Jonathan Consiglio wrote in post #2524002
Another thing that plays into this, as mentioned above, is how many blades there are.. The "nifty fifty" 50mm 1.8 only has 5 blades, and this is very noticeable in shallow DOF images, especially if there are lights in the background.. The higher end lenses have at least 8, and will create a much smoother blur..
="GyRob"]it tends to work on how many balde's the apeture has the more blades the creamyer look to the shot at a given apeture.
This is a very common misconception, but the number of aperture blades does not affect the quality of the bokeh.
It does affect the shape of the bokeh, especially out of focus highlights, but not the smoothness (or lack thereof) of the bokeh itself. It so happens that high end lenses often have more aperture leaves, but their superior bokeh is produced by other factors. And it's very easy to prove to yourself that aperture leaves have virtually nothing to do with bokeh -- just shoot wide open, and the aperture leaves won't be a variable at all. You'll still see a big difference between a top end lens and a cheap lens, and the aperture leaves are fully retracted, out of the path of light as it passes through the lens.
First, it helps to understand what it means for something to be in or out of focus. Say you shine a laser right at your camera lens. If that laser is in perfect focus, it will be rendered as a little disc with a certain minimum size on your film or sensor. Even if it's just a 'point' of light, like a laser or a distant star, it will still be 'disc' shaped and have a minimum diameter when in perfect focus. There are lens effects like diffraction and aberrations that prevent it from being focused as a perfect point. The diameter of this in-focus disc is what's referred to as a 'circle of confusion', and it's a measure of lens resolution. As something gets progressively out of focus the circle of confusion gets larger and larger. As you stop down the aperture the circles of confusion get smaller and smaller (and begin to take on a shape that resembles the shape of the aperture leaves).
So its these out of focus circles of confusion that have a certain quality or character that we regard as good or bad bokeh.
The principal contributor to bokeh is spherical aberrations in the lens elements. When spherical aberrations are overcorrected, you tend to see 'donut' shaped bokeh, with bright borders to the out of focus areas. Some lenses are very highly corrected for reproduction work, and they tend to have poor bokeh. On the other hand, when lenses have too much spherical aberration, they aren't as sharp and their resolving power is limited. Finally some lenses are cheap, and not much engineering work and thought has gone into striking a perfect balance between over and undercorrection.
The best lenses, particularly the German lenses (most famously Leica, but also Zeiss, Rodenstock, and Schneider) are famous for their exquisite care in engineering a perfect amount of spherical aberration so as to produce smooth, pleasing, 'creamy' bokeh.
As mentioned, the more out of focus something is in your photo, the fewer fine details (i.e. small circles of confusion) will be resolved and the more soft it will look. So wide apertures, close subject-to-lens distance, and long focal length will all contribute to having smooth out of focus backgrounds (by virtue of the fact that the DOF is shorter with those parameters). The more out of focus something is, the more likely its detail will melt away and all the huge circles of confusion will just overlap and blend into something unrecognizable. Of course this is a compositional choice and sometimes it's very good (and in many ways superior) to have an out of focus background that still has some recognizable features.
Here are some links for further reading about bokeh: