Show rI f 4 i 4 i r tt t 1 l I ITU TU i Study 4 Copyright 1119 by Seymour Eaton 4 t Directed by Prof Seymour Eaton Baton 4 c cof r 4 of I I i PHOTOGRAPHY PHOT GRAPHY GRAPEY FOR AMATEURS H ITHE THE PHOTOGRAPHIC LENS LENSBY LENSBY LENSBY BY L 1 B ELLIOTT Roch Rochester ter N Y Every one Is te more or leas lom familiar with the action adon of alens a lens s e yet an air alt altof of mistiness always to surround this most moat essential part of or the outfit The fundamental prin principles on which lenses are and which govern goern their use Ue ar are not at atall atall atall all difficult to understand The of ot a lens leas Ie In forming a picture depends upon the fact that right light travels m fir m straight lines JInes until It meets with t h sn n obstruction and ald It if the we obstruction is hi isone one through which light can enn pass passI i e e transparent the ray will be bent out of HB its COUrse t while passing pa ng through the substance and again diverted when wh n it emerges Into the air This ThiB phenomenon 7 I Pig Fig 1 constitutes ro refraction and is Es easily demonstrated by the familiar experiment exp rl ment fluent of ot placing a pencil in a glass of ot water and viewing It from above abO and from the side aide In Iti either case the straight rod appears to be bent In re reality reality of course only the rays of light reflected by the pencil have been bent by the water Had the he glass been filled with some fluid lIter than water such uth u h as ss alcohol the pencil would have appeared ippe red lean lessi bent while had the sub been heen more denee dense for tor Instance chloroform It would have apparently ii L Fig Tig 2 been bent more thus It Is that the he greater the difference between n the thin ddn sity of the air a d that of the refract lug ln substance e the tha greater the amount of or refraction opticians ana advantage of this fact in the construction construction tion Lion of lenses len e combining con dense and light glasses la es to give the greatest amount of refraction with the least die dis dispersion pension of ot the light lIht into its primary colors The subject of oC dispersion is isone isone Isone one of great interest to the photographer photographer but one which would require a treatise in Itself to explain thoroughly hence we shall shaH content ourselves with the Statement that if objects are viewed through a priam they appear fringed d with colors Tf j game same te hi true to a less lea extent if l an ordinary strong magnify magnifying magnifyIng ing lug lens Is substituted for the prism Ware these color fringes present even evenIn evenin In the smallest smalle t degree in the e image formed by the photographic lens they would cause indistinctness and render the lens useless If a ray of ot light be allowed to PAM pass through a 8 prism It will be found to have Dean Been by the prism m In such sue a manner that it is bent toward Fig 3 the thick end See figure 3 A lens Is is in effect eff two prisms pr with their I Fig 4 bases or r their edges together ether to astha as tM the case may be If It I the bases ba are placed d together rays of light passing through Pig Eg 5 each prism would cross crose at ac some point while In the other case the rays would continually diverge See Figs 4 and S 5 SIf L LIf If It the of the prisms were curved instead of flat fat their effect on light passing through them would not be l altered 1 Va therefore have Ve in inthe the th e example ithe basis of the two principal bO or o lenses n es the tile convex 7 0 Bg 6 converging tIlt the concave Mn o eave ave di diverging diverging diverging verging lens See Sec first and fourth lenses shown in Fig a 6 aThe 6 The Thu simplest St form of photographic lens ens ia ts a single convex lens A convex G ex lens will cause the rays of light coming from an object or group of objects to converge until at a fixed Axed distance they form a sharp clear eler image e This dis din distance distance tance Lance is hi called the tIre focal length of or thelen the lens len and Is greater or teas lens according to the curvature of the surfaces of the lens being shorter in a lens having strong strung curves and longer l when the curves are weak If we were to exam examine examine ine Inc the whole of the picture formed by b such auch a 3 single lens IeM It would be he found that while the circular image Ima is IcS quite large only a small portion in the cen ron center center ter would appear sharply denned defined while an amount of distortion would appear from front the center outward While no photographic lens lang has ever yet been made which would give absolute absolute absolute lute sharpness over its entire image circle the modern designed by b Professor Abbe of Jena very nearly do so It will wm thus thins be seen that one of the most valuable features of a lens is the ability to produce a It sharp non distorted image over owr the whole image circle and the more nearly a lens will do this thid th j more valuable it Is la Coupled I with covering power as it is called there should be rapidity The T e speed of ofa ofa ofa a lens depends upon the number of rays of light ight which It will converge upon a given area Light proceeds from every luminous object equally In all directions hence all aU lenses having the Ute same diameter will receive an equal amount of light It is the manner in which the lens converges the light which determines the speed The pho photographic photographic photographic plate is 18 affected more or less leS t quickly as the light which falls upon it is more or less Jas intense The intensity I of light varies inversely with the square quare of the distance from its It it source For I practical purposes we may consider con ider the source of or light falling upon the photo photographic photographic photographic graphic plate to be at the lens Then supposing two lenses of the same diameter diameter eter and having the same size image circles to differ in focal Cocal length by one half balf according to the law of intensity the Image formed by lW the lens having the shorter focus will be four fo r times as bright ae as the other the illumination will be four times as intense and tile the lens will secure a picture in the time required by the other For rapid photography and this In Includes Includes eludes the greater part of ot amateur photography it is I obvious that the lens should have the shortest