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20 February 2004

Minimum Shutter Speed Revised!

For a handheld 35 mm camera, the conventional rule of thumb is to select a shutter speed of at least the reciprocal of the lens' focal length to avoid blurring due to camera shake. For example, with a "normal" 50 mm lens, the shutter speed should be no slower than 1/50 of a second.

The basis of this rule of thumb is seldom discussed, inviting the question whether it is, in fact, correct. I have conducted experiments that indicate that although the reciprocal-of-focal-length rule may be adequate for small enlargements, sharpness can be increased with significantly faster shutter speeds. Indeed, buried within one of Ansel Adams' books is the suggestion that with a normal lens and a handheld camera, a shutter speed of anything less than 1/250 second may produce less than a completely sharp image. (See The Camera , Little, Brown and Company, 1980, p. 116.)

How steady is steady?

Just how steady can a camera be held? To answer this question, I used an enlarger alignment device which is essentially a laser mounted in a box about the size and weight of a 35mm camera. From 30 feet, trying my best, the laser spot would dance around within about a 1.5-inch circle on the wall. Using trigonometry, this corresponds to jittering within a solid angle of about 0.24 degree. (You might try the same experiment with a laser pointer securely taped to a camera.) [Caution: Use care to avoid pointing the laser pointer at a person or a highly reflective object.]

The next question is how rapidly the laser spot moved within the circle. Without sophisticated instrumentation, I chose to use published medical data for the frequency of normal human nervous system tremor. The normal frequency range is 4-12 Hz. I assumed that my tremor rate was in the middle of that range, 8 Hz. In half a cycle, 1/16 second, a handheld camera may be expected to swing through 0.24 degree, which translates to an average tremor of 0.24 x 16 =3.8 degrees/sec.

Film resolving power

How can we relate this jitter of 3.8 degrees/sec to the effect on image sharpness? Image sharpness is limited by the film's resolving power, which for Tri-X is 100 lines per mm. If the image projected onto the film during exposure moves less than, say, half of a line spacing, we've probably achieved the maximum sharpness.

With resolving power of 100 lines per mm, half the line spacing is 0.005 mm. For a 35 mm camera with a 50 mm lens, the angle of view is about 45 degrees and the dimension of the film is 35 mm. With a little math, jittering the camera 3.8 degrees/sec is seen to correspond to 3.1 mm/sec. To limit image movement during exposure to 0.005 mm, the exposure time should be 0.005 / 3.1 = 0.0016 sec. This would call for a shutter speed slightly faster than 1/500 second (compared to the rule-of-thumb speed of 1/50 sec).

Clearly, the conventional rule of thumb does not come close to achieving maximum image sharpness. It only works because the resultant blurring due to camera shake may not be noticeable for small enlargements.

Conclusion

I propose a revised shutter speed rule of thumb: use at least twice the shutter speed called for under the old rule. For example, using a "normal" 50mm lens, use, say, 1/125 second rather than 1/60 second. Even better results can be achieved by using up to ten times the old rule's shutter speed, e.g., 1/500 second in the example given.