I got some questions over the weekend about the details of the 1/f rule and I thought I’d share some of my answers with you. It is a simple formula which allows photographers to roughly estimate how fast a shutter speed they’ll need to prevent camera motion from blurring an image.
The “1/f rule” simply says that the longest shutter speed you can handhold a 35mm camera, with careful technique but without a tripod or other support, without getting blur from camera motion is about one second divided by the focal length of the lens (in millimeters). For a 100mm lens, the rule suggests that you’d have a shot at getting a sharp handheld shot at 1/100 of a second (or a 1/1000), but not a 1/10, or 1/50. This is fairly intuitive. Telephoto lenses magnify more, magnify motion more, than wide-angle lenses, and so you need a proportionally faster shutter speed to get a sharp image with the longer lens.
The rule is not particularly precise, though, and it is helpful to keep a few caveats in mind.
The most important caveat is that photographers vary widely in their ability to consistently hold a camera steady, varying by at least a few stops. The only way to get a real sense of how well you can handhold is to take, under controlled conditions, a shot from a tripod (using good technique), and then take a a series of several shots at each shutter speed near the 1/f rule and critically compare the results. As a result, if you’re going to photograph handheld a bit, you should go do this experiment, yourself. (I have, I figure I’m a little worse at handholding than this rule would imply.)
The density (or “pitch”) of your image sensor is a factor. The smaller the sensor pixels, the more your camera will be able to notice small motions because of movement. If you can consistently handhold a 6MP full-frame camera at 1/100 and 100mm, you’ll need a whole additional stop (1/200 of a second) to keep it sharp on a 24MP camera. Because the real issue here is the size of the pixels themselves, an 8MP camera with a cropped sensor will need (more or less proportionately) faster shutter speeds than an 8MP camera with a full-frame sensor using the same lens. In other words, it may be easier (if you are comparing cameras with the same number of megapixels, but different-sized sensors) to think in terms of “effective focal length”, multiplying the focal length of the lens by the crop factor. Because digital cameras have much smaller pixel pitches than they used to, sharp handholding is harder than it used to be. Because our cameras can resolve more detail they can also resolve more motion, we’ve gotten pickier about “sharp.”
Finally, the approximations to the lens equation that make this rule of thumb pretty darn good for most shots break down for wide-angle lenses and for macro shots–in both cases, making those types of shots a little harder than they might otherwise be. Think of it as a one-third stop penalty for very wide shots (17-24mm), and as much as a full-stop penalty for 1:1 macro shots.
Finally, image stabilization (also known as vibration reduction) can provide as much as a couple stops worth of assistance. These technologies are wonderful and I do recommend them, but in my experience the claims for how much they help can be just a little bit optimistic.
While this will seem like a lot to digest, with a little practice the 1/f rule becomes second-nature and a valuable tool for handheld work.