I am curious as to why many art museums disallow photographing a painting. Some museums will allow photography if you pay them, but there are still some paintings that are absolutely prohibited to be photographed. I know it doesn't apply to all museum, but most of them prohibit it.
Because they can. That is perhaps the only true answer to your question.
As to why they would want to, just follow the money:
- In the age of digital reproduction an unauthorized copy of a work of art can have a significant impact on sales of authorized prints or books that generate revenue for the gallery, artist, or owner of the piece.
- If the piece is part of a popular exhibit the goal may be to keep the lines moving through the gallery. The faster the lines move the more tickets they can sell each day.
- The incorrect assumption than because the UV rays in sunlight damage paintings, the bright burst of light from a strobe flash will also contribute to deterioration of a painting.
And of course there is always that other reason for why many things are done the way they are: "Because we've always done it that way."
The actual reason vary from place to place but I guess common reasons are:
Because they want you to buy a print in the gift shop
Light from a flash can actually degrade the painting (this may be a myth, I don't have the expertize to decide based on the evidence)
They don't want people standing in front of the painting too long and blocking the passage
It was that way in the previous museum I worked at
One key reason is that the museum may not own the piece, and therefore, only have the right to exhibit the piece. They can not transfer the right to reproduce or allow a piece to be reproduced, because that would cause them legal issues. This is especially true in museums that are exhibiting pieces that are not part of their collection.
As others have said, flash photography can cause premature aging of ancient works, and since most camera users have no idea how to stop their camera from flashing, it is simply easier to prohibit all cameras, rather than police stupid visitors.
Almost all museums don't allow flash photography because flashes will may cause damage to the work over time if UV is not properly filtered.
As far as non-flash photography, a great many museums are totally fine with it, including many of the best. The Louvre in Paris and the Smithsonian in DC both allow photography of anything in their exhibits. Those that are not do so for purely financial reasons. They own the copy of the work and want to control distribution of its image to help recover the cost of obtaining the work and to contribute to their on going work.