In the old fashion magazine, we can often see the head shot or full-body shot of models with blurred background. The telephoto lens that did this shots did two things. One, a telephoto effect that raises the background forward; second, a shallow depth-of-field that blurs the background.
Nowadays, Nikon does not even offer an ordinary 135mm lens anymore, but its proprietary AF DC-Nikkor 135mm f/2D, a hard to get item. The lens can controls the degree of blur (out of focus image) either in the background or in the foreground. Japanese call this blur, BOKEH. Other than the normal aperture control, the lens has a Defocus Image Control ring, to control the BOKEH in the foreground (F) or in the background (R). Although, the 135mm seems is out of fashion for the moment, but discussion of every telephoto lens always still include on how good the BOKEH looks for the particular lens. The BOKEH of the AF DC-Nikkor 135mm f/2D looks really good. We can even add BOKEH to the in focus image, the marvel of the DC lens. However, with the Defocusing Image Control, it is quite complicated to find the sharpest setting for the lens. This is what I set out to do this afternoon. The test is relatively easy. I put up a photo magazine cover on the wall and shot the image at f/2, 2.8, 4 with different Image Control ring settings. The objective is to find the best setting that gives me the sharpest in focus image with no BOKEH appears on it. As comparison, I also tested the AF-S VR Micro-Nikkor 105mm f/2.8G IF-ED and AF Zoom-Nikkor 70-300mm f/4-5.6 G.
The results are
135mm DC: f/4 and Defocusing Image Control at 2R
105mm Micro: f/4
70-300mm (100mm): f/8
Nothing surprise for the 105mm and 70-300mm, but it is good to know the sharpest setting for the 135mm DC lens. Diverting from this setting, I started to see small amount of BOKEH appeared on the image.
Another important point for the 135mm DC lens is focusing. Set at f/2, for a head shot, the depth-of-field is only about a few centimeters. The focusing plan of my 135mm DC lens is at the rear of the depth-of-field. Thus, when I focus the lens on the eye of the model, anything behind the eye is out of focus, the nose is still in focus. If the model is facing side way from me, it would be a good idea to focus the eye further away from the camera in order to keep the eye closer to the camera also in focus.