The lens for the photographer who has plenty of money to spend on his hobby is an anastigmat which will cover sharply at full aperture a much larger plate than that for which it is sold. As an example, it may be of interest to know that a 6 in. anastigmat can be obtained which will cover a 10x8in. plate at full aperture, viz. f6.3. Now a lens of focal length 6 in. is normally used on a 5x4 in. plate, so that the aforementioned lens would permit the maximum use of the rising front when used with a 5 x 4 in. camera. In cameras made with the so-called "universal" movements such a lens is essential, if good use is to be made of these movements. The maximum rise of front can be used at full aperture, and this is extremely useful to the architectural photographer where a stand camera cannot be erected on account of traffic, etc. It may be objected to the use of such a lens that a large part of the light it transmits is incident on the interior of the bellows from whence it is reflected to the plate. If this occurs it can easily be remedied by using a hood on the lens; but, care must be taken to see that no part of the picture is cut off by it. Caution is especially necessary when using the rising front. Hoods are made which are adjustable in length, and though expensive are undoubtedly the most useful, since their length can be adjusted to suit the amount of rise of front in use. A hood can be made very simply from an old pill box by knocking out the bottom, and giving the interior a coating of dead black. A 3 foci anastigmat used normally so as to include an angle of view from 500 to 60°, but which may be used equally well to include an angle of view of 900 is the best lens for all-round work, and all who can afford such a lens should purchase one. Such a lens is very useful to the worker possessing more than one camera, as he can use the lens as a wide-angle lens on his large camera, and as a normal-angle lens on the small one. In general, bear in mind what work the lens is required for. Do not get the 3 foci anastigmat if fuzzy pictures are your only desire.

Focal Length

When the type of lens is decided, the next thing to settle is the focal length. As the focal length increases, the perspective becomes more pleasing, and it has been found that a focal length of from one-third to one-half as long again as the longest side of the plate, is about the most useful length for all-round work. If the lens is to be bought new go to a reputable optician, and he will probably give you some good advice as to the make to select. Lenses by reputable makers are early all of good quality; but, some makers have a reputation for better workmanship. When the lens is obtained the actual focal length should be determined in the already explained. The effective aperture should also be determined, and it there is much discrepancy between the marked figure and that obtained by experiment, the lens should be returned, as it is worth considerably less than the price paid for it. The lens should also be tested for flare in the following way. The lens is taken into a room illuminated by a single candle, and the image of the flame sharply focussed on the centre of the focussing screen. The camera is now rotated so that the image of the flame moves towards the edge of the screen. If flare is present a bright spot will appear on the opposite side of the centre of the screen.

Testing Second-Hand Lenses

When buying a lens secondhand, greater care is necessary, and the lens should be obtained on approval before it is purchased. Even if the lens is marked with the name of a reputable maker, it should still be tested thoroughly, for unscrupulous dealers have been known to engrave cheap unknown lenses with the name of a good maker. The lens should first be tested for colour by looking through it at a piece of white paper. If there is a yellowish tinge, the lens should be rejected, as such a colour makes a considerable difference in the efficiency of the lens. The lens should then be tested for flare, and the above-mentioned test will also show whether spherical aberration is present. If it is, the image of the flame will not remain sharp as it approaches the edge of the screen. The next test is for chromatic aberration, and perhaps the best test is the following, recommended by the late Traill Taylor. A row of numbered cards is placed co-axial with the lens and at a distance of a foot or so apart. From the camera the cards appear as a fan, and the lens is sharply focussed on, say, number 8. A plate is exposed, and if on development, card No. 8 is not sharp, the lens is not corrected for colour. If the lens is alleged to be an anastig-mat, a further test is necessary. An object should be focussed at full aperture, and the camera rotated until the image of the object approaches the edge of the screen. If both vertical and horizontal lines are still defined sharply the lens is free from astigmatism. If they are both blurred, or one set sharply defined and the other set blurred, the lens is not free from astigmatism, and should be returned or a considerably lower price paid for it.