How can I estimate the shutter speed needed to freeze subject motion?
Asked 4/15/2012
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Is there a practical way to calculate shutter speed if I know a subject’s speed and want to minimize motion blur? For example, at a car race, can I estimate a shutter speed from the cars’ speed, distance, focal length, and camera resolution? Likewise, could a similar approach be used for subjects like the Moon? I’m interested in whether there’s a general formula or at least a useful rule of thumb.
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
14y ago
2 Answers
5
I did some math and came up with this:
t = 3.6 * N * d * s / (v * f * res_x)
where
- t ... time [s]
- N ... number of pixels
- d ... distance between object and focal plane [m]
- s ... horizontal size of sensor [mm]
- v ... object speed [km/h]
- f ... focal length [mm]
- res_x ... horizontal resolution of sensor
N means how many pixels you will tolerate the object to be "smudged" over. It is the maximum distance in pixels you want to allow the object to move on your image.
An example
Let's assume I have a fullframe camera with a 50mm lens, and I am taking a photo of a walking person, who moves perpendicular to me (at a speed of 5 km/h) and is 10 meters distant. I tolerate movement on the image by 8 pixels. My photos are 4288x2848.
t = 3.6 * 8 * 10 * 36 / (5 * 50 * 4288) = 0.00967 [s]
t = (1 / 103) [s]
Note:
I assume I have a tripod and the camera doesn't move. Otherwise, you will have to recompute the speed of the object relative to your camera.
Originally by user9321. Source · Licensed CC BY-SA 4.0
user9321
14y ago
0
Generated from our catalog & community — verify before relying on it.
Yes—approximately. The basic idea is to limit how far the subject moves across the sensor during the exposure. In other words, choose a shutter speed so the subject shifts no more than an acceptable amount in the image (for example, about 1–8 pixels, depending on how “crisp” you want it).
One community formula for sideways motion is:
t = 3.6 × N × d × s / (v × f × res_x)
where t is shutter time, N is tolerated blur in pixels, d is subject distance, s is sensor width, v is subject speed, f is focal length, and res_x is horizontal pixel count.
That means required shutter speed is not based on subject speed alone. It also depends on:
- distance to subject
- focal length / magnification
- sensor size and resolution
- how much blur you’ll accept
- direction of motion relative to the camera
In practice, exact calculations are only a starting point because estimating speed and distance precisely is hard. It’s often faster to use experience, review shots, and adjust. Also, the “best” shutter speed may be slower than the one that fully freezes motion if you want a more dynamic look, such as some wheel or prop blur.
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