The provenance of paintings can help resolve ownership disputes. For example, provenance between 1933 and 1945 can determine whether a painting was looted by the Nazis. Many galleries are putting a great deal of effort into researching the provenance of paintings in their collections for which there is no firm provenance during that period. Documented evidence of provenance for an object can help to establish that it has not been altered and is not a forgery, a reproduction, stolen or looted art. Provenance helps assign the work to a known artist, and a documented history can be of use in helping to prove ownership. An example of a detailed provenance is given in the Arnolfini portrait. The quality of provenance of an important work of art can make a considerable difference to its selling price in the market; this is affected by the degree of certainty of the provenance, the status of past owners as collectors, and in many cases by the strength of evidence that an object has not been illegally excavated or exported from another country. The provenance of a work of art may vary greatly in length, depending on context or the amount that is known, from a single name to an entry in a scholarly catalogue some thousands of words long. An expert certification can mean the difference between an object having no value and being worth a fortune. Certifications themselves may be open to question. Jacques van Meegeren forged the work of his father Han van Meegeren (who in his turn had forged the work of Vermeer). Jacques sometimes produced a certificate with his forgeries stating that a work was created by his father. John Drewe was able to pass off as genuine paintings, a large number of forgeries that would have easily been recognised as such by scientific examination. He established an impressive (but false) provenance and because of this galleries and dealers accepted the paintings as genuine. He created this false provenance by forging letters and other documents, including false entries in earlier exhibition catalogues. Sometimes provenance can be as simple as a photograph of the item with its original owner. Simple yet definitive documentation such as that can increase its value by an order of magnitude, but only if the owner was of high renown. Many items that were sold at auction have gone far past their estimates because of a photograph showing that item with a famous person. Some examples include antiques owned by politicians, musicians, artists, actors, etc. The objective of provenance research is to produce a complete list of owners (together, where possible, with the supporting documentary proof) from when the painting was commissioned or in the artist's studio through to the present time.