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==Film speed measurement systems{{anchor|Film|Film speed}}==
<!-- This section is linked from inside or outside this article, preferable by "#Film". Do not rename without keeping in sync. -->
===Emulsion speed rating criteria===
Five criteria for the rating of emulsion speed have been used since the late 19th century, listed here by name and date, these criteria are: threshold (1880), inertia (1890), fixed density (1934), minimum useful gradient (1939) and fractional gradient (1939). <ref>{{citation
| last = Allbright | first = G S
| publisher = The Journal of Photographic Science
| DOI = 10.1080/00223638.1991.11737126
| title = Emulsion Speed Rating Systems
| year = 1991}}
</ref>
====Threshold====
The threshold criterion is the point on the [[Sensitometry|characteristic curve]] corresponding to just perceptible density above fog.
====Inertia====
The inertia speed point of an emulsion is determined on the Hurter and Driffield [[Sensitometry|characteristic curve]] by the intercept between the gradient of the straight line part of the curve and the line representing the base + fog (B+F) on the density axis.
====Fixed density====
The fixed density speed point is determined by defining a fixed minimum density as the basis the emulsion speed (e.g. 0.1 above B+F).
====Minimum useful gradient====
The minimum useful gradient criterion places the speed point where the gradient first reaches an agreed value (e.g. tan <big>𝜃</big> = 0.2).
====Fractional gradient====
The fractional gradient is defined as the speed point at which the slope of the characteristic curve first reaches a fixed fraction (e.g. 0.3) of the average gradient over a range (e.g. 1.5) of the characteristic curve. <ref>{{citation
| last = Jacobson | first = Ralph E
| publisher = Focal Press
| title = The Manual of Photography (Seventh Edition)
| page = 410
| year = 1978}}
</ref>
===Historical systems===
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====BSI{{anchor|BS}}====
When [[British Standards Institution|BS]] 935:1941 was published during [[World War II]], specifying exposure tables for negative materials, it employed the same [[#Fixed density|fixed-density]] speed criterion used in the German [[Deutsches Institut für Normung|DIN]] 4512:1934 system. The British Standard also used ''logarithmic'' speed numbers, following the example of [[#Scheiner|Scheiner]] and [[#DIN|DIN]]. When the American [[American National Standards Institute|ASA]] Z38.2.1:1943 standard was published, it used a [[#Fractional gradient|fractional gradient]] speed criterion and ''arithmetic'' speed numbers, for
| last = Jacobson | first = Ralph E
| publisher = Focal Press
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| 33
| 1400 (1440)
| align="left" | Fujicolor 1600, [[Fujifilm_Superia|Fuji Natura 1600 and Superia 1600]], [[Neopan#Neopan 1600 Professional (Super Presto)|Neopan 1600]]
|-
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The picture shows an ASA/DIN conversion in a 1952 photography book<ref name="Singapore_1952"/> in which 21/10° DIN<!-- this convention used up to 1957! --> was converted to ASA 80 instead of ASA 100.
Some classic camera's exposure guides show the old conversion as they were valid at the time of production, for example the exposure guide of the classic camera [[Tessina]] (since 1957), where 21/10° DIN is related to ASA 80, 18° DIN to ASA 40, etc. Users of classic cameras
===Determining film speed===
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<ref name="http">{{cite book |title=ISO 2240:2003: Photography – Colour reversal camera films – Determination of ISO speed |url=http://www.iso.org/iso/iso_catalogue/catalogue_tc/catalogue_detail.htm?csnumber=34533}}</ref>
<ref name="iso.org">{{cite book |title=ISO 5800:1987: Photography – Colour negative films for still photography – Determination of ISO speed |url=http://www.iso.org/iso/iso_catalogue/catalogue_tc/catalogue_detail.htm?csnumber=11948}}</ref>
<ref name="Mulhern_1990_Letter">{{cite book |author-first=Charles J. |author-last=Mulhern |title=Letter to John D. de Vries |date=1990-06-15 |url=http://www.johndesq.com/westonmaster/edward.htm |type=[[Copyscript]] on John D. de Vries' web-site |archive-url=https://archive.today/20130103115812/http://www.johndesq.com/westonmaster/edward.htm |archive-date=2013-01-03 |quote= }}In 1931, Edward Faraday Weston applied for a U.S<!-- no dot here in web cite --> patent on the first Weston Exposure meter, which was granted patent No. [https://www.google.com/patents/about?id=ncNPAAAAEBAJ 2016469]{{dead link|date=June 2024|bot=medic}}{{cbignore|bot=medic}} on [8] October 1935<!-- written as "Oktober 8, 1935" in web cite -->, also an improved version was applied for and granted U.S<!-- no dot here in web cite --> patent No. [http://patft.uspto.gov/netacgi/nph-Parser?patentnumber=2042665 2042665] {{Webarchive|url=https://web.archive.org/web/20170226053630/http://patft.uspto.gov/netacgi/nph-Parser?patentnumber=2042665 |date=2017-02-26 }} on [7th} July 1936. From 1932 to around 1967, over 36 varieties of Weston Photographic Exposure Meters were produced in large quantities and sold throughout the world, mostly by Photographic dealers or agents, which also included the Weston film speed ratings, as there were no ASA or DIN data available at that time.
