i don't know, if photography is art. but when it is art, it is a lousy one.
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Lemme take a crack at this.
Most here are taking the position that photography is an art, and arguing that point. But Peter, you seem to have already claimed that photography is sometimes art - you said "when it is" not "if it is". So I'll start out by agreeing that photography is sometimes art. I read the crux of your statement to be that photography is lousy at being an art.
Here I also agree. Photography is a "lousy" art. How so? Let me contrast photography to several other disciplines thought of as being capable of producing art. First let's look at sculpture, no one would deny that there are many sculptures that are nearly universally thought of as art.
Sculpture can take many forms, from a small bust in a study to a huge amorphous blob situated in the heart of a great city. Sculpture knows no limits in material, form or size. Sculptures can be static, or they can move, they can be something one experiences from the outside, or you can get inside them. In some cases, you can even interact physically with a sculpture.
Painting is an art form. Paintings are close to photographs, but they are more. A painting begins as a blank canvas, and the painter adds to the canvas until the paining intended emerges. While the canvas is usually a rectangular piece of flat material, this need not be the case. A painting can be made on the interior or exterior of a structure, on a vehicle, even on a human. The subject of a painting is also limitless. Anything the painter can conceive - heaven and hell are popular topics - can be painted. The objects in a painting need follow no physical laws, indeed a painting need not even contain any objects.
Dance is not an art I spend much time thinking about, but it too is an art that enjoys many degrees of freedom. Traditional artistic dances, modern interpretive dance, and ballet are well defined movements of humans on a stage designed for exhibition, and in this aspect resemble photography perhaps more than other traditional arts. However the concept of temporal change by human beings - dance if you will - can be expressed in many different ways as well. The guy who walked a tight rope between the, then standing, twin towers comes to mind, as does the guy who shot himself in the foot in the 1970s (and ushered in the post-modern era.)
Photography on the other hand has few freedoms. We (photographers) are tied to an actual scene from which light is reflected. We need to collect that reflected light by chemical, mechanical or electronic means. That is, we require a machine or process to make the thing we produce. Nearly all photographs are bounded by a static, geometric border. And the processes we use to make the work requires tools outside our control. In the words of the US president, "we didn't make that". Of course we can take various parts of the process back - caffienol, van dyke, wet collidion/silver halide emulsions, but at some point photography relies on technology, be it high resolution electronic sensors, or chemicals produced by century old processes in factories located in Eastern Europe.
All in all, the boundries and limits that photography requires make it a "lousy" art. But it is exactly that "lousiness" that makes it expressive for the artist,
and artistic in and of itself.