What Determines the Quality of Bokeh?

What Determines the Quality of Bokeh? Bokeh may be creative or add an artistic flavor but clearly we should avoid bad quality of bokeh. Quite obviously, it is understandable that two factors – the body of the DSLR camera and the Lens are two variables. So, we should discuss these two variables and list their individual attributes. Those who are relatively newer in DSLR Photography, can read our previously published articles on bokeh, which might not be fully related to the current topic but definitely useful – Bokeh in Digital Photography, How to create perfect Bokeh with DSLR and Prime Lens, How to create gradient Bokeh effect, Tips to create custom Bokeh shapes.

Practically we can create bokeh with as basic phone camera as (now discontinued) Nokia N73 ME. Point and Shoot camera can create Bokeh too, but we are only discussing about DSLRs.

What Determines the Quality of Bokeh : Lens

Lens is the biggest factor for creation, sweetness, symmetry of Bokeh. All components of a lens can affect the quality of Bokeh :

1. Overall design of the lens
2. How the spherical aberrations are corrected by the design
3. The number of blades determines the edges of bokeh, shape of the bokeh
4. Focusing distance for a given lens – it is not focal length itself
5. The aperture which is  used or the aperture which is usable for a given lens for a scene
6. Depth of field
7. Circle of Confusion
8. Hyperfocal distance => H = (f/d) / c

The depth of field sets the maximum value for the circle of confusion, it does not describe what will happen to regions once they are out of focus. In these regions we get the bokeh. As the circle of confusion is not really a circle in most complicated lenses, larger and elliptical circle of confusion will render the Bokehs bad. In the days of Film photography, the Lens factor was the major determining factor for bokeh.

In general high quality bokeh is viewed by most photographers as the out of focus areas which are smooth rather than harsh. Most lenses that produce smooth bokeh also produce out-of-focus highlights which are round rather than elliptical or regular convex polygon. Since the lenses that produce round out of focus highlights with soft edges also tend to produce smoother out of focus areas in general, this is one of the easiest ways to evaluate the quality of bokeh that a lens produces.

Low quality bokeh is viewed as out of focus areas that have harsh transitions from the blur of one out-of-focus object to the another. Lenses with harsh bokeh also tend to to produce out of focus highlights which are not round. Some may also produce round out-of-focus highlights that have very hard edges or are even brighter near the edge than the center which also tends to make the bokeh very harsh for all out-of-focus areas.

Optically, if a lens with focal length below 50MM ( prime ) is mounted on the DSLR body with sensor size which is below the size of full frame – that is APS-C size sensor, it will tend to create bizarre effects – the bokeh in the middle will be near perfect while bokeh at the extreme corners will become elliptical. If a lens with focal length below 50MM ( prime ) is mounted on the DSLR body with sensor size which is bigger than the size of full frame (that is Medium Format DSLRs), it will tend to create smoother gradients. A very smooth bokeh has more or less a Gaussian illumination in the blur circle.

The iris blades make a smoother transition from transmitting to light blocking. Like the Sony (Minolta) STF 135mm has this feature (STF = Smooth Transfer Function). Old film camera lenses has good number of blades which are able to create excellent round bokehs. Hasselblad with digital camera back can be a winner depending on the budget.

What Determines the Quality of Bokeh : The DSLR

As possibly all the lenses excluding special lenses for the medium format cameras are manufactured calculating the optics of a full frame not a DSLR camera aided with APS-C size sensor (that is why crop factor exists), APS-C size sensor itself can be a negative point. As DSLR is a digital microprocessor controlled device, quite obviously, if a Sony (Minolta) STF 135mm is mounted on a Canon 1000D, the camera itself might add hundreds of other factors to overall distort the image including frank noise.

Last but not the least to mention – except few creative photographs, we need a good background not a photograph to showcase bokeh.