About Me

My photo
Cambridge, MA, United States

Sunday, March 31, 2013

Lens Bokeh Analysis


<<This topic is under development>> << Online version is only a draft >>

What characterizes the bokeh in a photograph? I mean not the shape of the bokeh, but the bokehness or the bluriness. We know that bokeh mainly depends on the aperture, focal length, and the focal distance. It also depends on few other things which will see later. The relationship between the size of the bokeh (circle of confusion) and the above mentioned parameters is a well established concept.

Here, I’d like to graphically plot and show how aperture, focal-length, distance and sensor size makes an impact on the bokeh. My objective is to provide a visual or a mental tool to quickly assess the blur characteristics given a set of parameters. And, perhaps help in understanding why bokeh in large-format photography is unique and often not reproducible in 35mm (full frame) format photography. We will also see how lenses such as EF 85mm f/1.2 and EF 200 f/2 differ in bokeh. Which of these lenses gives more bokeh? if you want to know the answer, please continue to read this discussion!

The size of the bokeh, let’s designate it by it’s diameter $C$, is given by

$C = \frac{\Delta }{{{s_1} + \Delta }}\frac{f}{N}m$

where, $f$ is the focal length, $N$ is the aperture, for instance $N=1.4, 2.8$ so on. $s_1$ is the distance to the subject, meaning, the camera is focused to the subject which is at a distance $s_1$. These parameters should be familiar if you are doing semi-serious photography. Now let’s look at less common terms: $m$ is the magnification and $\Delta$ is the relative distance. I will explain these two terms now.

Tuesday, March 19, 2013

Aperture Play – a customized EF 50mm f/1.4 Lens.


I wanted to play with the aperture of a lens, for instance to simulate anamorphic bokeh, gaussian aperture (apodization), and other aperture masks. By introducing aperture masks, we can control the blurring (bokeh) and hence control the appearance of the photograph. Carefully designed aperture masks can be used to perform computation on photograph that can reveal depth, for example using coded aperture: http://groups.csail.mit.edu/graphics/CodedAperture/.

I was inspired by the work of Markus Keinath here: http://www.4photos.de/camera-diy/Apodization-Filter.html. In order to access the aperture, the lens needs to be opened, which can be a pain. This especially hinders experimentation. I wanted a lens system where I can have constant access to the aperture. This way, I can see the live video from the DSLR while the aperture masks are being modified.

My Lens Modification

I modified a Canon EF 50mm f/1.4 lens by creating my own lens assembly. f/1.4 is quite large enough for the purposes. However, EF 50 1.4 costs about \$300, which is one downside of this choice. A cheap (\$75) FD 50mm f/1.4 would have also served the purpose, but this lens has mechanical compatibility issue with modern dslrs. The back element projects into the mirror assembly of the dslr when lens it focused a infinity.

Here is the regular lens and the modified lens:


The focusing is achieved using a small slider mechanism (with the knob as shown). The acrylic pieces were cut using a laser cutter. The metallic mount came from the original lens itself.

imageThe aperture can be introduced into the slot shown in the above picture. One issue with the mechanism is the light leak. The aperture region and other lens regions have to be covered with a black enclosure to avoid light leaking into the lens.

Aperture Masks


Using microsoft power point, I drew a bunch of aperture masks as shown above and printed them using a laser printer on transparencies. I then cut them into ‘inserts’ so that it fits into the aperture slot. Here is a printed ‘gaussian’ aperture mask:


Some Results

Image screen captured using EOS Record Utility. I know I could have taken a photograph, but this was more convenient in my setup (didn’t want to download images, edit etc). All images are straight from Live View video.







Issue – 1: Quality of Aperture Insert


Issue – 2: Cat Eyes


Issue – 3: Light Loss

Issue – 4:

Future Work

  • Analog film printing of aperture masks.
  • Colors in Aperture mask