The angle of view of the human eye (about 51 degrees) it’s called the normal angle of view.
Since I know how controversial this topic is I would like to point you to my sources at the bottom of my reply. I have been studying the technical aspects of photography and optics for more than 20 years and am not making anything i am going to write up. It is all thoroughly based on the current scientific view.
Contrary to a lens made of glass our eye contains a flexible lens
situated behind the iris.
This results in a variable refractive power
of 19 to 33 dpt for a healthy and young human. Additionally the light is refracted by the retina
, the anterior chamber
as well as the vitreous body. The whole system has a refractive power of 58 to 70 dpt, which equals a focal length of about 14 to 17 mm.
Therefore the maximum field of view looking straight ahead is roughly 95°. In 35 mm film terms (full frame sensor) this would mean our eye has the field of view of a 20 mm lens (when we are young). Since the iris can vary the aperture between 6.5 and 2.5 mm our eye has a variable focal ratio of f/2.4 to f/6.4. This conclusion should already show you that a "normal lens" has nothing to do with the field of view of the human eye and as I am going to show the term has solely optical relevance.
Normal focal length
In photography, the term “standard or normal lens” is understood to mean a lens with a focal length about as long as the diagonal of the image field (field size). Its purpose is to simply describe the characteristics of the lens's focal length on the used format, which boils down to the corresponding field of view. A lens that produces a wide field of view (wide-angle lens) has a focal length significantly shorter than the diagonal of the film format. If it is about the same as the long side of the format, the lens is considered to be a moderate wide-angle lens. Super wide-angle lenses are those with focal lengths between the length of the short side of the format and half the diagonal. Those with even shorter focal lengths are often referred to as extreme wide-angle lenses, though the delineation between “super” and “extreme” is fluid, of course, and to some extent a matter of taste. Long, normal and short focal lengths simply describe the field of view of a lens on a specific format relative to the format's diagonal. Basically it is just a way for manufacturers to put their lenses into understandable categories.
Of course, this implies that wide-angle lenses supposedly produce images that are perceived to be wide, but this is not necessarily the case. Human perception is quite a complex thing and even differs between genders. I would like to use following image to illustrate it a little more.
Le sentier de grande randonnée 10 (GR 10), Leica M9, 24 mm Summilux
Although it was taken with a wide-angle lens (24 mm on a full frame sensor) it is what I like to call a natural appearing landscape. I found the scene worthy a photograph at quite the same point of view and it represents pretty much what I saw. A normal lens would have cropped the image way too much to represent my view (the 24 mm lens was the only one I was carrying anyways). This is a topic I have talked to many fellow photographers about and many agree that wide-angle lenses can produce quite ordinary looking images. As photographers our job is to find a subject, a point of view, the best light and crop the image with a lens to come as close to what we are seeing with our "inner eye". Human perception can make an image that was taken with a long focal length look just as normal as one taken with an extremely wide lens. In studio work, for example, we often tend to use longer lenses to get more appealing images.
So why did the engineers decide to draw the line between wide and long right at focal lengths equal to the diagonal of the format? It has mainly to do with what is known as the normal viewing distance
for a print, which is about the diagonal of the print. Sounds familiar? The reason is a human limitation: the closer you are to a subject the more your eye literally focuses on detail. So if you were standing too close to a print you would not be able to see it as a whole. If you were too far away the image would get lost in its surroundings. Also don't forget that photography is always about enlarging the original image by reducing the resolution and after further fiddling with it displaying the result (print, screen etc.).
So let's return to the technical aspects. If you are not into mathematics just skip to Misconceptions
down below. The drawing above shows the light cone created by the lens and projected onto the film/sensor. Usually this projection has the shape of a circle and is known as the image circle. To cover the entire film the image circle's diameter has to be at least equal to the diagonal of the format (d). Traditionally this is defined by what is known as the angle of field (ω), which is exactly half the diagonal angle of view of the film format.
tan(ω) = d/2f
<=> ω = arctan(d/2f)
The diagonal field of view would have to be 2 * ω or 2 * arctan(d/2f).
Now what angle of view would a lens have to create to reflect the one from looking at the print from the normal viewing distance (diagonal of the print)? Through similarity of triangles we conclude that in this case f = d and therefore:
Normal field of view:
2 * arctan(d/2f) = 2 * arctan(d/2d) = 2 * arctan(1/2) = .93 rad = 53.13°
So this is where the approximately 50° for a normal lens, as Radu quoted, come from. Again, a normal or standard lens has nothing to do with representing the human field of view – it is merely a technical term for lenses where f = d.
It is determined by what thinkers found to be the normal viewing distance centuries ago.
1) A normal lens produces about the same perspective as the human eye.
Lenses do not determine the perspective. Only the position of the lens's nodal point relative to the photographed subject as well as a potential swing/tilt of the film plane affect the perspective. The closer you move the camera to the subject, the extremer your perspective. Vice versa, the farther away from your subject, the more you'll find the perspective to be compressed. Photographed from the same point of view a normal lens has exactly the same perspective as an ultra wide angle lens.
2) A normal lens has the same magnification as the human eye.
Magnification is solely determined by the size you print your image or display it on your computer screen and your viewing distance. Consider this example: You take a picture of a coin with a magnification of 1:1 (1 mm on the coin is projected as 1 mm onto the film). The image is then printed 100 metres wide. If you were standing right next to the print you would see a much larger magnification than 1:1, but from three miles away it would be quite the opposite. If you are arguing that the magnification of a normal lens is about the same at hyperfocal distance (far away objects) I'd urge you to take a picture with your standard lens and look at it from a normal viewing distance. As mentioned above our eye is more of a "wide angle zoom" – especially at hyperfocal distance.
3) Standard lenses are the easiest ones to design.
To some degree this is actually true. Moderately longer lenses often involve less complex lens designs and extreme wide angles are the greatest challenges for optical engineers.
Nevertheless many renowned photographers claim that normal lenses are the ones that come the closest to reproducing natural looking images in terms of magnification and perspective. One could argue for years over this extremely controversial topic. Personally I'd say there is some truth to it since it is all about perception anyways. What I tried to clarify simply is the technical aspect, although some might have a different opinion on that as well.
Applied Photographic Optics,
Sidney F. Ray, http://www.amazon.com/dp/0240515404
A History of the Photographic Lens,
Rudolf Kingslake, http://www.amazon.com/dp/0124086403
Lens Design Fundamentals,
Rudolf Kingslake, http://www.amazon.com/dp/012374301X
Distagon, Hologon, Biogon,
H. H. Nasse, http://blogs.zeiss.com/photo/en[...]Distagon.pdf
Experimentalphysik 2: Elektrizität und Optik,
Wolfgang Demtröder, http://www.amazon.com/dp/3540682104
Grundlagen der Optik in der Fotografie,
Jost J. Marchesi, http://www.amazon.co.uk/dp/3933131626