The good anastigmat lens is supposed to be free from all the foregoing optical defects. It has remarkable definition — too much in fact. It gives a flat field, is corrected for color, and besides, has more speed. It is an outgrowth of the discovery that glass of different composition has a different refractive index. Quartz glass will not bend the light rays the same as crown and flint glass. By combining lenses made of the different glasses and properly setting them together we get the modern anastigmat. They are made ever many different formulas and are necessarily expensive. The best I have found are the Carl Ziess Tessar, the Bausch & Lomb Tessar, and the Goertz Dagor. It is also a fact worth remembering that the slower anastigmats are the freest from optical defects, i.e., these working at F 6.3 and F.77 have more covering power and cut sharper than those working at F 4.5 and F 2.9, etc.
This brings us down to the important meaning of the diaphragm markings found on every good lens. That do we mean when we say we have an F 4.5 lens? It means that the lens works at F 4.5 when the diaphragm is wide open. What does this mean? It means that when the lens is focussed on some distant object or infinity, the diameter of the diaphragm opening is 1 over 4.5 or 2/9 of distance from the lens to the plate. What good is it to know this? It indicates that F 4.5 represents the angle subtended by the diaphragm when it is wide open, or in other words, gives us a clue to the amount of light that will pass through it. Some system of marking the diaphragm stops had to be adopted by the lens manufacturers in order for everybody to compare the speed of any lens with another, and to be able to use any lens. If you stop any lens down to 11, 16 or 22, they are all supposed to admit the same amount of light at that particular opening, irrespective of make or size. This is very important in figuring exposures. There must be some standard of comparison, otherwise it would be impossible to take out a battery of different lenses and use them with any degree of accuracy. It might be well here to say that the light admitting power of these diaphragm stops can be compared, one with the other, inversely as the squares of the numbers themselves. That is to say that F 8 passes twice as much light or is twice as fast as F 11, which in turn is twice as fast as F 16 and so on down the line to the end, which is usually F 45 or 64. Now supposing we figure our exposure should be 1/50 of a second at F 8, you can see that the plate will be hit with exactly the same amount of light if we stop the diaphragm down to F 11 and give it 1/25 of a second. Here we must do some figuring. If we stop it down one more, to F 16, we must increase the exposure to 1/12 or 1/10 of a second. We cannot hold the camera in the hand at such a slow shutter speed without the danger of vibration, which will ruin the picture. To overcome this we must resort to a tripod or rigid support together with a cable release, It is oftimes necessary in pictorial work, especially when filters are used, to give an exposure of from three to ten seconds. This is where the graflex and kodak usually fall down. They are essentially hand and speed cameras and should be used as such.
What is the advantage of stopping a lens down, under its maximum speed? If my lens works at F 4.5, why not always use it at F 4.5? I can stop a tennis ball at F 4.5 in good sunlight in 1/1000 of a second. The tennis ball will be in sharp focus, but everything on the near or far side of it will be fuzzy or out of focus. I have speed and have stopped fast motion, but I have sacrificed depth in my picture. If one were photographing a field of flowers he would have a different problem. You want the flowers near to the camera as well as those further away to be in sharp focus. This is accomplished by stopping down the lens, sometimes to 32 or 45. Your lens passes very little light at these openings, so you can’t stop motion. And be careful that motion stops and the wind is not blowing, or you’ll blur your picture. Any ordinary kodak below post card size set at 25 feet, diaphragm at 32, will give almost universal focus. In this connection, it is well to state that the depth increases as the focal length of the lens decreases. It is the main reason why such marvelous effects are obtained with the small motion picture lenses. These diaphragm markings and their use should be thoroughly understood before taking up photography at all.
