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Tlae lens aperture is simply the diameter of the lens opening, expressed .is a tractiun of Its fucal length. Thus a Lens of 4-Loch local length ivith a diameter nf nne inch has a relative aperture of 4/1, or i. The aperture designatiun is expressed aa t74. indicating that the aperture is the focal length/4. Another 4-inch lens that has a diamcierof Vi Inch YfouH bein 1/8 lens.

The aperture indicates ihe amouiu ol light that the lens will tranmii ro ihe film. Since (he ypenure is expressed as a fraction Of the focal length, jr/J knxCi ¿t!l flr ff 8 (Of HI y OibfF aperture} inm.t-mif tfic sfljui jjiretiijrjf of Ught ro the ¡Urn This amount of iLfjht is propurtuin.il to die mcj liE the lens aperture |and therefore to ihe SLjtiurc of ihe diameter}, die f/4 lent described is twice the diameter of the f/-B Lens, hut transmits lour times as much light.

The aperture inscribed un the front of the lens mount is the largest lor that I ens. For practical photography we need a means of reducing the apertUIC TO five us control of the intensity of the Hljht reaching the til in, In early days, a metal plate or separate tabs with holes of different diameters, known as Waterhouse stops, Wete frequently pmvEded. Dy sliding the plate or exchanging Nibs, different lens upenings could be selected. The adjustable aperture today takes. the form nf an iris diaphragm, a series uf metal blades dian make different uze lens opening depending on the setting of

ü control ring. The series of lens stops used almost universally toil ¡iV In provide j s land aid exposure sequence Is do fdluWI;

These standard apenute tumiteís ¿re known At major stops, or whcic siop.s, and are in geometric sequence, fjich ífop rrnn.Tmitj twice or onc-hdif the! jJJ]JOWJt üf fi'¡>ht oí tlifi ad/dt'ijni Baltic. Larger í-spnp numbers represíju ¿rnn í.'tir apertures: f/ll is a »mallei number than f/lfi, hut admits twice íS much lipht. The aperruie .scale on 1 lens Will usually alsu have inteimediale po&itLona between the whole-stop divisions, Ln increments of either one-half stop or one-third stop. ¡One-third stop iotcivals correspond to a change in film speed from one ASA indes number to tbe nexti see Pooki )

When Mi'ttmg the ipertnnc an a Jens ynu should always approach the f-stop from the ¿ame direction, moving the indeK nniík ¿íoivn the scale toward the desired setting, There may he a certain degree of slack in the mechanism that can cause slightly different lens upenings at the same setting, depending on whether the f-stop is appioached by stupping down from larger apertures or opening up from smaller one».

The f stop lelatcs exposure- to the effective diameter est the lens, hut disregards certa in oiher lac tors, primarily the efficiency of the lens in its actual transmission nf light, Since tenses of many elements art less efficient tlian those oí lew dements—because of ie-flection pi light at eaeh surface ¡ind the optical densily of the glasses—attempts have been made rxi develop a scale that indicates the actual iransmission of a lens. A scale of "t-stops" has some^ times, been substituted for t-stops to indicate light trans in issinn, These values are seldom seen today, except in some lenses fur cinematography, primarily because rbc elficiencv of lenses has been gieatly increased by lens coating techniques. The t-siop values, while fine for determining exposure, also distort other mathematical values that relaic di reedy to the true f-srnp, suc.h ¿is depth of field and bypeifocal distance.

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FOCUS AMI DF.PTH OF FIFI D

Ab the distance irom die cimtra to the sublcct changes, the distance behind the Lens where ihe image is shupjr Eocuaed also changes Thc image of a ncatby subject ir- sliarpLy Eocused farther behind (he s<r i ieuju 5-i lerts thin Lhat Of ,i distant subjec.t.<> Fotu-sing die Jens involves ad-

jM>tJlig its distance tD the til in to produce a sharp image of the Snbiect. incusing with a ntdaU camera is usually accomplished by rotating i ting on the lens hartelj with a view camera, ihe length of the bellows is uditinted by moving the front or rear standard.

Wi Cim acinic Critical foeui for only one plane m front of the camera,, iirld ail ubiecti m this plane will be iharp. hi addition, there will be an area iust in front of and behind this plane that will appear reasonably sharp iaccording to the standards of sharpness required for the particular phoLograph and ihc degree r>f enlargement □f the negative). Thi* total region ot adequate focus represent» the See i icuel zr ü depth of flefiM Ti is a property of lenses that as wt reduce the apermre used for exposure, the depth or held increases. Thus, if it ii Important m a phtitograph to have areas close co the camera appear approximately as sharp as more distant ones, we select a hmaJI aperture.

There arc two other Factors that aEfcci (In- depth of field the fncal length of the lens |change to a shorter Eoca] length tens If you need innre dtipth Ot fleldl and the subject distance jmnveaway frujn

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