Rated Focal Lengths of Lenses

Started 5 months ago | Discussions thread
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tjdean01
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Rated Focal Lengths of Lenses
5 months ago

We using m4/3s understand a lot about "crop factors" and adapting lenses.  If we take the Sigma 30, for example, regardless of what size sensor we use it on, it's a 30mm lens (yes, I know it's 60mm equivalent on m4/3s and 45mm equivalent on APS).  However, in our brains, we know it as a 30, which is a bit longer than our 'standard' 25s and a lot wider than the 45.

However, what I want to know is why they call the lens a 30?  Could we mount this lens on a huge medium format sensor or even 1' x 2' sensor and vignette the heck out of the resulting image?  Yes we could.  It would still be a 30mm lens.  FOV would still be the same as long as the sensor is large enough to vignette (FOV will only change when you crop the image circle).

So, let's look at two lenses.  A 50mm Leica and a 50mm off-brand lens.  They're both 50mm lenses, however, the Leica, to avoid vignetting, will produce a larger image circle.  So, since they're both 50mm lenses and the Leica image circle is a bit larger, wouldn't the resulting image of the Leica be cropped, thus making the resulting images from those lenses be more tele than the cheap lenses?

In short, how to they conclude the mm rating on lenses?  Some cctv lenses, designed for tiny sensors, are rated as 6mm.  But the image circle on some of them will cover a m4/3s sensor.  It certainly doesn't give us an awesome ultra-wide 6mm image, however.  I'm confused.  Thanks!

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