I photographed this agama lizard in Namibia with a 500mm f/4 Canon lens. I had to use extension tubes to reduce the minimum focusing distance because that enabled me to capture this frame-filling shot. It is completely uncropped. When you use extension tubes with a short macro lens, such as a 50mm, the amount of light reaching the sensor is reduced -- typically by about one f/stop. However, when tubes are used with a long lens, the light reduction is negligible and doesn't affect the exposure at all. The reason for this has to do with the millimeters of extension compared to the focal length of the lens. A 25mm extension is half the length of a 50mm lens, but it's only 1/20th the length of a 500mm telephoto. Because the depth of field was so shallow, I positioned myself so the head and the tail were equidistant to the lens to insure both ends of the reptile were sharp. My settings were 1/400, f/5.0, and 200 ISO. I used a tripod for this shot.