Posted on April 29th, 2015

Aperture Shape & Custom Bokeh blur

Have you ever looked at the out-of-focus areas in an image and wondered why all objects and more notably objects like a lights blur in a round shape?  Playing with my Camera Obscura got me thinking about aperture shape and Bokeh blur.  I think it is important to know that the out-of-focus parts of an image actually acquire the shape of the aperture. This got me thinking about creating a custom aperture shape. As I don’t fancy opening a lens up, its possible to mound an aperture modification to the front of a lens much like a filter, just so long as the new aperture is smaller than the original aperture in the lens(if not the shape is clipped by the old aperture). The only real problem is that once the new aperture is mounted, the original aperture becomes obsolete, so control over the aperture is lost. It’s also important to remember the max aperture will be stopped down a few (depending on the size of your shape) as this is now your new aperture. This becomes annoying as this whole project is about creating Depth of Field and out of focus areas.

So where to start?


I wanted to start simple, so just used some card that I cut to the size of my Cokin filters. This way I was able to mount them easily. In the center for each card, I cut my new aperture shapes and then could just slide them into my Cokin Filter Holder.


Let’s Take Some Shots


I found that my 50mm f/1.8 was the best choice of lens and simply whacked it out-of-focus and took some photos of some old Christmas lights.


What I Found


  1. The more out-of-focus an image is, the larger the shape shows. This allows you to control the size, but only when none of your image needs to be focused. Obviously ,when shooting with part of your image in-focus, depth of field controls/limits this


  1. Using a Cokin filter holder is a good way to mount the aperture filters


  1. As the filter becomes your new aperture exposure and DoF will be affected


  1. It’s worthwhile to make nice filters once and keep them in your bag

While searching online I also came across some pre made “creative aperture kits” like this one below. I Have not had a chance to try them yet but defiantly worth a look.
Lensbaby Creative Aperture Kit


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