24 July 2013

Another Lesson in Angle-to-Observer

To most of us, it's clear that lighting can have damaging effects on our identifications. It's a lesson that's soon learned when looking at the upperparts of adult-like gulls on sunny days. Take for example this group of Bonaparte's:

First summer (2nd cycle) Bonaparte's Gulls with similar aged Little Gull. Sheboygan, Wisconsin. 14 July 2013.
The Bonaparte's on the far left looks paler than the other five. Does it suffer from a pigmentation condition that's yet to be explained by ornithologists? Of course not. It happens to be the only individual (along with the Little) that is almost exactly parallel with the camera lens (i.e., the observer). The other five Bonaparte's are all facing towards the observer's ten-o'clock. A slight change in this so-called angle-to-observer can dramatically change our perception of the grays we're seeing. The effect is multiplied on sunny days.

Now note how this "pale" Bonaparte's becomes consistent with the group, altering its upperparts to match the others after changing its angle-to-observer:

Understandably, the Little Gull, which is not significantly paler than Bonaparte's (as the photo suggests) is blown out and overexposed. After waiting a few seconds for it to face towards the observer's ten-o'clock the effect is reversed:

Here the Little Gull appears at least one shade darker than the Bonaparte's which is not accurate (Kodak Gray Scale actual values: Little Gull 4.5-5.5, Bonaparte's Gull 5-6).
Now for an example with non-gray feathers. Consider this collage of the same bird. The images were taken one second apart with no post-processing alterations whatsoever:

Juvenile American Herring. New Buffalo, Michigan. 17 November 2012.
Although probably without any serious implications for identification, the face on the lower image is now darkened (or shall I say the face on the bird in the top image is now lightened?). Kenn Kaufman's reaction to this set verified my point with precision: "This is an outstanding example of the ways in which a single photo (or even a short series of photos) can be misleading...I'm dismayed when I see people arguing strongly for a difficult ID on the basis of a single shot; it's so hard to be sure which "field marks" are real and which are photographic illusions".