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.
Angle Of View. The question of angle of view has much to do with what has previously been said regarding the focal length of the lens; yet it is of sufficient importance to consider it separately. The angle of view of the resultant picture is determined by the focal length of the lens, compared with the size of the plate upon which it is to be used. The angle of view of the lens means the greatest angle that this particular lens is capable of giving when used upon the largest plate it will cover. Exactly the same results will be secured on a 4 x 5 plate when using a narrow angle lens of 5 inch focus, as a wide angle one of the same focal length. However, a wide angle lens of 5 inch focus may be stopped down to cover sharply to the edge of an 8 x 10 plate, or even larger.
A narrow angle lens of the same focus, regardless of stopping, could not be employed for any larger plate than it was originally intended to cover.
Dust. As dust causes pin-holes on the negative, you must be sure that your camera, plate-holders and carry-case have been carefully dusted. In this way you will save yourself a great deal of time and secure much better results.
163. In the majority of cases halation is regarded as a fault, and one that needs correction at any cost, but we disagree with that class of workers who hold rigidly to this belief. We do not, of course, advocate the wholesale spread of bright light that is bound to be suggested in bad cases of halation when photographing certain difficult subjects, but that a small amount of halation is perfectly natural and necessary for pictorial purposes cannot be denied by any one who uses his eyes and observes closely the phenomena of nature. Let any observer, for instance, look at the bright sky through a net work of fine branches or foliage. Not only is the spreading of light in the margins of the shadows observable with the eye, but this encroaching on the shadow outlines appears perfectly natural and correct. Why, therefore, every endeavor should be made to make this different in the rendering of the subject by photography, it is difficult to say. Halation is, to a certain extent, perfectly normal, and the lens sees no more than the eye. The dry plate, however, occasionally exaggerates what the lens sends to it, and it is for the correction of this exaggeration only that steps should be taken for dealing with halation.
164. If you will take a piece of ordinary glass, perfectly clean, and hold it at an angle so that the reflected image of a bright object is seen on its surface - the flame of a candle for instance - it will be observed that there are two images present and they do not coincide. (See Illustration No. 25). The brighter image is the reflection from the top surface of the glass, and the secondary image is reflected from the back surface. The distance that separates these two images depends on the thickness of the glass and the angle at which they are received. It follows, therefore, that the bright parts of the image projected onto the plate, after passing through the film, are reflected back onto the film again, and fog it. As this fog is not clearly defined it produces the effect called halation, and when a bright light strikes near the margins of a plate, this spreading of the light is generally worse, as the angle is greater.
165. Theoretically, of course, halation, under these circumstances, should not occur if the bright light is in the center of the plate, i. e., directly opposite the axis of the lens. There is, however, another form of halation, that also manifests itself in the modern dry plate, and one that no amount of backing can remove, and that is the lateral spreading of the light in the film itself, or irradiation. This will appear even in the thinnest film.