13.4 Simulating Motion Blur

Although motion blur is typically undesirable in bird photography, it can sometimes enhance rather than mar an image, especially if it affects only part of the scene, such as a hummingbird’s wings.  For birds in flight, if the background is too busy for your tastes, you might consider adding some motion blur to the background to strengthen the perception that the bird is moving.  This is the application that we’ll consider in this section.
    The figure below shows a male osprey hurrying back to to the nest with a meal for the family.  To reinforce the impression of urgency and great speed, we’ll apply the Motion Blur filter in Photoshop to the background; the impression we’re trying to create is that while the viewer maintains focus on the bird, the scene is passing by in a blur.

Fig. 13.4.1: Male osprey on his way back to the nest with yummy victuals.  Because the
camera was following the bird, the background was in motion, and is thus slightly blurred.
The Filter > Blur > Motion Blur effect in Photoshop can be used to exaggerate this effect.

    As usual, the first step is to separate the bird from the background, via the use of layers.  In this case we used the Quick Selection tool to quickly select the bird, and then used Select > Inverse to invert the selection so as to include the background instead of the bird.  Pressing Cmd-J / Ctrl-J then copies the background into a new layer.  (We could instead have duplicated the entire image and then used the background selection to initialize a layer mask).  The resulting Layers panel is shown below.  Note that we’ve locked the transparent pixels in the top layer by clicking on the checkerboard icon next to the
Lock label.

Fig. 13.4.2: The Layers panel after
creating a background layer.  The bird
has clearly been omitted from the
background layer.  We could also have
left the bird in the background layer
and then masked it out using a layer

    We then invoke the Filter > Blur > Motion Blur filter while the background layer is active (the blue highlighting in the above figure indicates which layer is active).  The figure below depicts the Motion Blur tool.  Then Angle setting was left at 0 since the bird is flying horizontally.  The trick is then to find a Distance value that conveys the desired sense of speed.  With the Preview box checked, changes to this slider should appear instantaneously on your screen so you can judge the aesthetics of the effect.

Fig. 13.4.3: The Motion Blur tool in Photoshop allows you
to indicate an angle and degree of blurring (
While the angle obviously needs to match the direction of
movement, the Distance setting allows some flexibility in
achieving the desired artistic effect.

    The figure below shows the final image, after application of the blur filter.  Note several things about this image.  First, the blurring of the background not only conveys the notion of movement and speed, but also reduces the detail in the background and thus helps to make the bird stand out more.  Second, note the dark shadow that extends a short distance left and right of the bird.  This is an artifact of the blur filter that arises because of an imprecise selection when creating the background layer.  The problem is that some of the darker pixels from the bird were inadvertently included in the background selection; the blur filter smeared these darker pixels out around the bird, resulting in the shadow.

Fig. 13.4.4: The result of applying Motion Blur to the background.
Note the shadow apparent to the left and right of the bird.  This is
an artifact resulting from sloppy selection prior to creating the
background layer.  Expanding or contracting the selection prior
to creating the layer can mitigate these effects, though if taken
too far you can induce other unpleasant artifacts (e.g., an
obvious margin around the bird that lacks any blurring).

    The leftmost image in the figure below illustrates a more extreme case of this.  Upon noticing this effect, the History tool (section 10.5) was used to back up to an earlier state of processing, and then the foreground selection was expanded by 2 pixels via Select > Modify > Expand.  The foregoing procedure was then repeated: the selection was inverted, the background was duplicated into a separate layer, transparent pixels were locked, and the blur filter was applied.  (The reason we expanded the foreground selection rather than contracting the background selection was to avoid edge artifacts around the outer margin of the image; because of edge effects, expanding one selection is not always the same as contracting its inverse selection).  The rightmost image in the figure below shows the result.  The shadow effect has been substantially reduced, due to the inclusion of fewer dark pixels from the bird in the background selection.  Further improvement could be achieved using either a larger expansion radius or by manually adjusting the selection boundary using the Quick Selection (or other) tool.

Fig. 13.4.5: Mitigating blur artifacts by adjusting the selection.
Left: dark pixels from the bird that were included in the background
selection resulted in dark smearing around the bird.  Right: expanding
the foreground selection prior to inverting it reduced the shadow effect
by allowing fewer stray pixels from the bird to fall within the background

    The next figure (below) illustrates the importance of locking the transparent pixels before applying the blur filter.  The left image shows what happens when the transparent pixels are not locked: the blurring has extended into parts of the image occupied by the bird, resulting in loss of foreground detail, and even the impression of a fog in front of the bird.  There may indeed be cases in which such an effect may be desirable in achieving a particular artistic goal, so it’s often worthwhile to try multiple settings of each parameter or button in a filter’s interface before settling on a final setting. 

Fig. 13.4.6: The importance of locking transparent pixels prior to blurring.
Left: blurring the background layer without first locking transparent pixels
results in a haze over the bird.  Right: locking the transparent pixels ensures
that the output of the blur filter can’t intrude into the area occupied by the bird.

    As we’ve already remarked too many times in this book, post-processing operations that dramatically alter reality in this way will be found objectionable by some people; as long as you don’t misrepresent your processed images by implying that they are unaltered photographs, there is no reason why you should feel ashamed to take advantage of digital tools to achieve an artistic rendering of a bird and the scene it’s depicted in.