We compare the size of the film and the focal length so that we can predict if the
Wide Angle Lens
When the diagonal measurement of the film plane is greater than the focal length. On a standard 35mm camera, lenses with the focal lengths below 40mm would be wide angle. These wide angle lenses are also called “fisheye” lenses because of the effects they create of displaying more than the human eye would see.
When the diagonal measurement of the film plane is equal to the focal length. On a standard 35 mm camera, lenses with the focal length between 40mm and 50mm are considered “normal”. These lenses create about a 47º field of view, the same as your eye.
Digital Focal Length Note
For the digital photographers out there, you should know that your CCD sensor is much smaller than film, but don’t worry. Digital Lens manufactures have made it standard to have 50mm lenses look “normal” although the focal lengths on those lens’s isn’t really 50mm. The CCD diagonal measurement is closer to 10mm, and each one is different. That’s why you see that your 35mm aren’t typically compatible with your digital SLR and digital SLR lens’ are made for specific camera models.
When the diagonal measurement of the film plane is less than the focal length. In this case 50 mm was called normal because the picture looks neither zoomed in or zoomed out. It looks just as it would if you were standing there, a field of view similar to human vision. You’re question is really so good it requires a whole new article that helps us understand focal length using a 35mm camera. Just like the one that took these pictures.
Arrgh! Not the old 50mm lens on 35mm camera = human eye field of view myth. There are plenty of articles out there explaining why this isn’t true.
Thanks for the additional information Matt. I agree that what you see through a lens isn’t ever what you see through your eye, that’s what makes photography awesome. However, I’m going to stand by the facts in the article, much of which is confirmed by the link you shared. Such as the diagonal size being about 43 mm because the height of the film is 35mm. And what we “see” is around 50mm. Clearly our depth of field is much more dynamic than most lenses and we focus on just small sections at a time, but the images have roughly the same perspective through a 50 mm lens as our vision or as your link tells us images look more “even” through a 50 mm lens.