Double cr*p! My second 105mm f/2 DC is also front focussing...

Started Apr 7, 2009 | Discussions thread
OP Brooks Lester Senior Member • Posts: 1,865
Hmmm....AF Fine Tune @ +20 seems to be correcting things...

I hate to be at the end of the range but that's better performance than I got from the first copy. A very nice poster on photo.net posted his exhaustive and informative experiences and findings with his 105DC, which are very similar to mine:

"I have done extensive testing of the focusing behavior of my 105/2 DC on both my D300 and D700, eventually building a large table of the focusing "error" based on aperture and DC settings. I also used live view to determine at which settings the AF system was selecting a focus that was not the same one I would have chosen. The focusing behavior was similar on both cameras; if anything, the focusing is substantially BETTER on the D700. Unfortunately, I didn't have this lens back when I was using my 8008, so I have no idea how it behaves on film cameras. Of course, all of my testing was done on a heavy tripod with exposure delay, remote shutter release, and copious note taking.

As a result, I can say that for a layperson, the focusing behavior of the lens is understandable, but very complex. First off, for this lens you must split the idea of focus into two components - I will call them edge focus and contrast focus. The DC feature has two effects on edge focus - first, it moves the external plane of focus in the opposite direction of the ring (i.e., F settings move the plane of focus away from the sensor plane), which is why refocusing is always required, and it also somehow increases the depth of field. I don't know enough about the technical details of spherical aberration to explain how this increase happens, but it is quite apparent when using a 45-degree focusing chart. It's also quite substantial at high DC settings - at f/2, the zone of acceptably sharp EDGE focus goes from about 2 cm (at N) to well over 10 cm (at 5.6 in either direction). As a result, there seems to be a larger range in which edge focus is acceptable, but contrast focus varies differently within that range, which is what causes the dreaminess. There is a plane of maximum edge sharpness, but it is less contrasty (i.e., more dreamy) - this is where the camera's AF system gravitates. There is also a plane of maximum contrast focus, but it is slightly less sharp on the edges - this is the one my test human (me) naturally selected when using live view. The distance between perfect edge focus and ideal contrast focus gets larger as the DC effect is increased, unsurprisingly, which is why the images get so very dreamy at high DC settings and wide apertures. At DC R2.0 and R2.8, with my lens, I can force focus on the contrast option closely enough with an AF fine tune setting of +20, and I leave AF Fine Tune turned off when I want more dreaminess. After all my testing, I now tend to use my lens at f/2.8 with a DC setting of R2.8 (if I want a slightly ethereal effect) to R4.0 (if I want more). At f/4.0 and higher, the dreaminess of DC begins to fade, so I no longer use it for that particular effect (but see below re:"tunneling"). It's also worth noting that the difference between "machine" and "human" focus exists even when the DC ring is set to match the aperture setting - I still haven't been able to determine why Nikon decided to give the settings on the DC ring aperture names, because nothing magical happens when they are lined up. They might as well have used letters or integers, as each combination gives a different effect. You just have to know exactly what they do before the operation of the lens ceases to be utterly obscure.

The other effect of DC is what I think of as "tunneling" - higher DC settings distort the outer portions of the picture and "wrap" the environment about the subject. This is most suitable, in my experience, for full-body portraits in natural settings, as the trees and other foliage almost seem to be bending to accommodate the subject. This is quite visible in specular highlights on the borders, which take on a cat's-eye shape and can ring the subject in a very interesting and pleasing way. Unlike the dreaminess, which decreases with smaller apertures, this effect maintains its intensity at all apertures, because it appears to me to be a form of lens distortion.

On the bokeh itself, DC definitely has an effect. Settings of 2.8 and higher render a gaussian bokeh in the targeted area with no sharp edges - it's a very painterly effect I can't simulate with any of my other lenses, and I love it - the background objects just melt into one another with no boundaries. Of course, the opposite side gets positively ugly, so you have to consider that. Again, better for portraits, where you can generally control what items are between the subject and camera (preferably none, if using DC).

Bottom line: After all my testing, now that I understand what this lens is capable of, I wouldn't dream of giving it up and I ALWAYS keep it in my bag if I may be doing portrait-style work. If you're looking for ultimate sharpness, I don't think you should be using this lens anyway, although to my eye it is capable of more sharpness than my 85/1.4 when stopped down to f/4 or f/5.6 (with DC at N). This lens was designed for flattering, magical portraits (of women and children, not men) and that is what it excels at. It is definitely not intended to be a general purpose "realistic" lens (although it can be, if you are willing to tolerate its high-maintenance aspects). Once I accepted and understood its characteristics it became a much more useful lens to me, as I stopped trying to make it into something it's not. As a side note, I also have the 135/2 Ai-S, which I understand is optically similar in design to the modern DC family lenses, sans DC of course. It is also very, very sharp, but takes much less intriguing pictures of people."

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