A good way for a beginner is to make a general practice of focussing any object which he intends to telephotograph first with his ordinary lens. This will enable him to get a fair idea of the degree of magnification which will be necessary in order to get a satisfactorily enlarged image. If he is working with a 5 X 4 camera, and the object as rendered on the screen with a 6-in. lens is 1 in. long by 4/5-in. high, it is obvious that at 5 magnifications it will fill the plate, and that probably 4 magnifications at most will be required. Having in this way obtained a pointer to the " power " which will give the desired result, and decided that, say, 4 magnifications will do what is wanted, the procedure may be as follows. Unscrew the ordinary lens and screw it into the front end of the tele-mount or setting, at the rear end of which you have already screwed in the tele-negative - which we will assume in this instance to be of 2 1/4-in. focus - with the plane side outwards, i.e., so that it will come nearest to the plate. Fasten on the tele-hood, but do not at present fully extend it, and then screw the whole tele-lens into the camera front. If necessary adjust the unipede lens support, but not rigidly, in case the camera may have to be tilted.
Now multiply 2 1/4 in. (the focal length of your negative lens) by the magnifications less 1 (4 - 1 = 3) and this gives you your camera extension, namely, 6 3/4 in. Rack out your camera until there is approximately this distance between the negative lens and the focussing screen. For the present do not touch the camera further, but get your object in focus by adjusting the rack and pinion or other movement on the tele-mount. If the picture does not appear to be the right size you must alter the camera extension, racking in to get a smaller and out to get a larger magnification, and subsequently refocussing by altering the separation of the positive and negative elements, that is, by working the rack and pinion, or slide, or lever of the tele-mount as before. But remember that, if you alter the extension, you also alter the magnification and, if you have done this appreciably, you will have to allow for it later in calculating the exposure. But we will take it that you have got the size of the picture right, and in that case all you have to do is to focus forthwith very sharply with the rack and pinion or other adjustment and the focussing magnifier. Do not attempt to get the final focus by altering the camera extension. In order to get good definition throughout stopping down to f/11 or f/16 may be necessary. While focussing extend the tele-hood as far as you can without cutting off any of the picture, and, if you are using an orthochromatic screen, focus with it in position. Remember that with many lenses you cannot safely focus at full aperture and then stop down. Personally, I habitually focus finally at the aperture at which I am going to work. If the magnification is high, or the light bad, a little lanoline smeared on the focussing screen is a great help.
In my own practice I seldom work at any but fixed extensions, and I advise all who possess two or three tele-negatives of varying foci to do the same. With, say, three tele-negatives of 2 1/4 in., 1 3/4 in. and 1 1/8 in. focus respectively, adapted to a tele-setting for use with a 6-in. positive and a 5X4 " Una," a little table can easily be made, showing exactly what each will do at the single or double extensions of the camera, and it only remains to set up the latter at one or other extension and use the tele-negative which gives the required "power."