aI think its a fair claim to say, that anyone fallen in love with vintage lenses, is also fallen in love with the bobles these lenses can produce. These bobles also called Circle of Confusion, or CoC can produce an artistic and dramatic effect to the picture.
Bokeh and Circle of Confusion is closely related. Where Bokeh are the out of focus parts of an image the Circle of Confusion is highlight spot, falling outside of the depth of field. In other words, when light entering a lens is not in focus, a point on the subject is focused into a circle on the sensor. This circle is the circle of confusion. The more out of focus a point is, the larger the circle of confusion becomes.
The size of the circles is depending on focus distance, subject distance and aperture. Stopping down the aperture creates smaller circles of confusion, which results in the depth of field being increased and more of the image being in focus
The good, and the bad
Different lenses render bokeh differently depending on their unique optical designs and qualities. Remember there is more to bokeh than just background blur; all lenses are capable of producing out of focus blur, but not all lenses are capable of rendering beautiful bokeh. On the site you will find plenty blogs on the bokeh of individual lenses, @danielellwanger wrote a very good blog post on this as well.
So what makes for a good or beautiful bokeh? a pleasing bokeh is a subjetive experience, but generally the background blur should appear soft and “creamy” sometimes with perfectly round smooth soap bubble alike circles of light and no hard edges.
Then how about bad or ugly bokeh? I’ll argue that such a term does not exist! I rather say displeasing bokeh, which for me is everything that distracts my eye away from the story or intent of the picture, in a destructive way.
What a bokeh brings
Some would like to turn the question if bokeh is important, into a question about you as a photographer. In reality they are asking if you see yourself as being an Artist or a Technician. I disagree with such perspective entirely, the choice on using bokeh or not comes down to how you want your spectator to see your pictures and which emotions you want to impose on the spectator.
Having a good bokeh isn’t necessarily the same as having a good photo. There is many other factors going in to that equation. But that isn’t an argument for that the bokeh is without importance, because it is a strong tool we as photographers can use to communicate.
A Bokeh can separate your photo in levels, allowing your viewer to see the photo in several planes. A Bokeh can also direct attention, and lead the eye. A bokeh can create a special emotion, like romantic feeling or introduce magic to the scene.
What creates the best Soap Bubbles
These out of focus highlights can be a result of light shining against trees, street lights, and reflecting light outside the focus of your image. In short what you are heading for, when making soap bubbles appear, is shooting is backlit scenes wide open. Primes is also a lot easier to work with, while they often are faster and their design better for this particular requirement. As you will be shooting into the light, is manuel focus, and manuel setting, mostly how such a scene should be captured.
Create your own Bokeh Project
Something as simple as a crumpled piece of foil can be the basis for a creative photo project. Position a still-life subject on a sheet of glass with a piece of dark material underneath it. Scrunch up the kitchen foil then smooth it back out and place it in the background. Shine a table lamp or torch on the foil and, with a tripod mounted camera, dial in the lens’s widest aperture. During the exposure, shine a flashlight onto the subject.
Check out Zeiss’ lengthy article on Depth-of-Field and Bokeh