Nikko 105mm f/2D AF DC

his was a test to see if the DC feature (Defocus Control) of the Nikon 105mm f/2D AF DC-Nikkor really made a difference. At first glance the effect was not clearly visible since it is rather subtle. The DC can also be found on the Nikon 135mm f/2D AF DC-Nikkor. I am not aware if the Canon Soft Focus lenses uses the same technology to achieve soft focus.

My first test was taken with a F80 using black and white film. Then enlargements were made on 8x10in paper. You could see the effect there but when scanning the negatives it was not as distinct anymore. So when I got my D70 I wanted to repeat the experiment. I do not use this feature very often, but it was fun to see that it made a difference, although small.

This is by no means an exhaustive technical test or review. It is merely an personal experiment that I wish to make available to the public since I have seen many questions regarding DC but not many really good answers. Now you can look at the pictures and understand yourself.

The lens

he lens itself is of excellent build and handles great. The large maximum aperture gives great bright view in the viewfinder and it focuses quickly due to the Rear Focusing construction.

The aperture diaphragm has rounded blades making the diaphragm opening very round and thus gives nice out-of-focus elements. The hood is built in and by pulling it out and turning it two times to the right you lock it into place.
What is DC? How does it work?

DC, Defocus Control, allows you to control the spherical aberration of the lens. Usually aberrations are not something you want in your lens but sometimes they can give effects that are desireable. In this case they can give softer or harder out of focus effects or when extreme a soft focus effect, similar to what you get with some filters.

To illustrate the following I recommend that you have looked at the excellent essay Understanding Boke by Harold M. Merklinger over at The Luminous Landscape. I will refer to the figures in this essay.

To understand this you should first understand how focus works with a camera. When photographing something only one plane is ever in focus. Everything in front of that plane or behind it is unsharp. The thing called Depth-of-Field is just an illusion. "What?! I have seen it", you claim.

Well, if you enlarge the image you will see that the Depth-of-Field decreases as the image is enlarged more and more, given that you are watching the image from the same distance. If you step back a few steps and repeat the experiment you will see that the same happens again but the Depth-of-Field seems to be a little bit larger this time at the same magnification rate as before.

Why? It is really rather simple. Our eyes can only resolve small detail down to a certain size. Instead of using size which is dependent upon how far away the object is, we can measure the smallest angle that we can distinguish details in. This exact angle is not really important, but what is is that we cannot distinguish a small dot from a small disc if the disc is small enough. Our eyes will make us believe that both are dots!

This is why you see the Depth-of-Field effect. Tiny objects close to the plane of focus will be portrayed not as dots, but as small discs. In figure 1 you see this illustrated with triangles. But if we, at normal magnifications and normal viewing distances, cannot distinguish these discs from dots, the objects will look sharp, thus creating an illusion that there is an area in the picture where objects are portrayed as sharp.

The further away from the plane of focus we get the larger the discs. Figure 9, just below the middle of the essay, shows how seven dots, three in front of the plane of focus, one on the plane of focus and three behind are portrayed on film. This lens has pretty neutral bokeh (I'll come back to the term bokeh later). How do I see this? Well the discs are uniformly gray with no really sharp and bright edges or centers. It is not exactly neutral since the right circles have a slightly higher edge contrast, but not much.

Bokeh is a japanese term that describes how the out of focus elements looks. It is pretty subjective but some factors can be measured and quantified. The effect contributed by the spherical aberration is one and the one that Harold describes.

Now, how does spherical aberration come into this. The simplest example would be if you take a look at figure 6. This image shows four light dots are placed, two in front of the plane of focus and two behind. The plane of focus is exactly in the middle of dot two and three. If this image was taken with the 105mm DC-Nikkor, the dial would be set to REAR to achieve this effect.

As you see the two dots in front of the plane of focus have distinct edges where as the centers are darker. Behind the plane of focus the dots have light centers and fading edges. Imagine that two dots are close to each other and behind the plane of focus. Then the soft edges would make it apperar that they are melted together whereas if they were in front of the plane of focus you would get two overlapping circles.

If you set the dial/ring to FRONT you just mirror this figure.

So how do you use this lens? If you have a background that you wish to have a softer background you first set the aperture. Then you turn the front, DC, ring to match the set aperture to REAR. Then you compose and focus. You must set the DC ring before focusing since there is a small shift of focus when you adjust the DC. If you have anything in front of the main object these objects will get an ugly out of focus effect.

Should I ever set this ring to FRONT? Yes. Imagine that the background is pretty even and soft. You might have a plain white or gray backdrop or just the sky, but in front of the subject you have some bushes or branches that pops up. Then you would want the branches to to be soft and the background doesn't contain any detail that would render as overlapping circles. Remember that this effect is most visible in small points or thin details where the contrast to the background is high.

More info at here