A single eye sees 120-140' with biscopic vision being able to sense movement maybe as far out as 220'.
However what we recognise as out usual gaze, or our field of active view, is actually less than 90'. Within this we both collect, process and percieve the most information.
The relative magnification which we percieve as normal and the brain calculates our spatial awareness on, is actually equivalent to a 90mm lens on an FF / 35mm( 135) camera. However a 90mm lens will give a "crop" or field of view to a mere 24'.
Why is this? Well I don't really know the answer but at a guess I would say the eye has both a relatively convex first lens - the cornea, and a powerful, malleable second lens coupled to a very concave focusing surface which greatly increases the surface area for image-capture: more megapixels if you like, and in addition a super sensitve sweet spot, the Fovea Centralis where even more sensor elements are concentrated.
In this way the circular focal plane created by the lens in the eye is better exploited by falling onto a curved, 3D plane rather than a cropped plane like a CMOS/CCD or layer of film.
On a four thirds frame, the reverse is true: the focal circle is somewhat under exploited from the quality lenses. see below for more discussion on this.
Kit and Budget Lenses from Olympus and their Relativity to Eyesight
14-42mm f3.5 Kit lens: (28-84 in FF/ 35mm) eq14mm: 70' and 1/3rd approximate eye magnification
eq84mm: 30' and 1/1.1 app "" "" ""
25mm pancake f2.8 "prime" eq 50mm 46' and 1/1.75 app
40-150mm kit lens f4.0 80 as above
eq 200 12' amd 2.2
eq 300 8' and 3.4 magnification.
Relative Field of View and Magnification
As a rule of thumb you could say that 100mm represents about 1.1 eye magnification, or as mentioned, 90mm is equivalent.
To capture near to all that the eye sees in field of view you would need to be using a 20 or 24mm (ff/135) but of course this both shrinks distance objects and distorts straight lines.
I very much like using 90-120mm because for either portraits or actually some landscape details, architecture and near-macro, the crop produced is what the eye and brain would have most perception of: the Foveal Crop you could say. Despite these lenses being either slower (f4 for example in a 40-150 zoom) or expensive ( f 2.0 or even 2.8 fixed focal macro/portrait primes) than say a 25mm prime or the 14-42mm kit lens, I prefer to use them because you can work at a little more distance from people. This gives you the chance to get the shot just before they are really aware that a photo is being taken.
Personal View on Four Thirds Economics and Technology
On a four thirds frame, the reverse is true from the extensive utilisation of the focal circle onto the retina in our eyes: the focal circle in this type of camera is somewhat under exploited with respect to these high quality lenses.
The latest mFT release from Panasonic, an 18 effective 16 CMOS with a panorame function which is broader than 4-3 demonstrates this to be the case. Some would say that a sensor bit-element dispersal of 4 to 5 microns, the sensors are too small and packed to offer equal quality to APS-C and FF sensors.
However, Olympus has pioneered compact-sized, near perfect telecentric lenses: this means that light is focused and travels almost paralell throught the later lens and aperture elements to an give an accurate re-dispersal to the focal surface, the resulting quality in image and reduction in scattered light and unwanted colour spectral effects means that the smaller sensor can acheive excellent results comparable say 10 mpx to 10mpx APS-C or even full frame cameras.
The quality of images producable even on the "cheap kit" lenses, is really superb and you have to spend a lot of money in getting two f-stops faster from the range of FT lenses out there.
Of course Canon and Nikon divotees will point to the previous noise in the FT sensors, now obvercome to give perhaps better results for Olympus and Panasonic operating at ISO 6400. Also these manufacturers have very good lenses which achieve a high degree of telecentricity.
However Olympus and the others in the FT /muFT group decided that less-must-be-more. By reducing materials they could reduce costs and pass that on to the hobby market they are aiming at. Also of course, the cameras are far smaller than most of the APS-C and all the FF ones on the market. mFT cameras are sold at a relative premium price, in line with the second tier of hobby dSLRs from the main competitors, but standing as mirrorless, half way compact-dSLR cameras.
What Olympus and Panasonic, Leica too, enjoy in better margins from this less-is-more, they can use to consolidate their leading positions in this market niche, or you may argue actually that is is a small sector, definded as: advanced compact digital cameras of superior qaulity lenses (interchangeable for all at the momeent) which even pros use. This means the producers can use their margin to continually innovate and offer ranges of cameras to suit consumer desires and pockets.