Fig. 6.
The best way to mark stops is, for example, f/8 and f/16, as these expressions are universally understood, but some persons think that the relative rapidities or intensities are better, others prefer to express the relative exposure necessary, and every system of numbering on these plans has a unit which is merely empirical, not one of them adopting the only true or scientific unit of f/1.
Zeiss has recently changed his unit from f/100 to f/50. Dallmeyer marks some of his lenses now with the practical expression. The following table may be of service to those who happen to have lenses with their diaphragms marked on any of these empirical systems.
f/ | Royal Photo- graphic Society. | Dall- meyer. | Paris Con- gress. | Zeiss (old). | Zeiss (new). | f/ | Royal Photo- graphic Society. | Dall- meyer. | Paris Con- gress. | Zeiss (old). | Zeiss (new). | |
| 3.16 | 1 | 1/10 | 18 | 32 | 8 | |||||||
| 3.2 | 1024 | 256 | 20 | 40 | 4 | |||||||
| 4 | 1 | 22.36 | 50 | 5 | ||||||||
| 5 | 2.5 | 1/4 | 22.6 | 32 | ||||||||
| 4.5 | 512 | 128 | 25 | 16 | 4 | |||||||
| 5.66 | 2 | 27.36 | 75 | 7.5 | ||||||||
| 6.3 | 4 | 4/10 | 256 | 64 | 31.62 | 100 | 10 | |||||
| 7.07 | 5 | 1/2 | 32 | 64 | ||||||||
| 8 | 4 | 36 | 8 | 2 | ||||||||
| 8.66 | 7.5 | 3/4 | 38.7 | 150 | 15 | |||||||
| 9 | 32 | 44.72 | 200 | 20 | ||||||||
| 10 | 10 | 1 | 45.2 | 128 | ||||||||
| 11.3 | 8 | 50 | 250 | 25 | 4 | 1 | ||||||
| 12.25 | 15 | 1.5 | 54.77 | 300 | 30 | |||||||
| 12.5 | 64 | 16 | 63.25 | 400 | 40 | |||||||
| 14.14 | 20 | 2 | 64 | 256 | ||||||||
| 15.81 | 25 | 2.5 | 70.71 | 500 | 50 | |||||||
| 16 | 16 | 71 | 2 | |||||||||
| 17.32 | 30 | 3 | 100 | 1 |
Image Angle.—The image angle represents what is called covering power. It may be expressed in terms of the focal length, and doubtless this is the best method, but it is not customary. It may be expressed as an angle, the angle formed when a line is drawn from each extremity of a line equal to the diameter of the circle covered, and caused to meet at a point distant from the base line equal to the focal length of the lens. The angle where the two lines meet is the image angle. But generally the covering power is expressed more roughly, as the ordinary size of the plate that sufficiently good definition can be obtained on.
Tele-Photographic Lenses.—If a negative (or dispersing or concave) lens is introduced between the ordinary lens and the plate, the equivalent focal length of the arrangement is greater than that of the ordinary lens alone, but the length of camera necessary is not proportionately great. It is possible therefore to obtain an image of a size that would otherwise require a lens of long focal length and a corresponding and perhaps impossible length of camera. But this is not the only advantage, for if the ordinary lens and the negative lens are separable to a variable extent, the amount of magnification of the image, or increase in the equivalent focal length of the optical system, is adjustable at will. For further details concerning tele-photographic lenses and their use, reference should be made to Mr. Dallmeyer's pamphlet on the subject.
There are two other subjects connected with the production of images by photographic lenses that must be referred to, though neither of them is of great importance if we exclude the use of hand cameras (which are separately treated of) and bear in mind the ordinary practice of to-day. These are depth of definition and the distortion due to the use of single lenses.
Depth of Definition.—It has already been shown that the action of the lens is to bring to a point in the image all the light that falls upon it from the corresponding point of the object. Now it is clear from fig. 2 that, if different parts of the object are at different distances from the lens, and this must be the case with solid objects, these different parts cannot be in focus at the same time. Still it is possible to get them so nearly in focus that the result is serviceable, and the ordinary method of doing this is to examine the image on the ground glass, and if the whole subject is not sharp enough, to reduce the size of the aperture. Depth of definition is increased by using a lens of shorter focal length or by reducing the aperture. If a large aperture has to be used, the focal length must be short if much depth of definition is wanted, or conversely, if the focal length must be long the aperture must be small. It follows that very rapid lenses that have a very long focus are of no use, for in portraiture, for example, this combination of properties would lead to the ear in the image being fuzzy if the eye was sharp.
If a lens were perfect and had a flat field, the depth of definition would depend only on the aperture and focal length. But if the lens gives inferior definition towards the edges of the field, it is quite obvious that there must be less depth of definition there, if a minimum of defining power is accepted. The definition at its best may be inferior to the minimum accepted and then obviously there is no depth. Depth of definition therefore at the centre of the plate depends entirely on the focal length and aperture, but away from the centre it depends also on the quality of the lens, and is much greater in a flat field anastigmat than in a lens of an older type. But depth of definition is not a quality apart, it depends entirely upon other factors, and it is better in examining a lens to determine these factors separately rather than to lump them together as depth.