Bokeh refers to the aesthetic quality of the out-of-focus areas in an image. To achieve the bokeh effect in photographs, you need to create a shallow depth of field, which separates the subject from the background and causes the background to blur out.
Photo by Ali Pazani
Following are some of the essential requirements to get the Bokeh effect in your Photographs
Use a lens with a wide aperture
A wide aperture (low f-stop number) allows more light into the camera and creates a shallower depth of field. A lens with an aperture of f/1.8 or lower is ideal for achieving a good bokeh effect. This does not mean that you just have to use this lens at any aperture. Set it at its maximum open aperture, be it 1.8 or 2.8 to get the Bokeh effect. More the number of blades in a lens Aperture, the rounder the Bokeh highlights.
photo griffin wooldridge
photo - Andrea Aliverti
Keep your subject close to the camera
The closer your subject is to the camera, the shallower the depth of field will be. This will create a strong bokeh effect in the background. The depth of field is directly proportional to the distance.
Keep the background far away from the subject
The further the background is from the subject, the more out of focus it will be, creating a more pronounced bokeh effect. So, to have a Shallow depth of field effectively you need to keep the subject closer to the camera and as much gap as possible between the subject and the background. What you see in the Photograph as Bokeh are the out-of-focus highlights created by the opening of the Aperture. This is the area that is not sharp. The distance which is focused by the lens is the sharpest of all. Anything further or before this distance is out of focus and as the distance increases both forward and backward, the scene becomes more and more blurred.
photo by Marcelo Dias
In this photograph, the light of the vehicle which is closer to the subject and the camera failed to create the Bokeh effect but as the distance increased the effect also is quite visible on the lights which are far away.
Photo by Alex Dolle
In this Photograph, the effect is most pronounced on the lights which are closer to the camera. The lights along the subject and the distance at which the lens has been focused are quite sharp and the effect is again visible as the shallow depth of field begins to drop. More open is the Aperture, shallower is the Depth of Field and hence more pronounced is the Bokeh effect.
Use a longer focal length lens
A longer focal length lens (50mm or more) can help compress the background, making it appear more out of focus and creating a stronger bokeh effect. Also, longer focal length lenses inherently have a very Shallow Depth of field. This is because a particular Aperture on a longer lens is a larger Physical opening as compared to the one on a wide lens. On a longer lens or a lens with a longer focal length, since the light has to travel a longer distance, the physical opening through which it travels is broader. On the other hand in the case of the lens with a shorter focal length, the light travels a shorter distance, so the physical opening is narrower. Effectively, on a particular Aperture setting, the same amount of light reaches the sensor irrespective of the focal length being used. For example, f8 will allow the same amount of light to reach the sensor, both in the case of a 200 mm lens and a 20 mm lens.
Fixed Focal lengths work well too!
You can also use fixed focal length lenses of 50mm or higher focal lengths which have a wide open aperture, anywhere between f1.2 to f2.8 Examples are 50mm f1.4, 85mm f1.8
Photo by Omar Houchaimi
More the number of blades better is the Bokeh
You will notice in these photographs that one has absolutely round Bokeh where as the other one has Pentagonal bokeh. Had the number of blades been a bit more the shape would have been octagonal and so on.
Photo : Alexey Demidov
You can clearly see the Octagonal shape of the Aperture. Had there been more blades, the opening would have been more circular. You can also notice in the following photos that the the aperture hole is more uniform in one as compared to the other lens.
Experiment with different light sources
The bokeh effect is enhanced when there are small, bright "Point" light sources in the background, such as Christmas lights or city lights. It wont work on broader light sources like Tube lights or Bill board lights. The light source has to be a point source like a typical house hold tungsten Bulb.
Photograph by Val Idjao
You will notice in this shot that only the point sources are creating the Bokeh effect, even though there are a lot of lights in the background.
Factors that control Bokeh in a nutshell
The fast aperture of at least f 2.8 is Ideal
Fast prime lenses also help in fast focusing
Use Long focal length lenses to create more dramatic bokeh
Shoot with the lens set at its widest open Aperture
Increase the distance between the subject and background, which allows the background to be more out of focus.
Move closer to your subject, which effectively reduces the Depth of field.
Take close-up portraits, which effectively means being close to the subject physically or optically.
Shoot macro images in nature with a brightly illuminated background
Notice that the highlights have a marked line in all of them. It's very important to keep the lens clean especially if you are planning to shoot images with Bokeh in the background. This might be a scratch or dust settled on the lens. You can actually remove the UV filter if you have one on the lens.
Remember, achieving the bokeh effect is not just about technical settings. It's about creating a visually interesting and pleasing image that draws the viewer's attention to the subject of the photograph.
In the nutshell, Position your subject in such a way that you have the "point" light sources in the relatively distant background and shoot with the longest possible focal length at the widest possible opening.
You can of course use a flash as well to illuminate your subject if it is poorly lit or sometimes not lit at all.
TIP - In case you do not have a very fast lens. You can get Bokeh effect even at f8 on a long lens, as long as you keep a whole lot of difference between your subject and the background.
200mm f/8 1/800 Canon EOS 400D Photograph by Munish Khanna