When 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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