There is a great deal of beautiful sculpture, both of figures and foliage, in the doorways (especially in the tympana and archivolts) of many cathedrals and churches, and this can undoubtedly be best recorded by means of telephotography. Accordingly, if much detail work be contemplated, a telephoto lens will form a very useful addition to the kit. The simplest and best way is to get a telephoto attachment (or negative lens with rack and pinion) fitted to the lens (positive) that is commonly used with the camera. For architectural work, a negative lens of about half the focus of the positive lens should be employed, and the two combined will form a telephoto lens of medium power. The chief point to remember in detail work with the telephoto lens, when the subject is above the level of the camera (which is nearly always the case), is that, when tilting the camera the back should not be kept vertical, as when using ordinary lenses. At all times, when only slight tilting of the camera is necessary, we strongly recommend that the back of the camera be allowed to tilt with the rest of the instrument. It will be found that this method gives good definition all over the plate at a fairly large aperture, and convergent distortion is negligible. Even when the tilt is considerable this may be the better way, in order to ensure good definition at top and bottom of the plate, and the distortion can be subsequently corrected when printing. If the back be kept upright when tilting the camera, the vertical lines of the subject will show divergent distortion in the photograph, the cause of which has been explained by Mr. A. Thomas as follows : "The reason of this is the alteration of the angle of the cone of light in the camera, through the magnification of the image, without a corresponding increase in the extension of the camera."
In practice we have found the following rule give results showing no appreciable distortion. Let the back tilt with the camera and, when the subject has been focussed, measure the camera tilt with a plumb line and protractor. Find out the proportion of the complete focus of the combination which lies in front of the negative lens, and set the back at an angle of the same proportion of the full tilt from the vertical. For example, suppose a 6 in. positive combined with a 3 in. negative attachment is being used at 3 magnifications, and the tilt of the camera from the vertical is 250. The focus of the combination is 18 in., and the camera extension) from back lens, is 6 in.; thus the proportion of focus in front of the back lens is 2/3. The tilt of the back from the vertical should then be 2/3 of 250 - namely 160 40'.
A tilting table, although convenient, is by no means necessary in most telephoto work, as the required tilting can be done with the tripod, provided one of the legs be set in the centre at the rear, instead of in the more usual position in the front.
When the subjects being photographed are not well lighted, focussing is sometimes difficult, as, even at fairly low magnifications, the tele-lens admits comparatively little light. In such cases a good focussing magnifier, used on a part of the screen, which has been rendered clear by cementing a piece of very thin glass to the ground side of it with Canada balsam, will be a great help; but the magnifier must be accurately adjusted.
E. A and G. R. Reeve.
When photographing details at the back of deeply recessed porches, what lighting there is is necessarily direct. This tends to flatness and lack of relief. The defect can be remedied to some extent by strengthening contrast, either by intensification of the negative, or by using a strong printing medium, such as gaslight paper.