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05-19-2010, 07:24 AM   #1
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Dust Alert and lens focal length

I was recently checking for dust on my K200D sensor using the Dust Alert function and with my Tamron 18-250 mounted. I always point at a clear, bright blue sky when using this function. The Dust Alert review is spotless from 18 up to about 125-150mm. At longer focal lengths, a spot appears (when shooting the sky I sometimes have to zoom to block out surrounding objects). The spot is consistently in the same location, and consistently only appears at the longer focal lengths. Can't see any dust spot in an image when simply shooting the sky with a small aperture, and can't visually see anything on the sensor filter. What do you think is going on here? What's the focal length got to do with what's happening?
BobG

05-19-2010, 08:04 AM   #2
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What aperture were you set at? If you were using 3.5 (at the wide end) it won't show dust as well as smaller apertures. As you zoom in (longer FL) the aperture decreases, revealing more dust. Try some shots at f16 or 22 and see if you don't get different results. If you were already using an aperture of f8 or smaller, then I'm stumped.
05-19-2010, 08:23 AM   #3
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Thanks for the reply, Dust Alert automatically sets the aperture to f16 regardless of focal length.

BobG
05-19-2010, 08:27 AM   #4
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QuoteOriginally posted by BobG Quote
Thanks for the reply, Dust Alert automatically sets the aperture to f16 regardless of focal length.

BobG
OK, then I stand by the second part of my post....... I'm stumped.

05-19-2010, 08:33 AM   #5
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You might be starting to see internal dust. Just a hypothesis.
05-19-2010, 08:47 AM   #6
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QuoteOriginally posted by BobG Quote
I was recently checking for dust on my K200D sensor using the Dust Alert function and with my Tamron 18-250 mounted. I always point at a clear, bright blue sky when using this function. The Dust Alert review is spotless from 18 up to about 125-150mm. At longer focal lengths, a spot appears (when shooting the sky I sometimes have to zoom to block out surrounding objects). The spot is consistently in the same location, and consistently only appears at the longer focal lengths. Can't see any dust spot in an image when simply shooting the sky with a small aperture, and can't visually see anything on the sensor filter. What do you think is going on here? What's the focal length got to do with what's happening?
BobG
I suspect that you have a very small spot on your sensor. Most cleaning recommendations that I have read suggest long focal lengths and small apertures. For what it is worth, here is my procedure with the K10 to locate any dust on the sensor.
  1. Set camera to ISO 100 (my lowest)
  2. Set camera to manual focus
  3. Set exposure to Av
  4. Set aperture to f/22
  5. Set Exposure Compensation [+/-] to +2 or more to give me
    1. A bright image, and
    2. A slow shutter speed 1 second or more
  6. Set focus to infinity.
  7. Take a picture of my evenly lit white wall from a foot or so distant, hand held.
  8. Examine the resulting image.
Anything that is on the sensor will show up. The combination of a slow shutter speed, infinity focus and hand holding ensure that none of the details (such as dust) on the wall are recorded. The overexposure gives me an image that is bright so that any dust particles stand out.

I have the additional step of loading the image into Lightroom to examine it, and to flip it myself, but the rest of the procedure should work just fine for you.
05-19-2010, 08:47 AM   #7
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Light rays from narrow field-of-view (long focal length) lenses are more perpendicular to sensor plane; dust on anti-aliasing filter positioned slightly above actual sensor, so wider angle lenses shine light at more of an angle onto dust, reducing contrast, similar to how dust (or worse) on front element is invisible on images.

PS Sharpening and increasing contrast can reveal dust not seen in non-PP'd image.

Last edited by rhodopsin; 05-19-2010 at 09:03 AM.
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