The Kodak Salesman . G and othersimilar pairs of rays will cross ona line through G which is at rightangles to the line through E. Lithis way the whole bundle of ravswill, after refraction, pass throughiwi) mnlually perpendicular lines atI and G >o that the best image ofI he ])()int T will be between E andCi. Tliis image will be an areawhich, at E, is reduced to a lineand at G is reduced to another lineat right angles to E. If we are photographing from aheight an area on the ground, let usimagine for the moment that a net-work of the sort shown in Eig. 11 islaid over this area. If the lens
The Kodak Salesman . G and othersimilar pairs of rays will cross ona line through G which is at rightangles to the line through E. Lithis way the whole bundle of ravswill, after refraction, pass throughiwi) mnlually perpendicular lines atI and G >o that the best image ofI he ])()int T will be between E andCi. Tliis image will be an areawhich, at E, is reduced to a lineand at G is reduced to another lineat right angles to E. If we are photographing from aheight an area on the ground, let usimagine for the moment that a net-work of the sort shown in Eig. 11 islaid over this area. If the lens weare using possesses astigmatism toan objectionable extent, lines on theground (edges of buildings, roads,etc. ) which lie along the radial linesin the network and those which liein the direction of the circular lines,will not be focused at the same dis-tance from the lens. This effectarises because of the astigmatismdescribed above. Lines lying inother directions will not be sharplyfocused anvwhere. We can so focus. Eig. 11 ^ KODAK SALESMAN \ £a&itu —^ 77 / V / astig nafisr o 1 24 „ -^ / -Veryash rapidi)jmarij increin. SSIf^ i Mode 21 0 ^ le ? / W 18 » / / / IS / / 12 / S \ 1 C 1 301 r idrl Axis( f IbuS FittlH CllKV ft 6 9 li / 15 \ 18 / 21 \ /Nod . \ \ i Zt V Vn xJe 27 Hawk Eye/ ,erial F/6 - \ Fiff. 12 tlie lens that radial lines are sharpor we can focus it so that tangen-tial lines will be sharp, but we cannot make both sharp at the sametime. Probably the best compro-mise is at an intermediate positionwhere both are unsharp to the samedegree. By properly combining sim-ple lenses into a photographic ob-jective, the optician can go fartoward eliminating this astigma-tism. Lenses in which this has beendone are called anastigmats. 6 In examining a lens for this de-fect a ])oint source (jf light is set upin front of it so that the rays pass-ing through the center of the lensmake angles of 3°, 6°, 9°, etc., witlithe axis. Fig. 8 (See April num-ber). The images of
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Keywords: ., bookcentury1900, bookdecade1910, bookidkodaksalesma, bookyear1917
