resin that is fade proof and extremely durable. Unlike many other gray reference tools, it is spectrally neutral. It provides white, gray, card-black, and absolute-black reference surfaces, along with a chrome ball for spectral highlight references.

Here’s an image of a SpyderCUBE, mounted on a small tripod, illuminated with a tungsten/halogen light source. Note that I’ve created a selection in Photoshop isolating the two gray panels on the SpyderCUBE (dotted lines).

I slightly under-exposed this image when creating a side-lighted setup. That is why the histogram is shifted slightly to the left of center. Note the two spikes in each color channel of the histogram. Each spike represents one of the gray faces of the SpyderCUBE. The SpyderCUBE is made of spectrally-neutral materials, so the spikes from each panel in the histogram line up perfectly in the composite (top).

Also note that the brighter face of the SpyderCUBE creates the histogram spike closest to the center, and the darker gray face creates the spike to its left.

Here’s a similar image, higher exposure, with selection of all of the faces of the SpyderCUBE:

Note changes in the histogram from the selection. On the left, the black trap/black card face creates what might be called a “double spike” in the histogram. We also see two spikes for the gray faces, and two spikes for white.

If the lighting were set up to illuminate both sides of the SpyderCUBE evenly, the white and gray spike-pairs would each merge into a single white and gray spike.

If one was shooting a wedding, and wanted to see texture in the bride’s dress, one would tend to use differential or moderate side-lighting – and the histogram from the SpyderCUBE would likely resemble the one we see here, with a noticeable difference in location between the primary white spike and the secondary one. If the two are too close together, that’s a warning of flat lighting, and the potential danger of losing the detail in that wedding gown. (Tip – take a setup shot of the SpyderCUBE which fills the frame – then look at the histogram on-camera or on the computer if shooting tethered.)

Similarly, if one wanted to see shadow detail in the groom’s tuxedo, one would want to be able to differentiate between the black card face and the circular black trap in the image preview on-camera, plus want to see the “double” spike for black in the histogram. This assures that the details in the tuxedo will be visible, and not lost into undistinguished black at the bottom end.

Additionally, in post-production, color correction made from the lighter gray face would be accurate and pleasing, regardless of the lighting used, as it would represent the primary light source. Once you learn to recognize the tight RGB spikes of the SpyderCUBE’s spectrally neutral materials, you may be able to pick them out in a camera histogram even when other materials are also in the shot.

There are additional articles in the SpyderBLOG regarding use of the SpyderCUBE; I encourage you to search for them, using the search term “SpyderCUBE”.

David Saffir is a commercial and fine art photographer and printmaker, located in Southern California, and a well-known speaker at workshops and conferences across the US.