When you buy a lens you have to believe the manufacturer when he defines its focal length. We can only buy two lens material types for our machines. Zinc selenide and gallium arsenide. It is possible to crudely verify the focal length of a ZnSe lens because you can see through it. However it will only be in the ball park with normal light because the real focal length is different at at our invisible laser wavelength and that will be how the lens was designed. No that is not the fallacy. There are many videos on Youtube showing you ways to find the focus . Very few people realise that they are not finding the lens's fixed focal point but something else. This session proves that the manufacturer's focal point is nothing more than an imaginary value as far as our technology is concerned because we can change parameters and influence its position.

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