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How does a focal lenght doubler work?

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It enlarges the image circle and as a result dims the brightness of the image in the process.


As an example to make this concept more clear imagine a movie projection say 10 feet away from a wall and it is projecting an image on a 50" screen. This is similar to the way a lens works, it projects and focuses an image at a certain distance onto a certain size sensor just like a projector.


Now take that projector and move it back to 20", now the image is much larger, 100" across, so the screen has much less image on it and the image that falls on the screen is enlarged. Also as you move the projector back the image gets dimmer because it is spreading the same amount of light over a much larger area.


This is essentially what a doubler does, it increases the size of the projection on the sensor which magnifies it. What once filled the entire sensor is now spread well beyond it and since a large amount of its light is no longer falling on the sensor the brightness of the image also decreases.

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