The Nikon 135mm f/2 and 105mm f/2 lenses include what Nikon refers to as “defocus control.” This “control” is a separate ring on the lens with settings that look like aperture numbers (F5.6, F2, R2.8, etc.). I’m not qualified to discuss the technical aspects of what this changes optically but it involves controlling the amount of spherical aberration in portions of the image. In simple terms it allows the photographer to change the character of the out-of-focus areas (foreground or background). For example, if you are shooting a photo with an out-of-focus foreground you can turn the ring to the appropriate F (front) position to give the foreground blur a bit more smoothness (I did say I wasn’t doing to discuss the technical aspects right?). When the ring is turned to the F position the the background blur suffers a little and when the ring is in the R position the foreground suffers. Nikon’s documentation states that the ring should be set to a number at or below your aperture. For example, if you are shooting at f/2.8 then you should never really have the ring set to anything higher than R2.8 or F2.8 (lower is fine). When the ring is set to a number higher than the aperture you start to introduce a soft focus effect. This can be a desired look but is typically seen as a cliche effect from the past. Many people criticize the two DC lenses because of this side effect but this is not their intended purpose. From what I have read, a true soft focus lens will introduce spherical aberration (normally something you don’t want in your image) across the entire field (foreground to background). The DC lenses however, introduce spherical aberration in only one area or the other. When used correctly the effect is extremely subtle. Many people purchase these lenses hoping to be able to shoot at f/5.6 but with the DC ring set to R2 and expect to get f/5.6 depth of field (and sharpness) with a f/2 background (super blur). That is simply not what these lenses can do (unfortunately!). In my personal experience I typically just leave the DC ring in the neutral position although this test has shown me that the ring is worth using (see below). To show the effects of the ring at various apertures I shot some test photos. These were all shot with the 135mm f/2 on a tripod in manual mode at the apertures indicated and at ISO 200. Shutter speed varied for correct exposure. The captions are below each image. There isn’t much of a foreground here but you can at least see what happens to out-of-focus highlights in the background. Each series starts with the DC ring in the F5.6 position. Click through the thumbnails to see what happens as you cycle back to the neutral position and then over to R5.6.
Examples shot at f/2
Examples shot at f/2.8
Examples shot at f/4
Examples shot at f/5.6
Up to this point I was under the incorrect assumption that the lower the DC number the “better” the bokeh. The opposite is actually true. One can clearly see that the out-of-focus circles are larger when the DC ring is set to match the aperture. It’s difficult to see in the f/2 series but compare the f/2.8 R2.8 photo with the f/2.8 Neutral photo and it’s pretty easy to see the difference. In practice you could probably even push the ring one stop past the correct number to achieve even greater dreamy smoothness but keep in mind that the sharpness of your subject will begin to suffer the consequences.
I do sometimes like to intentionally crank up the ring for special effects like this:
I will have a separate article in the future comparing the 135 and 105 lenses but I hope this article helps explain that mysterious feature known as “defocus control.” For further reading I’m including links to some other sites that show similar tests.
SLRGear DC Demo (excellent)
Defocus Control on the Nikon 105mm f/2D AF DC-Nikkor
135mm DC – Defocus Control Samples
Nikkor 105mm AF DC f2D – DC comparisons
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