Tuesday, August 9, 2011

Photography Basics - Reciprocal of Focal Length Shutter Speed Rule

This article is a very short excerpt from a book I am writing called, Moving Beyond Point-and-Shoot Photography: The Next Step – Learning to Use a DSLR or Interchangeable Lens Camera, due for release by Rocky Nook in the spring of 2012 in print and most eBook formats.

There is an important principle in photography that we should discuss, especially if you are shooting with a DX sensor. It is called the reciprocal of focal length shutter speed rule and affects the sharpness of your images. This impressive sounding rule simply means that you should use a tripod (no handholding) whenever the shutter speed in use is below the reciprocal of the lens’s focal length. What does that mean?

Simply that whatever the focal length (e.g., 18mm, 35mm, 50mm, 105mm) of the lens (or zoom position) in use, the shutter speed should not go below the same number as that focal length. In other words, if you are using a 50mm zoom position on your lens, you should not use a shutter speed below 1/50s without having the camera on a tripod. With a 105mm focal length the minimum handheld shutter speed is 1/100s or 1/125s—there is no 1/105s available, so you can use the closest one. If you are using a 300mm lens, you should not use a shutter speed below 1/300s.

Male Cardinal - Nikon D2X, Nikkor 80-400mm lens at 400mm, 1/250th of a second at f/5.6, on a tripod

The reason this rule exists is because a longer focal length (zoomed all the way out) tends to magnify the subject and any vibrations you introduce while pressing the shutter-release button.

With a shutter speed below the reciprocal of the lens focal length you can introduce movement into the camera just from your heart beat, reflex mirror slap in a DSLR (that clunking sound when you fire the shutter), or natural hand shakiness. If you are going to handhold images at slower shutter speeds, you need to learn how to brace yourself properly. The best thing is to use a tripod any time you have to shoot below the reciprocal of the lens’s length. Otherwise, you will be known for your well exposed, yet blurry images (from camera shake).

Today’s vibration reduction (optical stabilization) lenses and camera bodies will help control camera shakiness, so it is a good idea to use those lenses or cameras when possible.

With lenses using vibration reduction, an element in the lens moves to compensate for minor movements or vibrations of the camera. With camera bodies having built-in vibration compensation, the camera’s imaging sensor moves to counter vibrations. Technology is improving our ability to shoot handheld shots at slower shutter speeds; however, it is still best to use a tripod for maximum quality at slow shutter speeds.

The size of the camera’s imaging sensor affects the reciprocal of focal length shutter speed rule. The smaller the sensor, the more a longer lens magnifies vibrations. In today’s cameras there are various sensor sizes: from full frame (which is the approximate size of a frame of 35mm film from the old days) to 4/3rds (which is one of the smallest in a DSLR or ILC). We discussed sensor sizes in an earlier chapter.

If your camera is using an APS-C, DX, APS-H, 4/3rds, or comparable size sensor, you need to be extra wary of handheld vibrations. Instead of using the reciprocal of the lens’s length (50mm = 1/50s), you should use 1.5x the reciprocal. In other words, if your lens’s zoom postion is at 60mm, instead of using 1/60s, it may be better to use a minimum of 1/80s, or 1/100s (1.5x would be 1/90s, which is not available). The smaller sensor with a longer lens position tends to magnify the vibrations even worse.

When in doubt, use a tripod for maximum sharpness!


Part 1: Using shutter priority mode (S or Tv), take some pictures of quickly moving subjects, such as cars passing on a road (be careful to not look threatening). Use various shutter speeds from 1/30s to 1/1000s. Examine the pictures on your computer and see which shutter speeds stopped the action. Which left a lot of blur? Which left only a little blur?

Part 2: Set the shutter speed to 1/125s and try following the subject with your lens as it moves (panning) while firing multiple images. Use high-speed continuous shutter release mode (see your camera’s users manual) so that you can fire multiple shots while holding down the shutter release button. Examine how those pictures look on your computer monitor. Do you have an sharp images of the moving subject? Does the background have an interesting look, as if you can see motion in it? Do moving parts of your subject have blur, but not the rest of the subject? Panning with a medium shutter speed can provide very interesting, implied motion subjects.

Part 3: Set your camera up on a tripod outside next to a water faucet. Turn on the faucet and take pictures at 1/1000s, 1/125s, 1/15s, and 1s. Examine the images on your computer monitor and see what the different shutter speeds do to the water’s look. Does the water appear agitated and frozen at 1/1000s? Less so at 1/125s, normal looking at 1/15s, and wispy at 1s? Try this same thing at a stream the next time you are near one with cascades or waterfalls. Do you like the way water looks at 1/15s or do you prefer 1s?

Keep on capturing time...
Darrell Young

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