Lenses. Part 3

Description

This section is from the "Naturalistic Photography For Students Of The Art" book, by P. H. Emerson. Also see Amazon: Naturalistic Photography For Students Of The Art.

Lenses. Part 3

Diaphragms

Modified Diaphragms

Some ingenious workers have suggested modifications in the construction of diaphragms, with a view to improving the picture; one of these being a paper diaphragm, made translucent with castor oil; but we have not found any advantage in these novelties. It is, however, a legitimate field for experiment, and translucent diaphragms might be tried in indoor work and bright out-door effects.

Intensity Of Lens

The student will often see in photographic papers that a lens works at -F/8, or F/32, or some other number. This simply expresses the ratio between the working aperture and the equivalent focus of the lens, and is obtained by dividing the equivalent focus by the work ing aperture. F/8 - then means the aperture is one-eighth of the focal length of the lens referred to. The rapidity of lenses are compared in this way by squaring the denominators of the fractions thus obtained; when the results will give the ratios of rapidity. By "depth of focus" is roughly meant the sharp rendering of the different planes of a landscape, or any object with more than one plane in one plane. Needless to say, this quality, greatly sought for in lenses by photographers, is a thing to be carefully avoided in artistic work, as we shall show later on.

Flare Spot

By a flare spot is meant a circular spot on the focussing screen, which receives more light than the surrounding field; it is said to be caused by the diaphragms being wrongly placed. The same effect is produced when the sun shines into the lens, the light being then reflected from the brass tubing of the lens, and it is for that reason that the lens must be carefully shaded during exposure, when the sun is directly in front of the camera.

Angle Of View

The angle of view included by a lens is an important consideration, and we shall refer to this later on; here we shall only show how this angle may be determined when the student wishes to do so. The angle depends on two factors, the length of the base line of the picture, and the focal length of the lens. This is practically determined by ruling a horizontal line the actual length of the base line of the picture, and drawing from the centre of this line a perpendicular equal in length to the focal length of the lens. Completing the triangle, we have in the angle contained by the two sides of the triangle the required angle, which can be measured by an angle measurer. Experience shows that if the base of the picture is greater than or equal to the focal length of the lens, the angle included will vary between 53° and 90°; but if the base is less than the focal length, these angles will vary between 44° and 19°, or less. It will be seen, therefore, that the long-focus lenses give more suitable angles of view for pictorial purposes.

Hints On Lenses

Delicate optical instruments, like lenses, must, it is needless to say, be carefully protected.

A good lens should be free from scratches, striations, dull patches, due to imperfect polishing, and veins; but air bubbles do not affect its value, for it must be remembered that the shape of the hole through which the light passes does not affect the image, save only by cutting off some of the light. Thus, if a wafer be stuck to the centre of the lens, the image will be found unimpaired. Dust and dirt, however, though they do not seriously impair the definition of the image, yet cut off much light, as will occur to any one when he thinks of the difference between the light of a room, when the windows are dirty, and when they are perfectly clean. Lenses should not be left in bright sunlight, for this causes a change that slows them, the dark also injures them in certain cases, for, as all microscopists know well, darkness causes a change in Canada balsam, with which lenses are cemented together.

Mr. Dallmeyer insists that lenses should be kept dry and free from sudden changes of temperature, otherwise they may tarnish or sweat, as it is called. Any one who has been troubled with this sweating will never forget it. Our experience is that the best way to keep lenses is in small leather, velvet-lined cases. We generally keep with them a piece of soft chamois leather, or an old silk handkerchief. No compound of any kind should be used to clean lenses, if anything appears to be going wrong with them, they should at once be sent to the maker.