Originally posted by UncleVanya You haven't changed focal length you have simply restricted the opening
But isn't it like the focal length changed with respect to FOV? My simple minded way of thinking about this is that if the length of the hood increases, at some point it will cause vignetting, despite the opening remaining the same. FOV decreases as length increases.
I should probably stick to my day job, which obviously has nothing to do with optics, though these interesting conversations help me to better understand the factors affecting photography.