Helicon FB Tube – Focusing distances

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Focusing distances

A macro extension tube increases the distance between the sensor and the lens by 12/13 mm relative to its normal position. This brings the closest and the farthest focusing points of the lens closer. Impact on long focus lenses (200 mm) is not so big, but it’s very significant for short focus ones (24 mm).

Here’s the formula allowing to calculate the maximum focusing distance for your lens when using an extension tube:

1/s + 1/s' = 1/f, wherein s is the distance to the object, f is the focal length of the lens and s’ is f + extension tube length (12/13 mm)

The table below shows the result of practical tests and measurements for some lenses.

For instance, Nikon 105mm macro lens without a tube can focus at distances from 0.3m to infinity, and at 0.14 m to 1 m with the tube.

Depending on the distance to the object, one and the same step size of the tube will produce completely different linear size of the step and DoF.

For example, Nikon 105mm macro lens at the closest focusing distance of 0.14 m will make a linear step of 0.35mm, and 20 mm at the farthest focusing distance. And that with the same tube configuration settings