Using extension tubes with the 300mm f4 LWhen used on my Canon 20D, the Canon EF 400mm f4L lens will focus down to just under 2.5m. However, when taking photographs at close range it is helpful to reduce this distance by the use of extension tubes. The following images give an idea of the effect of using different combinations of tubes. For this exercise, the lens was switched to manual focusing and set to the minimum focusing setting. Camera and lens were then mounted on a tripod, and a focusing rail used to focus on the target scale. The distance given is measured between the target scale and the sensor plane (as marked on the top of the camera). 300mm lens only -
Area covered - 17.1x11.4cm Object distance - 247cm Magnification - approx 1:7.6 (x0.13)
300mm lens + 12mm extension -
Area covered - 12.25x8.15cm Object distance - 197cm Magnification - approx 1:5.4 (x0.19)
300mm lens + 20mm extension -
Area covered - 10.4x7cm Object distance - 177.8cm Magnification - approx 1:4.6 (x0.22)
300mm lens + 36mm extension -
Area covered - 7.95x5.3cm Object distance - 153cm Magnification - approx 1:3.5 (x0.28)
300mm lens + 36mm + 12mm extension -
Area covered - 6.7x4.4cm Object distance - 141cm Magnification - approx 1:3 (x0.34)
300mm lens + 36mm + 20mm extension -
Area covered - 6.1x4.1cm Object distance - 135.5cm Magnification - approx 1:2.7 (x0.37)
300mm lens + 36mm + 20mm + 12mm extension -
Area covered - 5.4x3.6cm Object distance - 128.6cm Magnification - 1:2.4 (x0.42)
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