Documents

A Tedious Explanation of the Aperture

Description
photography
Categories
Published
of 9
All materials on our website are shared by users. If you have any questions about copyright issues, please report us to resolve them. We are always happy to assist you.
Related Documents
Share
Transcript
  A Tedious Explanation of the f/stop  by Matthew ColeOur Home PageA History of Photography: The 2000sA Tedious Explaatio of !epth of   ieldPhotographers set their exposure usig a #ombiatio of shutter speeds  ad f/stops  to get the #orre#t amout of light o the film$sesor% The shutter speed regulates how log the film is exposed to light #omig through the les% The f$stop regulates how mu#h light is allowed through the les by &aryig the area of the hole the light #omes through% or ay gi&e film speed '()O* ad lightig #ombiatio there is oe #orre#t amout of light to properly expose the film% This amout of light #a be a#hie&ed with may differet #ombiatios of f$stops ad shutter speeds% !espite beig oe of the exposure #otrols i photography+ the f$stop remais a sour#e of #ofusio ad mystery to may photographers+ e&e to some who use it all the time% ( fid it iterestig that the lo#al #amera shop has pi#tures uder glass o the #outer showig a s#ee usig a rage of fo#al legths 'for a good example of this+ see my fried !a&e !ahms, -es o#al -egth Chart*+ photos showig the same s#ee prited at differet si.es ad photos showig a a#tio s#ee shot at differet shutter speeds% All that is assumed to be of iterest ad #omprehesio to the #ustomers% /hat they don't   ha&e is a set of photos showig depth of field+ or a s#ee shot at a rage of exposure #ombiatios where the f$stop,s effe#ts are show% Maybe it ust ta1es too mu#h explaatio% /ell+ too mu#h explaatio is what this page is all about+ ad (,ll go o&er the f$stop ad espe#ially its iitially#ofusig umberig at some legth% Fill That Bucket!  My fa&orite aalogy for exposure is fillig a bu#1et with water% A bu#1et is of fixed si.e ad eeds a #ertai amout of water to fill it+ ust li1e film or a digital sesor+ whi#h is of a set sesiti&ity 'the ()O* ad eeds a #ertai amout of light to optimally #apture a image% To fill your bu#1et+ you #a pour a small stream of water for a log time or a fast stream of water for a short time% Either way+ you ed up with the same amout of water% ( photography+ the si.e of the stream of the water is aalagous to the f$stop+ the legth of time you pour is aalagous to the shutter speed+ ad the si.e of the bu#1et is aalagous to the film speed% 3roadly spea1ig+ from the bu#1et,s poit of &iew+ it does,t matter whi#h #ombiatio of stream si.e ad legth of time you #hoose as log as the right amout of water eds up #omig i% ilm is the same4 withi limits+ it is idifferet to the #ombiatio of time ad amout of light as log as the right amout of light e&etually arri&es% Shutter Speeds  )hutter speeds are a bit easier to uderstad+ so (,ll start with those% 3oth exposure #otrols ru through a se5ue#e of settigs whi#h i&ol&e doublig ad hal&ig the amout of light rea#hig the film% )hutter speeds are measured i se#ods ad fra#tios of a se#od ad so the doublig ad hal&ig is fairly selfe&idet% Oe 5uarter se#od is half as log as oehalf se#od but is twi#e as log as oeeighth% Oe se#od is twi#e as log as half a se#od ad half as log as 2 se#ods% (t,s pretty easy+ ad this wor1s through the whole se5ue#e of shutter speeds% O my old   6i1o E+ for ista#e+ the shutter speed se5ue#e is: 7 se#ods 8 se#ods 2 se#ods 9 se#od 9$2 se#od 9$8 9$7 9$9 9$;0 9$<0 9$92 9$20 9$00 9$9000Ea#h of these settigs is #learly half$double the legth of time of its immediate eighbours 'O=+ ( 1ow+ 9$9th is,t exactly  half the time of 9$7th ad 9$92th is,t exactly  half the time of 9$<0th+  but it,s #lose*% This doublig$hal&ig is thus pretty simple to #omprehed for this exposure #otrol% F/Stops f$stops are a bit more #ofusig be#ause the umbers appear so arbitrary% This is the stadard se5ue#e of f$stops from f$9%8 to f$22% Although it may ot seem ituiti&e at first+ i this se5ue#e the f$9%8 settig lets i the most light while the f$22 settig lets i the