Depth of Field revisited…

While I’ve understood that a low aperture number = shallow depth of field – and vice versa – what I have been failing to get my head around is why that’s the case. (I am not technically minded, it has to be said). It didn’t make any sense to me that, if the lens was opening wide, less information was being captured. Surely the wider the lens (so the larger the actual aperture), the more room there should be for light and information to get in? Well, that was how my physics-challenged brain saw it. Talk about confused (which is ironic, really).

What I had failed to take into consideration was the way that light travels  and how the information either side of the focal point makes it to the sensor. The diagrams below helped me get my head around the whole angle confusion. The first shows a low aperture/f-stop – so wide lens – with the focus on Object 1. The information from Object 2 – the furthest away – reaches its sharpest focal point before the sensor. The information from Object 3 in the foreground reaching its sharpest focal point beyond the sensor. This creates what are known as circles of confusion. The larger the lens opening – so the lower the aperture – the larger the circles of confusion.


Conversely, the opposite happens with a large aperture, where the lens opening is much smaller. And while the sharpfocal points for Objects 2 and 3 aren’t quite hitting the sensor exactly, they’re near enough to create the impression that everything is in focus or thereabouts. The smaller the lens opening – so the higher the aperture – the smaller the circles of confusion. Aperture-DOF-21

Zig finally managed to get me to grasp the science behind this in class the other week – but the Dutch photographer, Reiner Van Houten’s excellent website, from where these graphics were sourced, also helped to finally ram the message home.

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