How we change what others think, feel, believe and do
Eye Avoids Blur
Blur occurs where the edge between shapes and against a background is not sudden, as with a sharp edge. A blurred edge is a gradient, with the transition between contrasting areas occurring
In making sense of the world around us, the eye tends not to stop at blurred edges and objects. The corollary of this is that the eye is attracted by sharp edges.
In the circles below, the eye is drawn more towards the circles with sharper edges and away from those circles with more blurred edges.
In the real-time process where we see and make sense of everything around us, it is easier to detect a shape that has sharp edges as the eye first finds the contrast at the edges and then traces the boundary so the shape can be identified. High-contrast, sharp-edged items hence are quicker to identify than those with blurred edges.
Things that are more distant, tend to be more blurred. Blurring is also created by haze or other material that will scatter light rays. Things that are closer to us tend to be sharper and those more distant appear blurred. This is partly due to haze and partly due to the limited resolution of the eye.
At the edges of our vision, things are more blurred. We might wonder if this could be dangerous as we do not see the sharp edges of an incoming attacker. However, this lack is compensated for by our skill in detecting motion. We may also note that if everything we see was sharp, we would find it more difficult to pay attention to the small area that is most sharp in the middle of our vision.
Photographers make use of this blurring-and-attention effect by using a wide aperture on the lens so the depth of focus is relatively shallow. In this way they tell the viewer where to look and where not to look. Typically, the foreground is sharp while the background is blurred. The blurred effect they get is called 'bokeh' and is shaped by the blades in the lens iris. Better quality lenses give smoother blur in the bokeh.
When you want to direct attention in images, show sharp edges to grab attention and use blur to reduce the priority of background items. Blur can also be used deliberately to confuse or suggest rather than tell, hence sustaining attention as the person thinks about what may be shown.
Photographers can use a wide aperture and careful focus to highlight the subject of the image.