least% Also+ ea#h of these f$stops has pre#isely the same hal&ig$doublig relatioship as the shutter speed se5ue#e% 9%8 2%0 2%7 8 %< 7 99 9< 22 O the fa#e of it+ goig from f$8 to f$%< does,t soud li1e hal&ig the amout of light% /hat,s more+ %< is a larger umber ad souds li1e it ought to be more light+ ot less% 6either does f$8 to f$2%7 soud li1e doublig the amout of light% ( fa#t+ ea#h of the umbers i this se5ue#e is a hal&ig$doublig of the amout of light from its immediate eighbours+ ust li1e the shutter speed settigs are% 6ot oly that+ but it ma1es sese+ as ( shall show below% The reaso that both the hal&ig ad doublig ad the smaller umbers mea more light thigs ma1e sese is that the f/stop is a ratio % The ratio is betwee the diameter  of the aperture i the les ad the focal length  of the les% The fo#al legth is geerally measured i millimeters+ so we,ll sti#1 with those as our uit of measure% O a 0mm les+ f$2 is sayig that the diameter of the aperture is 2mm% The ratio is: 0$2 > 2% That seems pretty straightforward% A good 5uestio might be+ what is the area of that aperture? /ell+ the aperture is usually a set of fi&e to fiftee blades whi#h form a roughly #ir#ular hole+ so we,ll use the formula for the area of a #ir#le+ whi#h as (,m sure you,ll re#all is @  radius 2 % or @ (,ll use ;%989B2<% O our 0mm les+ the aperture at f$2 has a diameter of 2mm whi#h is a radius of 92%mm% The area of the aperture is thus @  92% 2 + or ;%989B2<  9<%2+ or 8B0%B s5uare millimetres% This fa#t by itself is,t all that useful% (t is  useful i relatio to the ada#et f$stops% /hat is the area of the aperture at f$2%7? /ell+ be#ause the f$stop is a ratio of the fo#al legth to diameter+ our 0mm les at f$2%7 would ha&e a diameter of 0$2%7 > 9D%7<mm% emember+ we ha&e to di&ide that by 2 to get the radius of 7%B;mm+ so the area of the #ir#le thus formed would be @  7%B; 2 + or 20% s5uare mm% oudig off a bit+ that,s about 20 s5% mm at f$2%7 ad 00 s5% mm at f$2+ a double$half relatioship% AhaF )o that,s itF The area of the hole doubles and halves + it,s ust represeted by a ratio o the lesF 6o woder it,s so dar #ofusig%  Here,s a table of the aperture areas for the #ommo f$stops for a 0mm les: f/stop Diameter of aperture mm adius of aperture mm Area of Aperture s#$ mm f$9%0 0%0 2%0 9+B<; f$9%8 ;%D 9D%B 9+002 f$2%0 2%0 92% 8B9 f$2%7 9D%B 7%B 20 f$8 92% <%; 92; f$%< 7%B 8% <; f$7 <%; ;%9 ;9 f$99 8% 2%; 9< f$9< ;%9 9%< 7 f$22 2%; 9%9 8 'As show o les* '0mm di&ided by f$stop* '9$2 the diameter* 'pi  the radius s5uared* (f you loo1 dow the #olum of figures o the right+ you #a see the 'more or less* doublig$hal&ig goig o up ad dow the #olum% Gou #a see also how the big umbers ma1e for smaller areas si#e the f$stop umber is beig di&ided ito the fo#al legth+ the hal&ed+ the s5uared+ the multiplied by @% (t,s o woder this seems obs#ure%/hy ot ust #all for the area of the aperture dire#tly? A #ouple of reasos% irst of all+ if you ha&e a 0mm les o ad say ( shot this with my 0mm at 9$92th of a se#od ad a aperture area of <; s5uare millimeters you will impart #orre#t ad exa#t iformatio that pre#isely .ero  people will uderstad% (t,s mu#h easier to say ( shot this at 9$92th at f$%<% Also+ <; s5uare millimeters is f$%< oly with a 0mm les4 if your les is a ;mm+ or a 7+ or a ;00+ the ratio #hages ad the exposure is differet% That <; s5% mm is about f$8 o the ;mm+ f$B% o the 7mm ad f$;2 o the ;00% =owig oly the area of the aperture re5uires also 1owig the legth of the les to be iformati&e as to the amout of light #omig through the les% The f$stop figure i#orporates both of these i oe useful if iitially #ofusig measure ad the les legth is immaterial% (t,s shorthad+ i effe#t% /he you say f$7+ you mea for this fo#al legth 'the f?