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Photography Q&A
Autofocus mechanism
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<blockquote data-quote="Eyelight" data-source="post: 368114" data-attributes="member: 24753"><p>Not near as crazy as some things that are heard around the web, but the handle might be a bit farther away than it appears.</p><p></p><p>Wayne beat me to the high speed of the tennis ball, but just for kicks, lets assume the tennis ball travels across the sensor in the same direction and at the same speed as the shutter curtain. The shutter curtain and tennis ball travel across the view in 1/250 second. If the shutter speed is set to 1/250 second, the 2nd curtain will start to close at the same moment the 1st curtain has reached full open. The tennis ball enters the view immediately behind the trailing edge of the 1st curtain and exits the field of view immediately after the 1st curtain reaches full open. Hold that.</p><p></p><p>Assume the tennis ball is 3" diameter, so using Wayne's 20' field of view of 13.75' the ball is 1/55 the field of view. So, the ball occupies a ball sized spot on the sensor for less than 1/55 the amount of time the rest of the image is exposed or 1/13750 second. This ball, we will never see, nor will it leave an image on the sensor.</p><p></p><p>If the ball were traveling in the opposite direction of the shutter travel then it would only be in view for half the exposure, but would still be moving too fast to see or record.</p><p></p><p>To see or record the image of the tennis ball, we need it to move slow enough to be exposed similar to the background, which means it needs to occupy the same position in the view for a significant amount of the exposure time. So, for instance, instead of traveling 13.75', it travels 3", in which case we get a 6" oval shaped green blur that has background showing through. 3" travel in 1/250 second is 62.5 FPS (42.6 MPH).</p></blockquote><p></p>
[QUOTE="Eyelight, post: 368114, member: 24753"] Not near as crazy as some things that are heard around the web, but the handle might be a bit farther away than it appears. Wayne beat me to the high speed of the tennis ball, but just for kicks, lets assume the tennis ball travels across the sensor in the same direction and at the same speed as the shutter curtain. The shutter curtain and tennis ball travel across the view in 1/250 second. If the shutter speed is set to 1/250 second, the 2nd curtain will start to close at the same moment the 1st curtain has reached full open. The tennis ball enters the view immediately behind the trailing edge of the 1st curtain and exits the field of view immediately after the 1st curtain reaches full open. Hold that. Assume the tennis ball is 3" diameter, so using Wayne's 20' field of view of 13.75' the ball is 1/55 the field of view. So, the ball occupies a ball sized spot on the sensor for less than 1/55 the amount of time the rest of the image is exposed or 1/13750 second. This ball, we will never see, nor will it leave an image on the sensor. If the ball were traveling in the opposite direction of the shutter travel then it would only be in view for half the exposure, but would still be moving too fast to see or record. To see or record the image of the tennis ball, we need it to move slow enough to be exposed similar to the background, which means it needs to occupy the same position in the view for a significant amount of the exposure time. So, for instance, instead of traveling 13.75', it travels 3", in which case we get a 6" oval shaped green blur that has background showing through. 3" travel in 1/250 second is 62.5 FPS (42.6 MPH). [/QUOTE]
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