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Old Aug 31, 2008, 10:52 PM   #1
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Let's say I'm in a car at a 4-way intersection. Want to take a shot of city life. And would like the cars passing to be frozen. With little or no smear to show movement. Would 1/250th of a second be enough? Or would you guys recommend at least 1/640th? Assuming the cars are traveling at maybe 30-50 mph.

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Old Aug 31, 2008, 11:56 PM   #2
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You really have to know how far you are away and what focal length to get a better idea.

The more of the frame a moving subject occupies, the more blur from movement you'll see at a given viewing/print size (taking direction of movement into consideration).

That's because the human eye needs to see a certain amount of movement in an image (and there are studies that have calculated the exact distance needed) before it can detect blur.

So, you'd need a much faster shutter speed to freeze it if a car was filling a lot of the frame and moving across it, and you could get away with a much slower shutter speed if the car was only occupying a very small portion of the frame (as in a wider shot with lots of cars in it).

That's because if it's far away and occupying a smaller portion of the frame, it's not going to be moving very far across the image size you're looking at for the same subject speed of movement while the shutter is open, compared to if you were closer to it or zoomed in more.

Likewise, you could get away with a slower shutter speed for a smaller print than you could for a larger print (because that movement will be across a larger distance at a larger viewing size and you'll reach the threshold of what the human eye can detect faster).

For vehicles moving that fast, if you're right at an intersection, I'd probably lean towards something much faster than that though (around 1/2000 or so if you can do that without sacrificing depth of field too much or increasing noise levels too much via ISO speed increases).

There used to be an online calculator written by a nature photographer named John Dudak for that kind of thing at this link (where you could plug in subject distance, direction of movement, focal length, print/viewing size of the image, etc. and calculate shutter speed needed).


But it's not working anymore. I think this is probably the author (but, I'm not sure, and I can't find the calculator on this site):


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Old Sep 1, 2008, 12:31 AM   #3
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Thanks for the information Jim. I realize more information would have helped but obviously I'm not there at the intersection. So I was hoping to get a general idea. I will try 1/2000th and see what happens. And go from there. Do some experimentation. It's not like I'm trying to get this one shot at any specific intersection. For that once in a life time shot.

Beauty of digital photography is if you don't get it right you can just keep shooting and resetting till you do. With so much space in memory cards these days... (vs. using a roll of film, 36 exposures)

Have a good week!
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