The composition of an ad plays a vital role in determining which parts of the ad people look at. Advertisers need to leverage the principles of visual design to direct people’s attention.
 
When we look at a static visual, our eyes are drawn, initially, to its 'dominant element’ - whatever stands out most from the rest of the image. This could be due to size, colour, shape, texture, brightness, sharpness etc. The image here was inspired by the great Dutch artist Piet Mondrian. When people look at this image, their attention is initially drawn to the large, coloured square. Their eyes are then directed to the two smaller coloured squares - the secondary and tertiary focal points within the image.

In real life, whether the viewer can be bothered to spend more time looking at an ad depends on how much interest its dominant element generates and whether the ad has any secondary elements that also stand out from the background. Effective ads convey their message and connect it with the brand before people get bored with what they’re seeing and move on.
 
You may have come across the 'Z’ or 'F’ patterns used to describe how people’s eyes move as they look at a screen. However, these are based primarily on how people look at blocks of text, rather than creative images. The 'Rule of Thirds’, 'Golden Triangle’, 'Fbonacci Spiral’ and 'Golden Ratio’ are all useful concepts for composing an attractive, well-balanced image, but they all pale into insignificance compared to the principles of visual hierarchy, focal points and flow described above.