Effect on aperture of adding a front mounted TC.

Started Dec 19, 2010 | Discussions thread
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J C Brown Senior Member • Posts: 1,618
Effect on aperture of adding a front mounted TC.

Several recent threads have discussed the effect on aperture of adding a front mounted teleconverter. In some of these it has been claimed that due to the large difference in the diameters of the front and rear lenses of front mounted afocal TCs their use can result in an increase in speed of at least 1.5 stops.

From my limited knowledge of optics it seems from these posts that the behaviour of afocal teleconverters such as the Panasonic DMW-LT55, Olympus TCON17 and the highly respected Nikon TCs has not been fully appreciated.

As explained below, the use of an afocal TC will not IMHO result in either an increase or a decrease in the effective aperture of the camera lens.

Here it is assumed that the light received by the camera and TC is radiated uniformly from a sufficiently large circular area at a long distance from the camera. A circular area of uniformly bright sky would be a reasonable approximation.

The attached ray diagram is for a 1.5X afocal TC, a "Galilean telescope" consisting of convex and a concave lenses with their focal points coincident and their focal lengths in the ratio of 1.5 to 1. The traditional analysis assumes that the light rays entering the front lens are parallel as they come from infinity and those leaving the rear lens are parallel because of the optical arrangement of the lenses. Consequently the intensity of the beam leaving the rear element of the TC is greater than that of the beam entering the front element by a factor of 2.25, 1.5 squared.

In the diagram you will see that the black lines which show the traditional light path used in the analysis do not enter the front element of the camera lens, indicating some loss of light. You will also see that for the black, green and red lines the ratio of the diameter of the beam entering the TC to that of the corresponding beam leaving the TC is 1.5. It should be noted here that these green and red lines correspond to cylinders of parallel light which come to a focus at a single point at the centre of the sensor.

The most important point to note about the behaviour of the TC is that, as illustrated in the ray diagram, in addition to reducing the diameter of the cylinder of light passing through it by a factor of 1.5, the TC also reduces the field of view of the camera by the same factor. These fields of view are of course cones with their apex at the focal point of the camera lens. This is shown schematically in the diagram as a FOV of 7.76 degrees for the camera on its own and a FOV of 5.18 degrees for the camera with the TC attached.

Consequently at a suitably long distance from camera, the diameter of the circle which corresponds to the field of view of the TC is 1.5 times smaller than the diameter of the circle which corresponds to the field of view of the camera on its own. Due to the difference in the circular areas enclosed by their respective fields of view the total amount of light entering the camera without the TC will be greater than the total amount of light entering the TC by a factor of 2.25, 1.5 squared.

Thus the increase in intensity due to the reduction in diameter of the light passing through the TC cancels exactly the loss of intensity due to the reduced field of view, with the result that the total amount of light passing through the camera lens to the sensor is exactly the same with the TC as it is without the TC. There is therefore IMHO neither an increase nor a decrease in the effective aperture due to the addition of an afocal TC.

Hopefully the above explanation will help clarify the optical behaviour of afocal TCs.

Jimmy
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J C Brown

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