*+ gi&e me a aperture whose area is su#h that diameter of the resultig #ir#le goes eight times ito my fo#al legth% ortuately+ the les ma1ers figure out all these thigs ad ust mar1 the f$stops o the les for us% They,re doig us a big fa&or% So% does that mean f/& is the same brightness of illumination on the film/sensor regardless of focal length of the lens' Ges% That,s why they,re so hadyF (t,s also why hadheld light meters wor1they ha&e o idea what film si.e or #amera body or les you,re usig+ but they #a tell you that 9$92th at f$7 is the #orre#t exposure 'or at least oe of a whole host of e5ui&alet exposures+ we,ll get to that later*% (t does,t matter if you set that f$7 o a 20mm or a 800mm+ or if the #amera is from 9B8 or from last Iuly% (ot it% ) think$ *hat about other f/stop terms'  *hen people talk about a fast lens% +hat does that mean' -eses are referred to by their maximum aperture 'that,s the biggest hole+ the smaller umber*% Thus+ 6i1o made 'at least* four 27mm leses at oe poit+ a 27 f$9%8+ a 27 f$2%0+ a 27 f$2%7 ad a 27 f$;%% All four of these leses had f$8+ f$%<+ ad so o up to f$9<4 they were distiguished by the maximum amout of light they #ould let i% The 27mm f$;%+ oe of whi#h ( ow+ whe set to its maximum aperture of f$;%+ lets i oe third less light that the 27 f$2%7% The 27 f$2%7+ i tur+ at its maximum aperture+ lets i oly half the light of the 27 f$2%0 at its maximum aperture% The 27 f$2%0 lets i oly half the light of the 27 f$9%8 at its maximum aperture% -eses whi#h ha&e wide maximum apertures ad let i lots of light are #alled fast leses% -eses whi#h let i #omparati&ely less light at their maximum apertures are #alled slow leses% The 27 f$9%8 is a &ery fast 27mm4 27 f$2%0 would be a fast les4 the 27 f$2%7 would be sort of regular+ for whi#h there is,t really a ame4 the 27 f$;% would be 1id of slow% That 27 f$9%8 sold for eough ew that  6i1o did,t ma1e may4 o#e dis#otiued+ their pri#e i the used mar1et wet upF Gou #a fid them o e3ay from time to time goig for J2+00 or more% 6i1o i 2090 itrodu#ed a 28 f$9%8 les+ ust a bit wider+ so you do,t ha&e to buy used but you still ha&e to pay aroud J2+00% *h, +ouldn-t ,ou al+a,s use a fast lens' /eight ad expese% To get those larger diameter apertures meas you eed larger pie#es of glass mouted i #orrespodigly larger les barrels% They,re harder to maufa#ture+ the les  barrel 1eeps gettig hea&ier to hold all that glass i aligmet so it all gets weighty i a hurry+ they,re harder to autofo#us ad they,re more #hallegig opti#al desigs% ( the ragefider world there start to be #o#ers about the a##ura#y of the fo#usig si#e depth of field gets to be &ery limited at large apertures% Also+ there ha&e bee &ery fast leses made whi#h ha&e the reputatio of beig really i#e wide ope but 1id of doggy stopped dow% (f you ormally do ot use the fast les at its widest settigs+ if you are mostly at+ say+ f$7+ the you are #arryig aroud a hea&y ad expesi&e opti# whi#h may be uderperformig its #heaper brether stopped dow% The si.e pealty is really ob&ious i the log leses% The weight balloos ad the #ost s1yro#1ets% or ista#e+ ( used to ow a 6i1o ;00mm f$8% E!( les% The ( is iteral fo#us+ the E! had to do with the Extralow !ispersio glass used% (t was a sweet les+ ;00mm i legth+ with sil1y smooth 'maual* fo#usig ad weighed i at 2 lbs% 2%B o.% 'B7Bg*% (f ( stepped up to the ;00 f$2%7 les the weight wet to  lbs% 7 o. '200g*% 6ot fast eough? How about  6i1o,s ;00 f$2? (t weighed i at 9 lbs% <%B o.% 'D000g*% The ;00 f$2 pi#1s up 2 9$; stops o&er the ;00 f$8% ( owed+ but it ta1es a emietly hadholdable telephoto that fits i the #amera  bag ad ma1es it ito a uwieldly uit eedig a tripod+ re5uirig its ow suit#ase ad weighig se&e times as mu#h% The f$2s still sell used i the J20+000K rage% Gou also see the si.e i the .oom leses% Most moder #osumer .ooms are hady+ light ad slow% They are also ha&e a &ariable maximum f$stop a#ross their .oom rage% The professioal le&el leses ted to be fixed f$stop 'the 6i1o D0200 f$2%7+ for ista#e+ is f$2%7 a#ross the whole rage+ is a stadard pro les+ but it weighs a lot ad #osts J2+00*% ( e&erythig there are tradeoffs%
Search
Tags
Related Search
We Need Your Support
Thank you for visiting our website and your interest in our free products and services. We are nonprofit website to share and download documents. To the running of this website, we need your help to support us.

Thanks to everyone for your continued support.

No, Thanks