Camera shake, focal length and shutter speed.

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Submitted By: Bunyip

Joined: May 14 2007

Posts:

Camera shake, focal length and shutter speed.

Posted: August 24, 2008 - 6:59pm

Here's a question for the boffins.

It is often said that, in order to minimize the effects of camera shake, the maximum exposure time used ("shutter speed") should be the inverse of the lens focal length in use at the time.

However, using focal plane shutters, the actual exposure time is determined by the width of "the slit" between the first and second shutter curtain, NOT by how long this slit takes to travel across the film/sensor: this travel time remains constant for a given camera whether the expsure time is 1/1,000, 1/100, 1/10, or 1/4 of a second!

Hence, the film/sensor is exposed to the same degree of lens "shake" no matter which of the above exposure times is used.

So, unless using very slow "bulb" type exposure times, it would seem the "inverse of the focal length rule" can't be applied with cameras other than those with a leaf or an electronic shutter.

Is this right or not?

Submitted By: David B. Brooks

Joined: Apr 19 2011

Posts:

Re: Camera shake, focal length and shutter speed.

Posted: August 25, 2008 - 12:29pm

Focal plane shutters like those in dSLR cameras do function on the basis of a variable slit passing across the exposure plane, but with modern cameras the speed the curtain travels varies considerably, and the slit size only exposes with high shutter speeds small portions of the image at any instance, so the effect of shake or subject movement is really not all that different in result (blurring) than with diaphragm or other shutter configurations. It is not like in the days of the turn of the 20th century when large format reflex cameras with focal plane shutters were used and the curtain travelled slowly with a small slit and moving objects like automobiles were captured distorted with egg-shaped wheels in a photo.

Submitted By: Bunyip

Joined: May 14 2007

Posts:

Thanks David

Posted: August 25, 2008 - 4:42pm

Thanks David, I see your point.