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Camera Shutter Speed and Exposer Time Related to Blur

Posted on 2004-08-02
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I'm looking for information that relates camera shutter speed and expose time to blurring at certain altitudes and velocities. I believe shutter speed and exposure time are pretty much inversely proportionate but I'm not sure how to relate blur or how all of this can be predicted at different altitudes and velocities.

Any help would be much appreciated. Help in any form would be good: equation, web reference, code, best guess.....
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Question by:gfoote
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by:Callandor
Callandor earned 50 total points
ID: 11696601
Here's a page that discusses blurring and the factors of distance, shutter speed, and velocity: http://www.tpub.com/content/photography/14130/css/14130_212.htm
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by:potuncle
ID: 11708703
Shutter speed is the same as exposure time. For example if your shutter speed is 1/500 of a second then the film is being exposed for one 500th of a second. So a shutter speed of 1/1000 would expose the film for half the time of 1/500 and therefore reduce the motion blur by half. Since you mention altitude, I am assuming you are relating this to arial photography. When shooting arial photography there are the following variables:
- Altitude
- Velocity
- Field of View (this is determined by both the focal lenth of the lens and the size of the film)
- Shutter speed

The altitude and field of view determine the amout of area that is being photographed. Trigonomitry can be used to figure out the about of area in the field of view if the field of view is x degrees and the altitude is y meters.

In our example, let's say that the area in the field of view is 1 kilometer. Then, to keep things simple, let's say that our velocity is 1 kilometer a second. From this information we can calculete the amount of motion blur for any particular shutter speed. For example, if our shutter speed is 1/1000 of a second then the blur of our photograph would be one meter, meaning that a point in the landscape that was photographed will blurred 1/1000 of the length of the photograph.

The above may be a bit more complex of an answer that what you are looking for, but with more information, I would be able to answer your question more specific to youro needs.

Jason
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by:gfoote
ID: 11709664
Jason,

So could you say, in the above example, that a blur of 1 meter occurs because in the time it takes for the shutter to open and close once, the field of view has progressed 1 meter on the ground?

If this is the case, then decreasing your field of view by half or doubling the shutter speed, would decreasing the blur to 0.5 meter, correct?

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potuncle earned 450 total points
ID: 11710725
Gfoote, you are correct on both points. The one thing I didn't include in the equasion is vibrartion blur. If you are photographing from a airplane or helicopter then you and your camera are vibrating. Amount of blur due to this vibration is difficult to calculate, but increasing the shutter speed (decreasing exposure time) will reduce the vibration blur. Also lowering altitude while shooting the same area (increasing field of view to do so) would decrease motion blur.

A way to think abot that is to image yourself standing 10 feet from a wall with a dot on it. You have a 10 foot pole in your hand and are pointing it at the dot. Obviously, because it is impossible to hold something absolutely still, the tip of the stick is going to be vibrating within a certian radius around the dot. The steadier your hand the smaller the radius. Now imagine that same dot on the same wall, but it is 25 feet away and you have a 25 foot long stick (ignore the fact that this stick is probably quite heavier than the 10 foot one). The tip of the stick is going to be vibrating around a much larger radius around the dot. This is simply the principle of a basic lever. With the same about of motion, the tip of a long lever is going to move more distance that the tip of a short one. Think of your camera as the stick and something you are trying to photograph as the dot on the wall.

This doesn't really apply to arial photography, but is a useful tip and may help in understanding the lower field of view / more motion blur connection. When hand-holding and shooting a 35mm camera, as a general reference, the slowest shutter speed you may use without getting unacceptable motion blur is 1 divided by the focal length (in mm) of the lens. So if you have a 50mm lens then you would want to shoot at 1/50 second, or faster. If you are shooting with a 300mm lens, then you'd want 1/300, or faster. If you have a really unsteady hand, or you want optimal sharpness, then you would want minimum speeds to be faster than these. This is simply assuming that you are using a longer lens to be able to shoot something from a farther distance that you would have to be at to shoot it with the shorter lens.  If you are shooting at the same distance with both lenses, the amount of blur (proportinate to the subject) is going to be the same with both lenses, but the longer one, because of it's narrower field of view, will be showing much less of the subject.

Jason

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by:gfoote
ID: 11711964
Jason,

Thank you for your input it has been very helpful. Do you happen to know of any equation/quations that relate these ideas?

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