The case of this is that it does occur BUT most people do NOT use film fast enough to record it. Also when it does occur the rest of the film is exposed to such a great degree it would be hard to notice the difference in exposure changes from the center as opposed to the edges. Think of it this way. You decide to stare at the sun fot 1/16 of a second lets say. At the same time you look at the sun a person shines a flashlight into you eye for 1/128 of a second. Will you notice the flashlight? Probably not. What if the flashlight were open for twice as long? Same thing. 4 times...ect. Basically it is a a typical problem of systematic reading which is so "swamped" or overpowered by the actual reading the background reading does not exist. One way you could attempt to see this effect is to open the shutter and photograph a comply black room or subject while using slide film ( To avoid the same effect as exposing the paper ). But then you would still have to use VERY fast film and probably perform iris opening many times. Now for a fun question if one photographs things during the summer when things are hotter how much does the exposure time change? ( I bet Kodak and the other film companies have performed this experiment several times!! ) =:-) Bsteph