One of the most popular posts here at DIYP is the Create Your Own Bokeh post – this is the one that teaches you how to make nice shapes in the blurred area of your image – AKA Bokeh.
It is a nice thing because, usually, those hotspots are just taking away from your subject attention, and if you apply this technique, those annoying hotspots can become part of your artistic say.
Creating your own bokeh shapes is easy and fun, however there are a few repeating questions that I frequently get by email and comments. This is why I was really happy to learn that manimal magic has done some great thinking and have solutions for all questions. Some of manimal magic’s wisdom was found in the comments of the original post, and some was taken (along with the images for this article) from his (really awesome) Flickr stream. I am going to format this as a Q&A thing, cuz it really feels like he’s answered all the hard questions.
How Small Should I Make The Shaped Hole?
Well, that really depends on the aperture your lens have and on the focal length you are shooting with. The calculations below are based on the assumption that the black card is placed directly on the outer glass. In real life there is a bit more distance created by a filter, so the size needs to be a bit smaller
Because we are creating a new aperture, it must be smaller than the one inside our lens. This of course is how a lens works – it is one big aperture with a
smaller one inside. All we have done is supplement the old aperture :)
If we take a “large” aperture, say a value of f/2 – this just expresses
the ratio of the focal length to the diameter of the aperture.
In a prime lens of 50mm with an aperture setting of f/2 the aperture
will be 25mm, so your cutout shape must be less than 25mm in diameter
to get the effect.
A prime lens of 100mm at f/2 will have an aperture diameter of 50mm,
which is why they are so big and heavy and expensive, and your pre-cut
shape can be up to 5cm in diameter.
How do you know how big of a hole to make and what settings to use on any lens?
The shape you make must be smaller than the aperture in your lens.
Take the focal length that you wish to use (e.g. 100mm)
Divide this by the aperture value that is smallest on your lens (e.g. f/2)
In this case the diameter of the lens aperture is 50mm.
If you make your shape smaller than 50mm across the effect should work as shown above.
50mm f/2 – shape must be less than 25mm across
75mm f/2 – must be under 37.5mm across
100mm f/2 – must be under 50mm
50mm f/2.8 – must be under 17.5mm
50mm f/3.5 – must be under 14.2mm
Can I use this cool trick with my point and shoot?
Well, that depends on how small you can make your punctured hole. The focal length of a regular point and shoot is about 7 millimeters on the wide side and about 12 millimeters on the zoom side. (I’m talking real focal length and not crop factor equivalent).
That means that your hole will have to be about 2.5 millimeters at 2.8 wide. Not that trivial, but possible.
How do I make the hole in the dead center of the lens?
One of the key points to utilizing the entire “new” aperture is to place it on the dead center of the black card or cap of the lens. It’s real easy if you make two lines to form an “X” on the card board. See the picture to the left.
Once you have the cross to mark the center, simply place your hole on the middle of the “x”. As my good friend Indy use to say “X marks the spot”.