Angle of light and fine image detail

[Roger Clark]

After some heated discussion in the avian forum this weekend, I am posting a series of images to illustrate the the effects of phase angle on the detail in a subject.

Phase angle: the angle between the light source (usually the sun for outdoor photography) and the camera as viewed by the subject. A simple way to determine phase angle is to stand in front of or beside you camera and straighten both arms. Make one arm parallel to the lens axis, and point the other arm toward the sun. What is the angle between your two arms? That angle is the phase angle. When your arms are pointing close to the same direction, the angle between them is small so the phase angle is low. If your arms are 90 degrees apart, and the sun is low in the sky, you have side lighting. If your arms are close to 180 degrees apart, you have back lighting.

Many here on BPN advocate "point your shadow at the subject" but I say point your shadow close to but not quite at the subject. The closer your shadow is to the subject, the lower the phase angle and at phase angles less than about 15 degrees on birds (as well as other subjects) loses fine textural detail in the feathers, and loses color! I'll illustrate this effect with examples. The reason is when the sun is directly behind the photographer, shadows are minimized, and that includes the tiny shadows cast by fine details like grains of sand in a soil, or the structure in feathers, and from overlapping feathers, or an animal's fur. As we will see the results are quite striking!

When in the field I have used opportunities to create a series of images at different phase angles to show phase angle effects. The series here uses a great blue heron photographed at the south end of the pond at the Venice rookery. The circular nature of the pond and a nesting GBH enabled me to move position, keeping distance pretty constant but varying the phase angle, and all in a short time (5 minutes) so sun angle stayed essentially the same. Also, the bird stayed in almost the same position for this session.

Attached is the image showing the GBH at two phase angles: 10 and 30 degrees. The 10 degree angle had the sun behind me but overhead (I pointed my shadow at the bird). Two things to notice about this image. The 10 degree phase angle image appears flat. No shadows are there in the image to give form to the subject, Compare that to the 30 degrees phase angle image. The 30 degree phase angle image shows shading around the left side of the body giving form and adds to the dimensionality of the image. Next look at the green vegetation. The vegetation appears greener in the 30 degree phase angle image. This is because at low phase angles, first surface reflections contribute more to the observed brightness and less from light that has penetrated into the subject (e.g. the leaves). Same with the bird: the colors at 30 degrees phase are more intense (this will be more apparent in the full size image crops discussed next),

Next look at the series showing eye detail. In all images shown here, processing was the same. At 10 degrees, there are no small shadows around the eye. But as we increase to 20, 30 and higher phase angles, shadows are cast by the tiny detail, adding form and texture to that detail. The added contrast makes the impression of a sharper image. The detail in the 10 degree phase image is only due to color variations, and not shading.

Next look at the image series showing the feather detail in the neck. At 10 degrees, there is only a small hint of shadows and the image lacks perceived detail, even though the image is just as sharp as the other images. Moving to the 20 degree phase angle image, shadows now show between the tiny feathers, and subtle shading is showing on the left edge. Note the colors are more intense and there is a huge increase in fine textural detail.

Now look at the final image showing wing feather detail. The 10 degree phase angle image again shows low contrast and an apparent lack of detail, and is overall flat. Yet examining the image closely shows fine low contrast detail equal in size to the detail in the higher phase images. Now examine the 20 and 30 degree phase angle images. These images show the tiny shadows case between the feathers, and within the feathers. Color and contrast are higher. Now we see that the feathers are not laying flat on top of each other as they appear in the 10 degree phase image, but the have significant 3-dimensional structure. Moving to 60 degrees phase angle adds more structural detail.

Of course as one moves to higher phase angles, large scale 3-dimensional structure can make deep shadows. This is apparent with the bird's body at 60 degrees and is beginning to become apparent in the 30 degree phase image above. So there is a peak in optimum phase angle: avoid low phase angles, usually below 15 degrees so that you have some shading. If the light is directional, avoid higher phase angles (depends on how contrasty the shadows are), but above about 45 to 60 degrees shadows can become too deep.

The optimum phase angle is often in the 20 to 30 degree range, where there is enough shading to give form, fine texture, and better color than at lower or higher phase angles. What this means is don't point your shadow towards the subject if the sun is low in the sky. However, as Art Morris has said, if the sun is high in the sky, then pointing one's shadow toward the subject minimizes harsh shadows and is the way to go. But in that case the phase angle is higher anyway so you are already working above the low phase angle region.

Keep above 15 or so degrees phase angle to add form, texture, and better color to your images.

Roger