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{{Short description|Standard marking on film cartridges}}
{{Use dmy dates|date=June 2019|cs1-dates=y}}
[[File:Kodak DX Logo.png|thumb|DX Encoding Logo]]
[[Image:Dx135can.jpg|frame|right|135 Film Cartridge with ''DX barcode'' (top) and ''DX CAS'' code. The CAS code shows this is (top row) ISO 125 film, (bottom row) 24 exposures, +3/−1 f-stop exposure tolerance. The DX barcode reads 017563, showing [[DX number]] 109-12, 24 exposures.]]
'''DX''' ('''D'''igital inde'''X''') encoding is a standard for marking [[135 film|35 mm]] and [[Advanced Photo System|APS]] photographic film and film cartridges, originally introduced by [[Kodak]] in 1983. It includes multiple markings, which are a [[latent image]] [[barcode]] on the bottom edge of the film, below the sprocket holes, a conductive pattern on the cartridge used by automatic cameras, and a barcode on the cartridge read by photo-finishing machines.
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===DX film edge barcode===
[[Image:Dx-film-edge-barcode.jpg|thumb|left|DX film edge barcode; data track decodes to [[DX number]] 47-1 ([[Agfa-Gevaert|Agfa]] [[Agfa Perutz|Perutz Primera 200]]), frame 22A]]
Below the sprockets under each frame of 135 film is the DX film edge barcode. The barcode is invisible until the film has been developed. It is optically imprinted as a latent image during manufacturing. The barcode is used by photo finishers to identify each frame for printing. It consists of two parallel linear barcodes, one for a synchronizing clock called the "clock track", and the other encoding film data such as type, manufacturer and frame number, called the "data track".<ref name=":0" /> The barcode nearest the film edge (away from the sprocket holes) contains the data track. The data track sequence repeats every half frame, beginning with six start bits, followed by seven bits of DX Number Part 1, one unassigned bit, four bits of DX Number Part 2, a seven-bit frame/half-frame number, one unassigned bit, one parity bit, and finishes with four stop bits.<ref>{{Cite journal|last1=Gschwind|first1=Rudolf|last2=Zbinden|first2=Erwin|last3=Trumpy|first3=Giorgio|last4=Delaney|first4=John|date=2017|title=Color negatives at the demise of silver halides|url=https://doi.org/10.5167/uzh-142330|journal=ICOM-CC 18th Triennial Conference Preprints, Copenhagen, 4–8 September 2017|volume=|pages=188–191|doi=10.5167/uzh-142330 |via=[[University of Zurich]]}}</ref> The seven-bit frame/half-frame number is called the "DXN" number (different than the [[DX number|"DX Number Part 1" and "DX Number Part 2"]]), and is an extension on the original DX edge code, patented by Eastman Kodak in 1990.<ref name=":0">{{cite patent|country=US|number=4965628|title=Photographic film with latent image multi-field bar code and eye-readable symbols|status=patent|pubdate=1990-10-23|gdate=1990-10-23|inventor1-last=Olliver|inventor2-last=Johnson|inventor3-last=Patton|inventor1-first=Richard|inventor2-first=Cortlandt|inventor3-first=David |assign=Eastman Kodak Company}}</ref><ref>{{cite patent|country=EP|number=0838718B1|title=Method of printing frame numbers from DX-coded photographic film not having frame number codes|status=|pubdate=|gdate=|invent1=|inventor1-first=}}</ref>
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===DX Camera Auto Sensing===
[[File:Codage DX.jpg|thumb|right|upright=1.5|DX CAS code positions; this film is rated at ISO 400/27°, has a 24-exposure length, and an exposure tolerance of ±1 stops.
The outside of film cartridges are marked with a DX Camera Auto Sensing (CAS) code readable by many cameras. Cameras can then automatically determine the [[film speed]], number of exposures and exposure tolerance.▼
The DX Camera Auto Sensing code uses a 2×6 grid of contact points on the side of the metal cartridge surface that are either conductive or non-conductive. Electrical contacts in the camera read the bit pattern. Most cameras read only part of the code; typically, only the film speed is read, and some cameras aimed at the consumer market only read enough bits to tell apart the most common film speeds. For example, 100, 200, 400, and 800 can be distinguished by reading only S1, S2, and ground.▼
<ul>
<li>1: Ground</li>
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<li>11–12: Exposure latitude</li>
</ul>]]
▲The outside of film cartridges are marked with a DX Camera Auto Sensing (CAS) code readable by many cameras. Cameras can then automatically determine the [[film speed]], number of exposures and exposure tolerance.
With 135 film cartridges, the DX Camera Auto Sensing code uses a 2×6 grid of rectangular contact areas on the side of the metal cartridge surface; these areas are either conductive (bare metal) or non-conductive (painted). The left-most area of both rows (with the spool post on the left) are common (ground) and are thus always bare metal. Electrical contacts in the camera read the bit pattern. Diagramatically (with spool post to the left):
The film speed codes are in binary order if the first three bits (S1, S2, S3) are considered to be a "base" identifying a trio of speeds and the last two bits (S4 and S5) are considered an adjustment of +{{frac|3}} or +{{frac|2|3}} stops within that trio. For example, ISO speed 25/15° is encoded as 00010, while 32/16° is 00001 and 40/17° is 00011. These share a common encoding of 000xx for the first three bits, differing only in the last two bits. Similarly, the next three speeds are encoded 10010 (50/18°), 10001 (64/19°), and 10011 (80/20°); it is clear from examination these all share the same 100xx encoding for the first three bits. By comparison to the preceding set of three speeds, the encoding for 25/15° (00010) and 50/18° (10010) have the same xxx10 encoding for the last two bits; likewise, 32/16° (00001) and 64/19° (10001) share the xxx01 encoding, which indicates +{{frac|3}} stop compared to the xxx10 encoding, and 40/17° (00011) and 80/20° (10011) share the xxx11 encoding, which indicates +{{frac|2|3}} stop compared to the xxx10 encoding.▼
In the second row, the first three bits represent eight possible film lengths, although in practice only 12, 20, 24 and 36 exposures are encoded. The remaining two bits of the second row give four ranges of exposure tolerance, or latitude.▼
{| class="wikitable"
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* "Tx" are the exposure tolerance contacts
▲
The encoding scheme is illustrated in the truth table below using letters and color.▼
====Film speed====
* "G" is ground▼
The five bits after the ground contact in the top row can be encoded to a maximum of 32 different film speeds, but only the 24 speeds from ISO 25/15° to 5000/38°, inclusive, spaced in intervals of {{frac|3}} step, are used.
* "T" represents the contact is connected to ground▼
* "F" represents the contact is disconnected from ground▼
▲The film speed codes are in binary order if the first three bits (S1, S2, S3) are considered to
====Film length====
▲In the second row, the first three bits represent eight possible film lengths, although in practice only 12, 20, 24 and 36 exposures are encoded
====Exposure tolerance====
The remaining two bits of the second row give four ranges of exposure tolerance, or latitude.
▲The complete encoding scheme is illustrated in the truth table below using letters and color.
▲* "G" is ground.
<div>
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