possible focus and at the seine same time be able to pro produce produce produce duce a 8 perfectly sharp image over the whole plate The difficulties of the op optician optician optician begin when he attempts to com con combine combine bine th desirable features To Th return to our single convex lens lene a portion of its indistinctness can be he done away with by placing an opaque stop with a small circular opening in it In front of or the lens Jens The smaller the open o n nI I ln I the larger I er the area of sharpness wilt will ill be at the same time the volume of lIght will be correspondingly reduced and arid the lens made cade slower The practical practical cal solution is the use of ot a stop top of moderate size and a lens having ah an image circle much larger than the ding diagonal diagonal onal of the plate of or the camera thus allowing the indistinct portion of the image circle to fall tall outside the limits of to be ui u used ed L Under these Immutable conditions it will be seen that only a 8 lens of or very limited speed s ed can be had when hon the tho double convex form Is employed This IB is the cheap eat oot form of lens made The Pimple simple meniscus lens is an Improvement on the double convex form which bleh it Ft re resembles los with the exception that thet the surface toward the object is curved cUred in inward Inward Inward ward instead of outward This In Increases Increases increases creases the area arH of sharpness in the image circle and permits its the use of a alens alen alens lens len at shorter focus and greater aper aperture aperture ture The rays of light which fall upon the portions of a lens farthest from the center and at the most moat oblique angles I are the ones which are most acted upon by the lens 1 and which unless some special provision Is ia made for them in inthe inthe inthe the construction of or the lens cause in indistinctness indistInctness distinctness dl in the image and make It necessary Nry nece to shut hut them out by means of stops therefore the lens which can bring the greatest number of these marginal rays to aid in the formation of ol a sharp image will be the most rapid and give the least distorted d image Ima e eThe The next step in this process ia is the employment of two lenses of ot different shape cemented together to form one lens len One lens lena is of crown glass ass with comparatively low lorn refracting retracting power and the other of flint glass of great re refractive refractive refractive action The resulting lens lena Is called an achromatic lens and is su w superior superior to either the double douile dou le convex or meniscus as its image circle has a larger sharply defined denned area and a 8 much shorter focus may m be employed on the same plate thus giving greater I speed These single achromatic lenses are alaJ made double convex and me meniscus nl cu of which the meniscus is the better Having proceeded so far tar it is easy to see that still further results can be obtained by the proper combination of two achromatic e lenses Jen es constructed fer the purpose into one compound lens lena The be simplest form la hi that known as 5 rectilinear In which two meniscus Fig 7 achromatic lenses lenges called systems are mounted in a tube with their concave surfaces facing each other othe The rectilinear lens is vastly superior to the single lens excelling ec lUng in all the properties propertied which we have hoe mentioned in connection with lens lenses s If all objects could be photographed at a distance of or lee feet or more from the camera sin single single singie gle gie lenses might do quite well but when it is I desired to make at one time pictures of all aU objects from im immediately Immediately mediately in front of the lens to the greatest greate t distance it is important to have a lens which will do so without unduly reducing its speed Depth of focus as 8 this property is called varies directly with the focus of the lens be being beIn beIng ing In greatest in the lenses len es The depth of ot focus can be In Increased increased creased crea ed in any lens by stopping it down that is cutting off of the mar marginal marginal ginal rays raya and reducing the speed Rectilinear lenses lensea have only half balf the focal length of single lenses having the same image circle hence bence are much more rapid will make a 8 much larger lar er picture and have greater depth of or fo focus focus cus cua They also possess one other fea lea feature feature ture tare of ot greater importance the abil ability ability ity to reproduce straight lines Unes correct correctly ly In the single bunsen l the distortion of the image la in such sueh that when wh n photo photographing photographing graphing a building or other object having parallel p lines linen the lines appear crowded together at the margin of or the picture while in the rectilinear lens le S Sno no such distortion dl occurs The rectilinear rectilinear linear lens leu has bu its defects being un unable unable unable able to reproduce lines at different an angles anglee glee Clet with equal distinctness In the manufacture of lenses a varia variation variation tion Lion of ot of or an Inch is as read readily readily ily Ib detectable as a hands breadths difference In ia ordinary measurement All defective lenses lens are rejected Here Herein Herein Herein in lies the snare for the unwary pur purchaser purchaser purchaser chaser The unscrupulous manufacturer may use lenses l with slight de defects deI fectO fe such as the layman could not I himself detect with the result that the purchaser of such a lena would find himself pi an Instrument from which J it Is ia impossible to obtain the best re results i suits although It may be apparently perfect It is for this thin reason that only lenses len by of oC established Integ Integrity rity should ever aver be considered Courses of Instruction Spring Term 1550 I Mondays American Political Par Parties Parties i ties Tuesdays Tue dYs Twenty Lessons in French Conversation and nd Recent Re ent Set entitle c Discoveries Wednesdays and aad Thursdays Golden Ages of Liters Litera Literature ture Fridays Photography for Ama Amateurs Amateurs tents Saturdays Biographical Stu Stun lea leti for Girls These courses will con continue continue continue until May 31 1900 Examinations will be bo held at their close as U 1 a basis for the granting of certificates 1 |