By Georg Wacker

9 October 2008

Rainy day outside? Bored? Fear not, we will build a vignette lens cap today!

I stumbled across this very nice tutorial at DIYphotography.net and improved it a bit.

You may ask why to do this? Well, it's fun and nearly costs nothing and it will give you a new creative effect to play with. The goal is to change the appearance of the highlights in our beloved bokeh. Depending on your lenses aperture blades you will see different shapes, but mostly a n-gon. By holding a shaped vignette in front of the lens you can alter its shape.

All you need is one piece of black paperboard, scissors, glue strips, a good ruler and 20 minutes.

As you can see in the photos I used blue paperboard, which just works fine, too. Also, you might want to use non-reflective glue strips on the inside.

First you need to decide which lens to use, a luminous prime lens with f/2 or below is good, because by using our vignette (which, optically speaking is just another aperture with a fixed 'blade') your effective light stop will be higher.

I'm using the Pentax-M f/1.7 50mm, which will allow me to use cutouts up to 29mm in front of the lens (50mm : 1.7 equals ca 29.4mm).

Now let's start off with some measurements. Focus your lens, so that it has the smallest depth. Take a piece of paper and measure the circumference of the lens, by wrapping it around it. It should slide of while preserving a nice grip. Mark the spot where the paper overlaps; you now have the length (210mm) of our lens wrapping. Next, measure the depth (30mm) that'll give us the width. Cut out a piece of our black paperboard accordingly plus some overlapping space (220x30mm) and glue it together, to get a nice ring that slides on your lens.

Measure the diameter of the ring (60mm) and add two times the width (2x30mm) of it; you now have the diameter (120mm) of our big square to cut out. Keep in mind, we want a square that fits on the ring and four flaps to put it together. You'll get the idea when looking at the pictures.

Next, cut out a square in the middle, I used a slightly bigger value than my maximum cutout value, a 30mm square. Fold the flaps and use the glue strips to put it together. I added two thin stripes of paperboard on top to hold our interchangable aperture shape-sheet.

Cut out the actual shape on a piece of paperboard that fits between your cramps, so that the hole is completey covered. Cut any shape you like, but keep the max. size in mind (remember? 29mm in my case). Make sure to center it properly on the sheet. Using scissors will work fine, a special paper puncher will give much cleaner results, though.

Slide the finished sheet between the clamps and equipp your lens with the new vignette lens cap.

Now, any shots with articulated bokeh highlights (that is, the aperture blade shape) will get a special touch!

Experiment with different shapes and incoperate them in normal motives or as a starting point for abstract pieces. Have fun!

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