[MUSIC] Welcome back. We're going to look at that strange word normal, and put it in the context of focal lengths, so let's get going. Normal. Now, there's a word that we use quite a lot In reference to a lot of different things. We've got normal temperatures, normal music, normal food, normal manners, and many other things that we characterize with that word. In those descriptions, we're using the word in a subjective manner. What might be normal music to my ears, might be very abnormal to you. When it comes to lenses however, the word normal has some very specific meanings. They mainly have to do with things that we can see. With the size of our cameras and sensors and with just a little bit of math. The human eye is a remarkable organ, one that we can think of as a camera obscura made of living human tissue. Like a photographic camera, it is a dark chamber with a light sensitive surface on the back and a lens on the front. On average, the focal length of the pupil, the lens, of most of the eyes of mature adult humans is about 17 mm. Matched with the size of the rods and the cones which are the sensors in the eye, that 17 mm focal length creates a picture that renders space in the way that we humans all perceive to be normal. Things that are three feet apart or one meter apart, look three feet or one meter apart in the picture in your mind, just as they do in my mind. If we could insert a longer focal length pupil, our eyes would act like a telephoto lens compared to normal. Hawks and eagles have longer focal length lenses than would be normal and thus their vision is more telephoto. We introduced this concept in course one and you may remember Rosie, who was are subject for those examples. Here she is with a diagram showing how a normal lens might render her image on the sensor at the focal plane, when she was at a given distance from the camera. A telephoto lens is one that has a longer focal length, than normal, and objects at any given distance will appear larger at the focal plane. And you know a wide angle lens is one that has a shorter focal length than normal and objects at any given distance with that lens will appear smaller at the focal plane. I'm sure you remember those introductory concepts. We're going to add to that knowledge and actually go further into the effects of focal length changes on perspective in the next few lessons. But let's define what is normal first. The traditional definition of a normal lens involves another important word. And that word is perspective. By perspective, we mean that the manner in which an image renders relationships between objects that are at near and mid ground and far distances. A normal lens renders those relationships of space normally, and best from a normal vantage point. In the same way as the light passing through that 17mm focal length of our pupil creates an image on the rods and cones and sensors, so to speak in the eye, and how that renders space. In this portion of the painting, Kublai Khan on the Hunt, painted in 1280, we can see a very different approach to perspective. Notice how the figures in the mid ground are so of very much larger, both the horses and the people, than those in the foreground. Were they really bigger, or was there a very different purpose like making an important man appear clearly important. Could that be done today without the artist seeming to be untrained, now that the perspective rendered by the camera is so much a part of our DNA? In this photograph made with a normal focal length for the camera, the relationships between the children and the background are rendered in a manner very similar to the way our mind's eye would perceive them. With the telephoto lens, see how child and trailers look closer together. You'll also remember that whether any particular focal length lens is considered normal or not depends on the size of the sensor. For a full frame sensor, the normal local length is said to be 50mm. For a 27 by 23mm sensor It's 30mm, and for a four-third sensor, it is 25mm. Here's another example you have seen before, as we laid the groundwork in course one. It's a photograph made with a full frame sensor and a 50mm lens. This would be the effect of using the same 50mm lens but a smaller sensor, the APS-C, creating a telephoto effect in making the subject larger. If an even smaller sensor, a fourth-thirds size, had been used, the 50mm would be even more telephoto, more enlarging in its effect. In general, the normal focal length lens for any film or sensor size can be determined by simply taking its diagonal measurement. In this chart, you'll see the diagonal measurements for some standard film sizes and in all instances the focal length lens accepted as our normal is exactly that focal length. The 6 x 6cm has a diagonal of 80mm. 4" x 5" film has a diagonal of 150mm and 8" x 10" has a diagonal of 300. 35mm has a diagonal of 43mm. Now, the only focal length that you might recognize as a bit off, in terms of what we associate with the film size, is the 43mm lens as normal for 35mm film. While that lens is mathematically more normal than a 50, it is the standard 50 that, through tradition, has become the accepted norm. Knowing the normal focal length for your particular camera sensor is a very valuable bit of information which of course you can only truly know if you have the information on the size sensor in your camera. If you don't know the focal length if it is normal for your camera you also do not truly understand what focal lengths are shorter and thus wide angle. And what focal lengths are longer and thus telephoto. Knowledge such as this, is essential for purposeful control if you are going to take your photography to the next level of excellence. There's one more thing to note. It's important to understand that when we refer to normal lenses we're not talking about the angle of view, also known as the field of view. We're not talking about a lens that frames in the same sweep of area from left to right, that our eyes take in. The horizontal angle of view of the human eye that is, left to right is approximately 114 degrees. As an example of the difference between lenses and eyeballs, the angle of view of a normal 50mm lens with a full frame sensor is only about 40 degrees. The normal lens of the camera covers far less of that horizontal view, about 75 degrees less than the human eye. Don't let the fact that we see a greater angle of view with our eyes than the normal lens does, confuse you. It has nothing to do with the main point, and that is the appearance of near, middle, and far space. Now that we know what normal is, let's move on to differences. [MUSIC]
