What is camera shake

Camera shake is when movement of the camera results in an image that is blurry. While camera shake and focus are independent, a camera shake would often lead to an out-of-focus image. Even if an image is technically in focus it may be blurry or lack clarity but the light from the object is being recorded multiple times at different locations on the sensor.

Camera shake is almost universally bad. It is right up there with an out-of-focus image. Avoid camera shake at all costs. 

What causes camera shake

Many factors can contribute to camera shake. It isn't just slow shutter speed. Anytime you shoot handheld you run the risk of introducing blurriness due to camera shake. The problem is that humans cannot hold perfectly still. The blood circulating in our body and our breathing makes us tremble just about enough to not be able to hold the camera still enough. 

Here are some factors that cause camera shake. 

• Shutter Speed. Slower the shutter speed, the greatest the chase of camera shake. 
• Focal Length of the Lens. Longer the lens (like a telephoto lens - say 200mm) the greater the camera shake. 
• Lens size. The bigger or heavier the lens, the greater the camera shake.
• Camera Support. Shakier your grip, the greater the camera shake. Hence tripods help!
• Shutter Release. The messier your click, the greater the camera shake. 
• Mirror slap. SLRs mechanically depend on a mirror for you to preview your picture. When you click the shutter release, the mirror lifts up and finally drops back. The drop can be heavy and cause shake. 
• Camera Type. Some cameras are designed better to reduce shake. 

Reciprocal Rule

The reciprocal rule states that in order to avoid camera shake - The shutter speed of your camera should be at least the reciprocal of the effective focal length of the lens.  

This means if you are using a 50mm lens, the closest full shutter speed is 1/60th of a second so that should be your minimum shutter speed. If you are using a longer lens like a 100 mm lens then you want to shoot at least 1/100th of a second. For a 200mm lens - it is at least 1/200th of a second and so on. 

If you are not shooting "full-frame" (where the size of your sensor is smaller by about 1/1.6 of the standard size) then keep in mind that a lens rated at 50mm is really an 80mm lens so you really need to be at 1/80th of a second or faster.

Now add ISO to this. These ratings are for ISO 100. So, if you shoot ISO 200 then you can shoot around 1/15 and 1/30 and greater on the same lens without getting a shake. And if you shoot ISO 400 then you can shoot 1/8 - 1/15 and higher and so on.

Reciprocity rule only holds between shutter speeds between 1 second and 1/1000th of a second.

Image Stabilization

Image Stabilization (or simply IS) is a group of techniques used to reduce blurring associated with the motion of a camera or a lens. The central idea is that motion can be detected by using motion sensors and then corrective action can be taken to compensate or correct for that motion. 

Generally, IS is built into the lens. A lens with IS can give you 2 "f-stops" worth of light. Meaning to say that if there is enough light to shoot only up to 1/250th of a second then with an IS will probably help you to shoot at 1/60th. A very expensive lens's IS might let you shoot about 3 stops lower. However, shutter speeds lower than 1/60 are probably still going to be hard to manage.

Motion Blur

Motional blur is not the same as camera shake. Motion blur is a technique using which a photo can communicate a sense of motion. A critical difference between motion blur and camera shake is that in motion blur only those subjects are blurred that that move in real life and static objects are static. Whereas in camera shake - everything seems blurred or moving - even things that don't move - like lampposts or tree trunks or rocks. 

Motion Blur is often desired and can be "cool". Camera shake is rarely if ever desired. 

For some additional techniques when you don't have or cannot use a tripod check out this post.