This section is from the book "Complete Self-Instructing Library Of Practical Photography", by J. B. Schriever. Also available from Amazon: Complete Self-Instructing Library Of Practical Photography.
Size Of A Lens. Some beginners think because a certain lens includes more of a subject on the plate than others that a larger and more bulky instrument must be employed. A student once expressed himself that he could not use a wide-angle lens on his camera, as the regular lens he had was so large he could hardly get it on the front board; and as he knew the wide-angle lens would take in more of the subject, he thought it must, of necessity, be a larger and bulkier lens.
660. What is Covering Power? - The lens as ordinarily fixed to a camera produces an image sufficiently large to cover the whole of the plate, or ground-glass. That is, the purchaser of a cheap 3 1/4 x 4 1/4 camera, on focusing a view, finds that the picture extends to the corners of the ground-glass focusing screen. Under normal circumstances then the covering power of the lens is sufficient. Suppose, however, that you take this lens off and attach it to a 12 x 15 camera. An enlarging camera will do if you cannot borrow a landscape camera from a friend for the experiment. The front of the camera will, of course, be only about 5 inches from the ground-glass when the picture is sharp, and you will find the image you get is circular, being about 7 or 8 inches in diameter. At the edges of the focusing screen you will secure no image at all, and, therefore, no light.
661. Now you have some indication of the covering power of the particular lens you are experimenting with. It will cover a 3 1/4 x 4 1/4 plate, but not a 12 x 15. If you will carefully examine the disk, or round image, on the large focusing screen, you will find the definition is much better in the center than at the edges. You will also notice that the image is brighter in the center than at the edges. The definition falls off towards the edges of the image, because the lens is not corrected for astigmatism and curvature of field. If you will stop it down to F. 16 or F. 22, a reasonable amount of sharpness of definition will be produced up to the edges of the image. But, the stopping down will not increase the size of the disk, or, in other words, the circle of illumination. You will also find upon carefully examining the brightness at the center and on the edges of the disk, that much greater equality exists. Why is this?
Equality Of Illumination. Open the lens to the full aperture, for instance F. 8, and removing the ground-glass place your eye - one eye only, closing the other - opposite the lens. You will observe from this position that the full aperture appears as a circle. Now move the eye slowly towards one corner of the camera back and notice what change takes place in the apparent shape of the lens aperture. The circular shape gradually changes to an ellipse, which soon, owing to the ends of the lens barrel interfering, becomes narrower and more elongated, until before the eye reaches the corner of the camera back no direct light can be seen at all. The quantity of light received by any portion of the plate is, of course, proportional to the size of aperture as seen from that portion. When the eye can see no direct light, you have reached that part of the plate onto which the lens projects no image. Now repeat this operation when the lens is stopped down to F. 22, and it will be found that the eye can be moved a considerable distance from the center towards the corner before any part of the small circle of light seen through the F. 22 aperture is obstructed by the mount.