<ref name="Goodwin_1938_emulsion_ratings">{{cite journal |author-first=William Nelson Jr. |author-last=Goodwin |title=Weston emulsion speed ratings: What they are and how they are determined |journal=[[American Photographer]] |date=August 1938}} (4 pages)</ref>
<ref name="Roseborough_1996_PHSC-PC-22-3">{{cite journal |author-first=Everett |author-last=Roseborough |title=The Contributions of Edward W.<!-- TBD: Certainly in the title, but can someone confirm his second surname W.? This is the only source I could find mentioning it. --> Weston and his company |journal=Photographic Canadiana |volume=22 |issue=3 |date=1996 |url=http://www.phsc.ca/excerpt22.html}}</ref>
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<ref name="Graves_1996_Zone-system">{{cite book |title=The zone system for 35mm photographers |author-first=Carson |author-last=Graves |publisher=[[Focal Press]] |date=1996 |isbn=978-0-240-80203-9 |page=124 |url=https://books.google.com/books?id=nHgZHXvqy5sC&pg=PA124}}</ref>
<ref name="ISO-2721_1982">{{cite web |url=http://www.iso.org/iso/iso_catalogue/catalogue_tc/catalogue_detail.htm?csnumber=7692 |title=ISO 2721:1982. Photography — Cameras — Automatic controls of exposure |type=paid download |location=Geneva |publisher=[[International Organization for Standardization]] (ISO) |archive-url=https://web.archive.org/web/20080807163442/http://www.iso.org/ |archive-date=2008-08-07}}</ref>
<ref name="Leica_2002_ISO_R9">{{cite web |publisher=[[Leica Camera AG]] |date=2002 |url=http://en.leica-camera.com/assets/file/download.php?filename=file_2048.pdf |title=Leica R9 Bedienungsanleitung / Instructions |language=de, en |id=Leica publication 930 53 VII/03/GX/L |location=Solms, Germany |page=197 |quote=Film speed range: Manual setting from ISO 6/9° to ISO 12500/42° (with additional exposure compensation of up to ±3 EV, overall films from ISO 0.8/0° to ISO 100000/51° can be exposed), DX scanning from ISO 25/15° to ISO 5000/38°. |access-date=2011-07-30 |archive-date=2013-04-15 |archive-url=https://web.archive.org/web/20130415104604/http://en.leica-camera.com/assets/file/download.php?filename=file_2048.pdf |url-status=dead }}</ref>
<ref name="Leica_1996_ISO_R8">{{cite book |publisher=[[Leica Camera AG]] |date=1996 |title=Leica Instructions – Leica R8 |location=Solms, Germany |pages=16, 65<!-- |access-date=2011-07-30 --> |quote=The DX-setting for automatic speed scanning appears after the position "12800" […] Film speed range: Manual setting from ISO 6/9° to ISO 12,800/42° (With additional override of −3 EV to +3 EV, films from 0 DIN to 51 DIN can be exposed as well.) DX scanning from ISO 25/15° to ISO 5000/38°.}}</ref>
<ref name="ASA_1961_PH2-12_12500">{{cite book |title=ASA PH2.12-1961 |chapter=Table 2 |page=9}} (NB. Showed (but did not specify) a speed of 12500 as the next full step greater than 6400.)</ref>
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<ref name="Nikon_2012_ISO_D4">{{cite web |publisher=[[Nikon]] |url=http://www.nikon.com/news/2012/0106_flagship_01.htm |title=Nikon D4 |access-date=2012-01-06}}</ref>
<ref name="Ricoh_2014_ISO_Pentax_645Z">{{cite web |url=http://www.dpreview.com/news/2014/04/15/ricoh-announces-medium-format-pentax-645z |title=Ricoh Pentax 645Z specifications}}</ref>
<ref name="Nikon_2014_ISO_D4s">{{cite web |url=http://imaging.nikon.com/lineup/dslr/d4s/spec.htm |title=Nikon D4s specifications |access-date=2014-02-25 |archive-date=2014-03-02 |archive-url=https://web.archive.org/web/20140302023757/http://imaging.nikon.com/lineup/dslr/d4s/spec.htm |url-status=dead }}</ref>
<ref name="Sony_2014_ISO_Alpha_ILCE-7S">{{cite web |url=http://www.dpreview.com/news/2014/04/06/sony-announces-alpha-7-series-full-frame-mirrorless-with-4-2-2-4k-video-output?ref=title_0_1 |title=Sony α ILCE-7S specifications}}</ref>
