You're right, it's is a loaded question :smile:
As phigmov says, the human eye sees very wide. It also essentially sees in fisheye, which the brain then sort of software corrects (who said it was cheating?!).
But then the human eye is very sharp in the centre and very soft away from the centre, so in terms of what you actually see, you're looking at 35-50mm (full frame equivalent). But then the human eye tends to see a specific subject with an awareness of the surrounding context, so in that regard we're closer to 35mm than 50mm, which slightly isolates the subject.
In terms of a 'neutral' view - that is, one that isn't magnified when you bring a 100% 1x viewfinder to your eye, my understanding is that it correlates to the diagonal of the sensor/film. Hence Pentax's legendary 135 format 43mm Limited.
Not sure exactly what the diagonal of a MFT sensor is?
Perfect example! I was considering mentioning the Pentax 43mm, I thought about it, and it's actually the reason that I typed 43mm above as the limit of normal focal length :smile:
By the way, the 4/3 diagonal of 22.5mm is technically the ONLY parameter that is fully specified, which is why even though the current sensors are 4:3 aspect, to maximize the lens capabilities, any other aspect that preserves a 22.5mm diagonal is still considered 4/3. That's my understanding of the system, anyway.
human eyes are wide angle but most of it is not in focus so maybe it's equivalent of 12mm at f1.4 or so on full frame.
Kind of. You need to introduce the caveat that with the human eye, only the center is in focus, while a 12mm f/1.4 lens would have a plane of focus parallel to the image plane, considering that it isn't a tilt/shift lens, and is functioning correctly, etc. So maybe it's more correct to say parallel
My understanding is that we also see with very deep depth of field (or, perceived depth of field, due to the eye's ability to focus very rapidly), so probably not an f1.4...
Actually, does some smart person here know if the human eye does actually have a measurable 'focal-length'?
I think his f/1.4 statement is because of what I mentioned above, about a lot not being in focus, but saying that it's like f/1.4 isn't really correct, because with the eye, a portion of the image is in focus, while with a lens, a plane is in focus.
I think we established pretty well above what the focal length of the eye is. If you're considering what you can focus on, it's about 45-55 degrees field of view, although this isn't true for all people, in all situations. I've read online somewhere that the human eye has about a f/2 aperture. Stick your finger 5-8cm away from your eye, focus on it, and enjoy the bokeh