>>4123202>Does that mean for example a 50mm 1.8 lens might have the equivalence of a 50mm 1.4 on a smaller mount, say the Canon RF?No.
Forget about their marketing shit, the engineers didn't write it.
Recall that teleconverters are a magnifying glass on the back of your lens, and speedboosters are a reverse-magnifying glass.
Both of these will affect the actual f/stop (which is calculated by dividing focal length by the size of the entrance pupil).
If we have a 2x teleconverter, our 100mm will become 200mm because we're just zooming in on the 100mm focal length. We're just projecting the light in a different way, and light is all the camera knows, it doesn't use a ruler.
But our entrance pupil remains the same regardless, so if it's 25mm then our 100/25 = f/4 lens becomes a 200/25 = f/8 lens.
If we have a .7x speedbooster, our 100mm becomes 70mm and because of that our f/stop goes from 100/25 = f/4 to 70/25 = f/2.8, our lens became "faster".
This is basically just because the speedbooster scoops up light from a larger-format lens, light which is usually wasted on the sides of the sensor, and then focuses the light onto the smaller sensor, which has the effect of "zooming it out" because the larger lens is naturally "zoomed" or "cropped" when its projected on a small sensor.
So maybe its possible for them to design their lenses the way you asked about, but as long as the focal length is the same (and you can tell it's the same), then the number on the box is accurate. 1.4 canon = 1.4 nikon.
