Motion is a possible cause of blur in photos, either because the subject or the camera moves during exposure time. When exactly they occur and how you deal with them, when the blur might even be good for your photos, is the topic of this page.
Motion blur is caused by the subject moving during exposure time.
Whether that's good or bad for your photo depends. For example, consider the two photos of splashing waves (hit the + to enlarge)... which one looks better? One image captures the many drops that the eye would otherwise not notice, the other expresses the movement of the water visually much better.
There is no standard limit time as to when motion blur becomes visible. What matters is the movement within the image frame during the exposure time – as many pixels as your subject covers with its movement will appear blurred.
It depends on the subject's speed, the direction of movement, the distance and also your angle of view. An airplane in the sky moves much slower through the picture than a train pulling in next to you because of the distance.
A few rough guidelines are:
There is no way around trying it out yourself, gathering your own experience and also observing and learning anew in every shooting situation.
Consider whether it is really appropriate to want to freeze movement. Motion blur is the simplest way to express motion in a still photo.
If your subject, such as a car, doesn't look any different when it's in full motion than when it's stationary, your photo doesn't really win if you make it technically perfectly sharp. It's a different story with athletes, who look strained and adopt particular body postures.
If you decide to use motion blur in the picture, don't be too stingy with it – a little motion blur looks... well, just blurry, like unintentionally unsharp; it doesn't create yet the impression of movement.
If you follow a moving subject with the camera so that it stays in the same place in the frame, it will appear sharp and the background will get blurred instead. This is a common technique to create the impression of movement in a picture.
For this it is better to allow rather longer exposure times of e.g. 1/15 or 1/30 s to let the effect appear clearly. This technique needs practice and several to many attempts until you get sharp pictures. Track your subject well before you press the shutter release in order to adopt the right speed.
With the orange vehicle you can see another effect that may appear when panning: distortion in perspective. The center is sharp, the beginning and the end are not. The car was driving diagonally towards me, so at the end of the exposure time it was closer to the camera = wider in the frame than at the beginning of the exposure. This also brings blur.
As an aid you can use autofocus, when you focus on the car with a small focus point you can try to keep this focus point on the same spot of your subject.
At short distances, the flash with its extremely short burn time can help freeze movement. It can give another interesting effect – the main subject is bright and sharp from the flash, but has a kind of shadow of motion blur from the longer exposure with ambient light.
More on this under its own heading Flash.
Blur can also be caused by the camera moving during the exposure time. You can see two examples on the traffic sign; I deliberately moved the camera once sideways and once upwards at an exposure time that is usually quite good for taking sharp photos.
With millions of pixels on tiny sensors, fractions of millimetres are enough to create visible traces.
Blur can be easily detected on sharp edges, where it creates shadow-like patterns; stronger blurs become zig-zag patterns.
The risk of blurring depends on the focal length – telephoto lenses show a smaller image section and therefore blur more easily than wide angle lenses.
A rule of thumb | A common rule of thumb is ”1 divided by focal length“. According to this rule, the exposure time should be no longer than the reciprocal of the focal length – e.g. no more than 1/100 s for a focal length of 100 mm, no more than 1/60 s for 50 mm (don't be too picky, allow some rounding).
Here, the equiv. focal length (=actual focal length x crop factor) should be used as the focal length if the camera has a smaller sensor than full format.
The rule is simple, immediately provides a precise time as a point of reference and dates back to the times of analogue photography, where it has endured for decades. It still has its justification today in digital photography and often quoted. Camera manufacturers also use it, as I have observed with models from Nikon and Sony: If the camera automatically raises the ISO sensitivity to avoid long exposure times, the longest exposure time allowed is roughly based on this rule.
Nevertheless, this rule of thumb should be used with caution:
Image stabilisation | Most modern cameras have image stabilisation techniques to compensate for camera shake. The lenses or the sensor in the camera are moveable, sensors register the camera motion and try to move the lenses or the sensor in the opposite direction. This technology actually delivers what it promises and allows significantly longer exposure times without shaking.
Tips to reduce camera shake: