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I just bought a Canon 2X Extender and I always believed that It would only result in 2 Stops loss of light and not change the aperture.
When I tested this extender on a Canon 135mm F/2 lens, the maximum aperture I could set went down to F/4. So It kind of confused me because if I step down the aperture to F/5.6, is it actually setting the aperture to F/5.6 or F/2.8 (i.e. 1 stop below F/2)?

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2 Answers 2

up vote 12 down vote accepted

This is really simple when you think about it. The additional element changes the focal length of the lens, without changing the apparent size of the aperture. That means that the relative size of the aperture decreases, so the f number does in fact actually change. (If this is unclear to you, see the bit about f numbers in this other answer.

This is also why real wide-angle converters can go the other way, effectively increasing the aperture. (See How can a speedbooster improve the light performance of a lens? for more.)

Some converters communicate intelligently with the camera body, so the aperture displayed will be correct. This is the case with the Canon extender you have, but might not be with third-party ones. This explains the part you were confused about: the camera is aware of the change already and the numbers it is showing you are what you will actually get. When you set the aperture on the camera to f/5.6, the aperture on the lens is set to the same position that would be f/2.8 without the extender (but which genuinely is f/5.6 with it).

Note that teleside converters which go on the front of the lens do change the effective aperture (see What's the difference between real and effective aperture?), so they don't change the f number. (They are usually lower quality, however, and can introduce vignetting.)

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Another way to think about this is that you are now fitting 1/4 of the light through the same size hole. You extended the focal length by using a teleconverter (basically a magnifying glass). You are now only looking at 1/4 of the original image on the sensor. This means you lose two stops worth of light that you are now discarding.

The opening for the light didn't change any, but the focal length was lengthened which means less light makes it to the lens. Technically at the point of the aperture, it's still the unconverted focal length, but since the effective focal length is doubled, this drops the f number since the f number is based on the focal length divided by the size of the entrance pupil.

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