My 105mm f/2 DC: normal or not?

Discussion in 'Nikon' started by davidlong, May 6, 2008.

  1. I purchased a 105mm DC lens recently and have been using it on a D40 and (with an adapter) on an Olympus E-1. While I've been generally happy with the lens, I've noticed a bit more chromatic aberration than I expected. It's most annoying when there's a high-contrast edge in a part of the image that is slightly out-of-focus. The aberration varies in color depending on whether the focus is slightly in front of or behind the edge. Focus in front produces a blue-green cast, and focus in back gives a red cast. The aberration is present even in the very center of the image, i.e., it's not just lateral chromatic aberration which could be easily corrected in post. The aberration is greatest at f/2 and gradually disappears as I stop down, though it's still noticeable in some photos at f/4. I haven't seen it in real photos at f/5.6, but then again I'm mostly interested in using the lens at f/2.8 and f/4, so I haven't taken many pictures with it at f/5.6. My question is whether or not this is normal behavior for this lens. If it's normal, I'll just deal with it in post as best I can, but if it's not, I'll send it to Nikon and see if they can fix it. I have created three montages that illustrate the issue, one at f/2, one at f/2.8, and one at f/4. These are using a black-and-white 2D test target so as to show the problem clearly. Each montage is a 3x3 arrangement of center crops taken with my D40. The rows correspond to slight front focus, in focus, and slight back focus, and the columns correspond to different settings of the DC control (neutral, rear, and front positions). For the rear and front DC settings, I used the position corresponding to the shooting aperture, e.g., R2.8 and F2.8 for the shots at f/2.8. Full-resolution montages are in my gallery. Here are reduced-size versions.
    00POtw-43336884.jpg
     
  2. Here's f/2.8
    00POtz-43336984.jpg
     
  3. And here's f/4
    00POu3-43337084.jpg
     
  4. Do you re-focus after you change the DC control? You should, that's how it works....
     
  5. Yes, I do that. These tests were made column by column. That is, I set the aperture and DC control, then rotated the focus ring so that the lens was front-focused. Then I gradually turned the focus ring bit-by-bit, taking shots throughout, until the lens was back-focused.

    What I see when I do this is shown here: as the lens approaches focus, the features start to resolve, but they're green; when in focus, the aberration is gone; and as lens goes past focus, the features turn red as they start to blur again.
     
  6. I'm not sure what, if anything, Nikon can do other than completely redesign the lens. It has no ED elements to minimize CA. The lens was designed long before digital sensors came along to reveal every flaw in otherwise excellent lenses that were top notch performers with film.

    The most practical solution would be to fix the CA during post processing.
     
  7. David, not sure if you are going the right away about assessing this lens. This lens is by design uses flawed zone both in DC-front and DC-rear setting. Here is some more info about the lens if you haven't seen it already.

    http://www.stacken.kth.se/%7Emaxz/defocuscontrol/

    105 DC was fused to my D1x when I had that body, never saw a visible flaw when used correctly
     
  8. Maybe I'm not being very clear. As I mentioned at the beginning, I'm generally happy with the lens, and I never even bothered with much testing after first buying it. Only after encountering occasional situations like the one below did I start trying some more systematic exploration. If I get confirmation that the behavior is normal, then I'll just live with it and fix the CA as best I can when it happens to be a problem. My concern was just that I might have gotten a bad copy. I don't want to go to the trouble of sending it to Nikon only to have it come back a month later with a "performs as designed" note. Regarding the DC-front and DC-rear settings during the testing, that was mostly to verify that the neutral position of the DC ring was not misaligned (I've read of some lenses having that problem).
    00PPH6-43344984.jpg
     
  9. May I ask a stupid question here? Do DC lenses offer something to old-time
    photographers who had DOF scales on all their lenses and routinely changed the focus
    off the target?
     
  10. Bjorn Rorslett's review of this lens notes slight color fringing in out-of-focus highlight areas as a result of the rear-element-focus design and states this is found in other internal focusing lenses as well. Perhaps he can comment on whether your lens displays this to an excessive degree.
    HWD
     
  11. Kari, not really. There is a scale but it has only for f:16. Useless!

    Dave, so you use the same aperture stop on aperture ring and the same aperture on DC ring?

    I get result like posted here(Rudolph) when I use different apertures on the two rings. Nikon itself says you can do this to introduce softness over all the focus range...
     
  12. That amount of CA doesn't appear excessive, especially for a telephoto lacking any ED elements, and can easily be corrected during editing. I suspect that sending it to Nikon will just be a waste of shipping costs and risk loss or damage.
     
  13. I always use the same setting on the DC ring as on the aperture ring (or use the neutral position). Generally I shoot in aperture-priority or manual mode. I've made the mistake of having the two inconsistent, and it's very noticeable. The particular crop above is from a picture where the focus is on the person in the blue sweater that you see behind the nose. They were rendered fine with no fringing that I could see. But being focused just behind the high-contrast edge of nose against sweater gives a redish fringe.

    Based on the comments here and on another forum, my current guess is that this is just normal behavior for the lens.
     
  14. Just out of curiosity I rechecked some older photos taken when I first got my D2H and noticed CA with some lenses that had never showed any problems on my film cameras. All test photos were of a lakeside dock with white handrails, taken in sunlight.

    Sure enough, the 28/3.5 PC-Nikkor and even 105/2.5 AI showed some traces of CA on the D2H. The 105/2.5 showed less CA, but it was still visible.

    The 35-70/2.8D AF Nikkor showed little CA, despite having no ED elements.

    The 24-120 VR showed virtually no CA. However it's not quite as sharp as the 35-70/2.8D or 105/2.5 AI, so the lack of CA did not make it a superior lens.

    The 300/4.5 AI ED Nikkor (non-IF version) showed little or no CA. It definitely lives up to the hype.

    While not all older lenses will have problems with CA, and the significance is likely to vary with different dSLR models, it's simply a problem we'll have to deal with. Fortunately, it's usually easy to correct. For example, Corel Paint Shop Pro X, a simple image editor comparable to Photoshop Elements in features and price, offers simple one-click corrections for conventional lens CA and the purplish fringing that's peculiar to digital cameras. These fixes make a significant difference in, apparent sharpness, in some cases eliminating the need for any additional sharpening since the inherent sharpness of the lens is restored.
     
  15. I'd say the lens seems to behave in the expected fashion. The internal focusing brings with it a tendency for longitudinal CA (LCA), the amount of which was generally not a problem on film but does show up on a DSLR. This is because the digital imager records everything on a surface not into colour-coupled layers of the film base.

    Nikon has been clever in making their newer lenses virtually free of LCA. Thus, the 24 PC-E is a prime not a zoom, has internal focusing, but there is no LCA. The longer telephoto lenses behave similar. Were they to redesign the 105 DC, I'd expect a LCA-free lens. But this is likely not on the top of their to-do list, I'm afraid. And you can always stop down a little more to reduced the impact anyway.
     
  16. Thanks to all for your comments. Normal behavior is the consensus.

    Again, I'd like to emphasize that I'm generally happy with the lens, and I don't want to dissuade anyone from buying one just on the basis of this test. I'd estimate that only about 5-10% of the shots that I'd consider keepers based on other factors (light, pose, critical focus, etc.) have exhibited enough of this issue for me to even think about trying to fix it.
     

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