<ref name="Canon_2015_ISO_ME20F-SH">{{cite web |title=Unsichtbares wird sichtbar! Canon präsentiert die ME20F-SH für Full-HD Farbvideos bei extrem wenig Licht |publisher=[[Canon Deutschland]] |language=de |date=2015-07-30 |work=Press release |url=http://www.canon.de/about_us/press_centre/press_releases/consumer_news/camcorders/me20f_sh_colour_video.aspx |access-date=2015-07-30 |archive-date=2015-08-02 |archive-url=https://web.archive.org/web/20150802060136/http://www.canon.de/about_us/press_centre/press_releases/consumer_news/camcorders/me20f_sh_colour_video.aspx |url-status=dead }}</ref>
<ref name="Sony_2009_ISO_Alpha_DSLR-A500-A550">{{cite web |work=Sony Europe Web page |url=http://presscentre.sony.eu/Content/Detail.aspx?NewsAreaId=2&ReleaseID=4744 |title=DSLR-A500/DSLR-A550 |date=2009-08-27 |quote=Dramatically reduced picture noise now allows super-sensitive shooting at up to ISO 12800, allowing attractive results when shooting handheld in challenging situations like candlelit interiors. |access-date=2011-07-30}}</ref>
<ref name="Sony_2010_ISO_Alpha_DSLR-A560-A580">{{cite web |work=Sony Europe Web page |url=http://presscentre.sony.eu/content/detail.aspx?ReleaseID=6109&NewsAreaId=2 |title=DSLR-A560/DSLR-A580 |archive-url=https://web.archive.org/web/20100830033236/http://presscentre.sony.eu/content/detail.aspx?ReleaseID=6109&NewsAreaId=2 |archive-date=2010-08-30 |date=2010-08-27 |quote=Multi-frame Noise Reduction 'stacks' a high-speed burst of six frames, creating a single low-noise exposure that boosts effective sensitivity as high as ISO 25600. |access-date=2011-07-30}}</ref>
<ref name="Pentax_2010_ISO_K-5">{{cite web |work=Pentax USA Web page |url=http://www.pentaximaging.com/slr/K-5/ |title=Pentax K-5 |date=2010 |quote=ISO Sensitivity: ISO 100-12800 (1, 1/2, 1/3 steps), expandable to ISO 80–51200 |access-date=2011-07-29 |archive-url=https://web.archive.org/web/20101206023448/http://www.pentaximaging.com/slr/K-5/ |archive-date=6 December 2010 }}</ref>
<ref name="Fujifilm_2011_ISO_X100">{{cite web |work=Fujifilm Canada Web page |url=http://www.fujifilm.ca/products/digital_cameras/x/finepix_x100/specifications/ |title=Fuji FinePix X100 |date=February 2011 |quote=Extended output sensitivity equivalent ISO 100 or 12800 |access-date=2011-07-30 |archive-date=2011-08-27 |archive-url=https://web.archive.org/web/20110827132520/http://www.fujifilm.ca/products/digital_cameras/x/finepix_x100/specifications/ |url-status=dead }}</ref>
<ref name="Lambrecht_2003_Way-Beyond">{{cite book |title=Way Beyond Monochrome |author-first1=Ralph W. |author-last1=Lambrecht |author-first2=Chris |author-last2=Woodhouse |publisher=Newpro UK Ltd |date=2003 |isbn=978-0-86343-354-2 |page=113 |url=https://books.google.com/books?id=Q7M0zOHcUxsC&pg=PA113}}</ref>
<ref name="Kodak_2000_Print-Grain-Index">{{cite web |url=http://www.kodak.com/global/en/professional/support/techPubs/e58/e58.jhtml |title=Kodak Tech Pub E-58: Print Grain Index |date=July 2000 |publisher=[[Eastman Kodak]], Professional Division}}</ref>
<ref name="Ilford_2006_Delta-3200">{{cite web |url=http://www.ilfordphoto.com/Webfiles/200613019405339.pdf |title=Fact Sheet, Delta 3200 Professional |location=Knutsford, U.K. |publisher=[[Ilford Photo]] |access-date=2008-05-18 |archive-date=2008-05-28 |archive-url=https://web.archive.org/web/20080528143940/http://www.ilfordphoto.com/Webfiles/200613019405339.pdf |url-status=dead }}</ref>
<ref name="iso12232">{{cite web |url=http://www.iso.org/iso/en/CatalogueDetailPage.CatalogueDetail?CSNUMBER=37777 |title=ISO 12232:2006. Photography — Digital still cameras — Determination of exposure index, ISO speed ratings, standard output sensitivity, and recommended exposure index |location=Geneva |publisher=[[International Organization for Standardization]] (ISO) |archive-url=https://web.archive.org/web/20080807163442/http://www.iso.org/ |archive-date=2008-08-07}}</ref>
<ref name="CIPA_DC-004">{{cite web |url=http://www.cipa.jp/english/hyoujunka/kikaku/pdf/DC-004_EN.pdf |title=CIPA DC-004. Sensitivity of digital cameras. |location=Tokyo |publisher=[[Camera & Imaging Products Association]] (CIPA) |access-date=15 June 2008 |archive-url=https://web.archive.org/web/20120417120837/http://www.cipa.jp/english/hyoujunka/kikaku/pdf/DC-004_EN.pdf |archive-date=17 April 2012 }}</ref>
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