"ADF" is an acronym for "Automatic (or Automated) Document Feeder". An ADF is an electromechanical device with a tray that holds a stack of documents. The ADF feeds documents one at a time into an office machine, i. e., a photocopier, a scanner, a fax machine, a printer, a multi-function device that combines all of the aforementioned functions, or a more specialized device such as a photo processor or printing press.
An ADF is common standard equipment on most office machines these days. Only the cheapest office machines lack an ADF, and the difference in cost is generally not worth the loss of convenience and productivity. An ADF allows processing of a dozen or more documents without human attendance. It can take up to 60 seconds to print a high-quality color document, so the time savings are significant if you do such things often. However, if you needs are largely confined to processing a single document or two, then you may consider an office machine without an ADF.
An ADF is rated according to its speed, which is expressed in page per minute or ppm, and the capacity of its document tray, typically 10 to 200 pages. Both speed and capacity strongly affect the price of an ADF-equipped office machine.
Two kinds of ADF are capable of duplex (both sides) processing. A Reversing ADF (RADF) scans one side of a page, flips it over, and scans the other side. A Duplexing ADF (DADF) scans both sides in one pass. A Duplexing ADF is much faster, obviously. However, a DADF is considerably more expensive than a RADF because the former must incorporate two scanners, one for the top side and one for the bottom side. RADFs and DADFs are rated in images per minute or ipm.
While an ADF increases productivity when it works, it can also bring production to halt when it fails. The many moving parts in an ADF are more subject to failure than a simple flatbed scanner. When buying any office machine make sure that it includes a flatbed scanning surface as well as an ADF.
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