This is the first of a four-part series on lenses which attempt to control bokeh. If you want to easily navigate to the other parts, at the bottom of this post (below the comments) you’ll find pingback links to each of them.
Bokeh is a term, originally used in Japan, that describes a camera/lens system’s rendering of out of focus (OOF) objects. Since the OOF parts of a scene can be thought of as the superposition (linear addition) of the OOF renderings of each point in the scene, almost all that is necessary to define a lens’ bokeh is the shape and size of the OOF rendering of a point source. Once the lens is focused at a particular distance, the shape may vary with object distance, but that’s a second-order effect. The size will certainly vary with object distance. When it’s placed well away from the focus distance, the diameter of the blur circle of a point source can be estimated quite precisely by conventional DOF methods, both in the image field (on the sensor) and the object field (in the scene). For a thorough explanation, look here. If you get interested in the behavior of OOF blur near the point of focus, or when lens aberrations and diffraction are taken into account, have a look here. The preceding link is to the start of a rather long-winded and many-part exposition. To see the parts, go to the bottom of the first page (below the comments) and click on the pingback links.
This post is not about the quantity of the OOF blur, but rather about its quality.
I have two lenses that are designed to produce unusual bokeh. One is the Sony 135 mm T/4.5 (f/2.8) STF, and the other is the Nikon 135 mm f/2 DC-Nikkor. STF stands for smooth transition focus, and DC stand for defocus control. The Nikon lens has a control that allows the user to turn the defocus control feature on or off, so it can act like a regular 135 mm f/2 lens. The Sony lens is a more special-purpose device, and the STF feature is baked into the design of the lens.
I tested both lenses on a Sony a7RII.
Here’s an image of a small LED flashlight about 25 feet from the DC-Nikkor, with the lens wide open and focused at its minimum distance, which is a bit less than four feet:
There is a very slight radially symmetric falloff that starts about two thirds of the way from the center to thee edge. Longitudinal chromatic aberration (LoCA) contributes to the color fringing at the edge. Otherwise, the image is altogether unremarkable.
Leaving the focus at the same place, and dialing in the appropriate defocus control for objects further than the focused distance gives us this:
There is more falloff from the center to the periphery, it is smother, and it starts sooner. All this should produce smother bokeh.
If we want to be perverse, and twist the defocus control in the wrong direction, we see this:
Now you can see that the falloff as the radial distance increases actually reverses as you get near the edge. That’s a recipe for jittery bokeh.
Wide open, and focused to about the same distance, the STF produces an image like this:
The rolloff in brightness as you get away from the center is much more dramatic. There is still a sudden drop at the very edge of the circle, but the light has fallen off by a whole lot by then. This lens should produce really smooth bokeh.
Stop the Sony lens down a stop, to T/5.6 (f/4ish) and we get this:
This won’t produce quite a smooth bokeh as the lens does wide open, but it should still be pretty darned smooth. Note that you can see that the aperture is not quite round when the lens is stopped down.
Stopping the Nikkor down to f/2.8 and setting the defocus control correctly gives us this:
I never did like the way this lens worked at f/2.8, and now I know why. The odd way the blades close in the lower right part of the image is reduced at f/4, and disappears entirely by f/5.6.
It is clear that the Nikon’s control of bokeh is much more subtle than the Sony’s. What’s all this mean in the real world? We’ll shoot some pictures next.