Motion blur

[chris_4622]

I was on my way home last weekend when I stopped to take a picture of the grain elevator. While setting up I heard the train horn and hurried to get ready. I wasn't expecting the train and having about 15 seconds to take a reading, set the aperture, load the holder and pull the dark slide I gave little though to the motion blur, though I was expecting some.

I notice the front of the train is where the blur shows with little or none as you move further back. I'm curious why there is this effect.

[Charles Carstensen]

It would be helpful if the photo is larger, like 900 pixels @ 72 ppi on the longest side.

The closer you are to a moving object, the faster that object is traveling to your fixed positiion.

[Daniel_Buck]

Relative to the camera, the front of the engine has changed position a greater amount than the rear of the train, in terms of what you see. An object 10 feet away that moves 5 feet is very noticable. An object 30 feet away that moves 5 feet isn't as noticable. Similar to how oncoming traffic on a freeway looks slow in the distance, but as they pass you they seem to get faster and faster. We deal with this in 3d a lot, we often have to explain why something looks a certain way, to make sure that it's realistic looking :)

[chris_4622]

Thanks for the explanation.

Charles I haven't figured out how to put a larger picture up. There is a limit on the size for jpegs. I see others put pictures that fill the screen but I'm at a loss as to how it is done.

[ljsegil]

I think the cause is that at constant velocity the closer an object is to the film the greater will be its linear movement across the plane of the film while the shutter is open, blur being a function of that movement. I'm really dredging now (like over 35 years worth of silt), but I think that the movement across the film will be proportional to the sine of the angle subtended by the motion of any point during shutter opening, the angle being determined by the lines from the image point on the film to the positions of a point on the train at the opening and closing of the shutter. For a linear object like a train, the distance traveled by a more distant (from the film) point of the train will subtend less of an angle than a nearer point moving the same distance, and the smaller angle will have a lower sine value, thus less movement across the plane of the film, and consequently less blur than the nearer point. The object must be moving at an angle to the film plane for this effect to be observed. An object moving directly toward the film would not subtend any ange at all as the shutter is open, and the sine of an angle of 0 degrees is zero, so there would be no motion blur (although there could be plenty of focus blur as the lens-object distance changes). A train moving parallel to the film plane would show the same motion blur throughout its length, as the motion of any part of the train would subtend the same angle relative to the film and thus have the same sine value throughout. No focus blur problems, however, as no change is occurring in the lens-object distance.
Now I'm thoroughly confused, I think I should go back to my picture books.
LJS, (MIT '74, though after this obfuscation I'm sure the Institute would deny any knowledge of my existence).

[Ralph Barker]

Chris - those posting images larger than the 650x650 limit on uploads here are doing so with links to externally-stored, but Web-accessible files - often on their own domain or site.

[Ole Tjugen]

Motion blur is defined by the shuter speed and the angular velocity, which is why something long and fast like a train will show different blur in different parts.