Internal dust in focus?

Discussion in 'Nikon' started by kenneth_smith|7, Jan 23, 2014.

  1. I'm using a Nikon PB-5 bellows with a Micro Nikkor 55mm @ f/11. Exposure is with flash. Nikon D600.
    What I thought was sensor dust turns out to be something utterly baffling. Is it possible that I'm focusing the internal lens dust? The camera sensor checked out O.K. It had a few soft spots, but nothing like the hundred or more hard black dust spots that clog both ends of the frame length wise. I'm using a focus staking technique and noticed that the out of focus areas have sharper dust spots, and they are many. What to do? I'm truly surprised to discover this problem, and it occurs with both the d7000 and the d600. I don't think it's the camera.
     
  2. Is it possible that I'm focusing the internal lens dust?

    How can a lens focus inside itself?
     
  3. ShunCheung

    ShunCheung Administrator

    A follow up posted to a parallel thread:
    Geoff Mower [​IMG], Jan 24, 2014; 01:23 a.m.
    Could it be internal lens dust? Extremely unlikely! I cannot imagine how any set-up could produce sharp spots from dust on or in the lens.
    Anything that produces a hard clear black spot is going to be on, or very close to, the sensor (i.e. on the anti-aliasing filter.) My guess is that the extremely small effective aperture that you get using a bellows is showing up sensor dust that you simply don't see at more conventional apertures. You may end up pushing the limits of sensor "cleanability"! :)
    (However, if you tell me that the dust spots are in exactly the same positions with both cameras, I'll retract the above and rethink!)​
     
  4. Could you post an example or examples Kenneth?
    I take it there's no filter on the front of the lens that could be harbouring dust.
    It's possible to get a soft image of large dust specks on the rear element of a lens at small apertures, but I doubt that they'd ever be sharply focused. So the only thing that springs immediately to mind is that the dust is actually present on the sensor, but you can only see it under macro circumstances.
    If you're using a bellows, then the lens must be at about 1:1 or greater, and the effective aperture number will change in proportion to the magnification. It's likely therefore that the marked aperture of f/11 is actually becoming an effective aperture of f/22 or smaller, and is much more likely to reveal dust spots. If you're working at a mag of 2:1 then the effective aperture becomes f/33 and you'll see any dust spots much more sharply because of the narrowness of the cone of light coming from the lens. The higher the magnification, the smaller the effective aperture becomes.
     
  5. [[I cannot imagine how any set-up could produce sharp spots from dust on or in the lens.]]
    Not to bog this down in minutia, but on the topic of focusing on dust that is on the lens, many of Canon's travel-zoom cameras focus down to 0 cm. (Example: SX50 HS.) Which means you could actually focus on the dust on the very front of the lens (not internally, of course).
    For this particular problem, I agree that it's very likely dust on the sensor you are not able to see with conventional methods. If you put a pinhole body cap on both cameras you'd see the same thing.
     
  6. "Canon's travel-zoom cameras focus down to 0 cm." - Then Canon don't understand the convention of scale focusing. The focused distance is (or should be) given as the distance between focal plane and subject, and definitely not as the distance from the front of the lens.
    It may well be that the camera will physically focus to the point where the lens is touching the subject, but the minimum focus should never be given as "0 cm". That's a nonsense; it should be given as whatever the thickness between focal-plane and subject is at minimum focus.
     
  7. [​IMG][/URL][/IMG]
     
  8. Well that worked swell. Where's the pic? Copied from PhotoBucket to the tree?
     
  9. [​IMG]http://s159.photobucket.com/user/Alden56/media/darkandsharpspots_zpsc048d5fb.jpg.html
     
  10. Kenneth, i'm trying to separate the dust from the subject on this pic, but that's not easy...
    Maybe it would be helfoll if you coud make a pic of a blank piece of paper, with possibly a mark in the middle for focussing ?
    That would show the specs that you are writing about a lot better and mak it easier to Judge ?
     
  11. I did as much cleaning as I could, but at 100% there's still more than I want to repair. I have hundreds of shots. The upper right shows the most obvious at normal viewing. Just the Anti-Aliasing filter brought into focus I guess. Looks like I'll need to learn how to use a dust mask, though I don't have LR or upper tier Photoshop.
    I'm also wondering what I'm leaving out as far as posting a photo goes.
    [​IMG]<img%20src="http:/i159.photobucket.com/albums/t151/Alden56/DSC_2329_zps53e15de2.jpg"%20border="0"%20alt="Dust%20Example%20photo%20DSC_2329_zps53e15de2.jpg"/>" alt="" />
    00cKqt-545068284.jpg
     
  12. The posted photo doesn't reveal much. I have about fifty dots at 100% that have to be cleaned up.
    Last question, does anyone do that process where you shoot a mask then overlay it to clean in a batch process? Can you do this with Photoshop layers? Is there a background brightness level that suits this?
     
  13. Dust example - now move the paper and the sign 2 inches away from the camera. Take a defocused picture.
    See the dust spots better.
    I did my first thorough sensor cleaning a month ago. My camera needed 5 rounds of cleaning and checking. At each round I got about 70% of dust spots gathered. Eventually I did not need to do any wet cleaning.
    I did not mind trying to get 70% of the one last remaining away ;-)
     
  14. I think this is optically possible......in a similar way to the way the human eye is capable of seeing 'floaters'....whilst being focussed somewhere else.
     
  15. when on assignment i sometimes check my sensor for dust by setting the camera to f8 or f11 and take a shot of the bright sky.
    zoom in to 100% and scroll over it. you will see when dust is on your sensor.
    i did alot of makros using nikon pk3 and some other rings and a micro nikkor lens.
    for whatever reason i forgot to check the bellows for dust.
    it was used when i bought it and i was so eager to test it that i did not care about anything else but go out and shoot.
    this ended up killing one of my d3s's
    i got dust in..big dustbunnies that then were scratched over the sensor.
    i had to send it in and pay around 600euros to have nikon replace the lowpass filter since it got scratched.
    happened to my d3 also, but i do not repair this camera since it is almost worthless (fell down quite some times and has around 400k shutter...)
    so to sum things up
    when using a bellow
    be extra extra carefully.
    clean it
    and be sure it will stay clean.
    i know it might sound stupid but i use a cover one uses on largeformat cameras as a cover for light and change lenses and bellows underneath it.
    since then i never had problems again.
     
  16. this ended up killing one of my d3s's
    i got dust in..big dustbunnies that then were scratched over the sensor.​
    I don't understand. Dust and oil DO NOT damage a sensor. The incorrect cleaning technique however, that's a whole different story....!
     
  17. I don't know how I managed to create that huge gap in the postings. Sorry Photo.net
    I've had the bellows for decades and had it in clean storage, but I think the advice is good anyway, because dust gets everywhere. The camera D600 was a rental from LensGiant, and I think they did as good a job as they could. The good news is that by using the camera sensor cleaner six times and my rocket blower at time exposure I was able to knock out 90% of it. That initial snowstorm may have been my doing with me and my dusty bellows. One freakin thing after another. C'est la vie.
     

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