Is there a linear relationship between shutter speed and speed of subject to freeze motion?

Question

Is there a calculation that I can do that will calculate the required shutter speed if I know the speed of my subject and want to get a crisp image e.g. freeze motion?

For example if I go to a car race and focus on a particular corner, where I know the max speed of the cars - is there a calculation I can do to work out my shutter speed before I setup?

Or if I know the speed at which the Earth rotates can I devise a perfect shutter speed to freeze the moon?

Would be amazing if you could devise an optimum shutter speed per subject chart, like a shutter speed cheat-sheet?

Asked by Rob

Answer

Shutter speed can be calculated - all you have to do it calculate the time it takes the subject to move one pixel in the image.

Unsurprisingly, other people already made those calculators, there's one here and another here

I've even seen a cheat sheet like you want but I can't find it right now (I was sure I bookmarked it)

Answered by Nir

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Why are the area of aperture, focal length, and amount of light specified in these terms?

Question

So I'm looking at the wikipedia article on Aperture, and I'm a bit confused. I understand aperture to be the diameter of the hole that lets light in. In the article, someone states that

The amount of light captured by a lens is proportional to the area of the aperture, equal to:

Area = pi ( focal length /(2 * f number) )^2

But in the f number article, they define f number as

f = focal length / aperture

It then seems trivial to substitute in:

Area = pi ( focal length / (2 * focal length / aperture) )^2

Area = pi ( focal length * aperture / (2 * focal length) )^2

Area = pi ( aperture / 2)^2

Area = pi ( radius )^2

So, my question is: Why did they bother putting something so blatantly obvious into the article? Didn't we already know that the area of the aperture was pi * r^2? Is this just me reading too much into a wikipedia article?

Asked by Chaosphere2112

Answer

The focal length and f-number are photographically relevant quantities, so the formula is expressed in terms of those convenient variables. Photographers don't generally know the radius of their lens aperture for every f-stop.

Answered by coneslayer

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