When I first arrived in the Brooks Range to do my Master's Thesis field work my advisor, who was able to join me for only the first day or two of my summer mapping, commented to me that sometimes the lighting really matters--that some folds, like the ones just there, in the Kayak Shale on the ridge above my camp, will only show up when the light is perfect. Having said that, he promptly took out his camera and begin taking photos. Being a dutiful student, I too took photos of the ridge, and, at the time, I was quite certain I saw the minor parasitic folds we were recording for posterity.
However, after he left I spent two more months in the field, tromping around my field area working on my map, and each evening, when I returned to camp, I'd look up at the ridge above my camp at the patch of shale, scratch my head, and wonder what we'd taken photos of--all I could see were a few sheep trails and no sign of any folds.
It wasn't until my very last week in the field that the sun was at the exact correct angle once again, and I looked up at my ever-present ridge and lo and behold, there were the folds, clearly visible! Needless to say, I hurried to grab my camera and once again took photos, to be certain that I had a record of them--they were so clear and easy to see.
The next day I looked back up the hill, and was not in the least surprised when I could not see the folds. My advisor was correct, sometimes the light does have to be at exactly the correct angle.