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BokehMeasurement.jpg
11-Nov-2013

BokehMeasurement.jpg


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Lensrentals13-Nov-2013 23:30

The rendering of out-of-focus areas by a lens, also known as bokeh, is often considered an important characteristic of a lens. However, you will rarely find a quantitative measure of bokeh in a lens test, and at most a few examples might be shown as part of a larger image. The layout shown here is an example of what I hope gives a quick and valuable overview of bokeh for one lens, by isolating these defocused shapes.

A lot can be learned about the lens just by looking at the bokeh. For example, this lens has:
-A 6-bladed aperture, demonstrated by the hexagonal bokeh shape when stopping-down.
-Little vignetting, indicated by the fact that the shape of the bokeh does not change much as we move away from the center of the sensor.
-Longitudinal chromatic aberration, seen as red edges on one side of the focal plane and green on the other.
-Smoother bokeh behind the focal plane, without the inner rings that appear in front.

The images were taken with a NEX-6 with bellows, using an extension of 155-135mm. The bokeh images were created by placing a bright light behind a small pinhole, then adjusting the focus to blur this point. The pinhole itself has a finite size seen in the in-focus images of about 0.1mm, which was the smallest I was able to produce using available tools (i.e. a pin and paper). The final image is limited in that only shows bokeh at one magnification, but I hope that someone finds it interesting enough to take a closer look at their own lenses.
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