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Thread: dust detecting explaination

  1. #1
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    Default dust detecting explaination

    I already know the "how to"
    (iso 100, f/22, light uniform surface)
    But what i wonder is why that setting?
    why must be small aperture??
    i thought small aperture is for lens dust?
    small aperture never affect the dust which is already on the sensor itself?
    but according to all the source i've been reading on internet, they all advice the same setting
    amazingly, i've test with 2 different lens, same sensor dust on the 20D, so the method must be working coz the chance for 2 different lenses to have same dust pattern is 0.00...01%

    anyone know why? just curious
    Last edited by htthach; 24th November 2006 at 03:23 PM.

  2. #2

    Default Re: dust detecting explaination

    can we use the same settings to do assessment on lens dust/smudge/scratches?

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    Default Re: dust detecting explaination

    so no one have the answer?

  4. #4

    Default Re: dust detecting explaination

    When the lens aperture is small, the light exiting from the pupil of the lens is acting like a small light source. Imagine a torch shining on a ball, we would have a small and hard shadow. In this case, the dust would be analogous to the ball and so, a visible shadow is captured on the CCD.

    But when we open up the aperture, the light exiting from the pupil of the lens is acting like a giant light source relative to the dust. Think of it as a diffused light source, the shadow casted by a diffused light source is large, diffused and of little contrast. Therefore, we can't really see the dust shadow on our CCD when captured.

    Now, when we extrapolate these characteristics to checking dust on a lens, we wouldn't be surprise that we are not likely to see the lens-dust's shadow on the CCD when captured. This is because the lens-dust is just too far from the CCD and the shadow casted would be too diffused to be noticed on the CCD.

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    Default Re: dust detecting explaination

    thanks
    that's sound like a great explaination to me

  6. #6
    Senior Member asterixsg's Avatar
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    Default Re: dust detecting explaination

    Quote Originally Posted by william_wwong View Post
    When the lens aperture is small, the light exiting from the pupil of the lens is acting like a small light source. Imagine a torch shining on a ball, we would have a small and hard shadow. In this case, the dust would be analogous to the ball and so, a visible shadow is captured on the CCD.

    But when we open up the aperture, the light exiting from the pupil of the lens is acting like a giant light source relative to the dust. Think of it as a diffused light source, the shadow casted by a diffused light source is large, diffused and of little contrast. Therefore, we can't really see the dust shadow on our CCD when captured.

    Now, when we extrapolate these characteristics to checking dust on a lens, we wouldn't be surprise that we are not likely to see the lens-dust's shadow on the CCD when captured. This is because the lens-dust is just too far from the CCD and the shadow casted would be too diffused to be noticed on the CCD.
    Bro, thanks for that wonderful explanation. Makes total sense. This is what happened to me when I took some pics during a recent trip to China. In some pics, there were black spots in a few places. In other photos taken a few minutes apart, same lens, there weren't !!! It puzzled me for a while, until I checked the settings and figured out that bigger the f-stop number, black spots were seen.

    That begs my next question which hopefully you or htthach can help answer.

    How to test if there is dust on the lens or on the sensor ? It would be a shame to shoot a 100 pics, download to your PC and then see the black spots all over the photos.
    You mentioned something like ISO100, f22 etc. If you can explain that a little bit or point me to some article on the net where I can find more info, it would be a great help.

    Cheers...
    The trouble with the world is that the stupid are cocksure and the intelligent are full of doubt....

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    Default Re: dust detecting explaination

    oh
    to what i've read on the net
    the technique is quite simple
    iso 100, f/22 or smaller depends on lens
    shutter long enuf to get proper exposure

    point to any uniform light color surface (ex, your wall, white or bluish sky)
    manual focus to out focus (ensure that u don pick up dust on the wall)
    shoot, hand shake is also welcome.
    one landscape, one portrait
    load to computer
    rotate back the portrait
    if you see spot at same location on 2 images => dust
    ------
    to differentiate btw sensor dust and lens dust, i guess, either use 2 different lenses or cameras.
    but i never try to find dust in lens (other than using my eyes to look at the lens haha).

  8. #8
    Senior Member asterixsg's Avatar
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    Default Re: dust detecting explaination

    Bro,

    Thanks for the tip.
    Yeah, I've gone through both the experiences - dust on sensor and dust on lens. Thankfully, most of the times, it has been during my test shots and I didn't lose much. My first traumatic experience was when I encountered some great scenery enroute to Sailimu Lake in Xinjiang. Clicked a few pics and immediately I could see black spots in the LCD screen itself. Xinjiang can be quite dusty at places and I guess I didn't take enough precaution.

    Like you said, black spots in the same location - even with different lenses ===> dust on sensor. It felt so stupid to sit in the van and clean filters, lenses and sensor when everyone else was happily clicking away

    Bro, I know that this thread has served its purpose. But rather than start another thread, I thought if I can use it to get your opinion or your experience with cleaning agents. Hope you don't mind me hijacking this thread.

    I rely mostly on lens tissues and lens solution and a small blower that I got from CP when I bought my drybox there. Sometimes I feel that its not good enough. Have been looking around for better stuff.

    Did you or anyone use the "Perfect Pixel Camera Parts" PPCP cleaning accessories ?
    Am thinking of getting these. The prices seem quite reasonable.

    PPCP Sensor Swab Kit
    Premium Grade Lens Tissues
    PPCP Sensor/Lens Solution
    7" Jumbo Rocket Blower
    Hoya Microfiber Cloth
    The SpeckGRABBER

    Any feedback most welcome.
    The trouble with the world is that the stupid are cocksure and the intelligent are full of doubt....

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    Senior Member AxeLa's Avatar
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    Lightbulb Re: dust detecting explaination

    Quote Originally Posted by asterixsg View Post
    Bro,

    Thanks for the tip.
    Yeah, I've gone through both the experiences - dust on sensor and dust on lens. Thankfully, most of the times, it has been during my test shots and I didn't lose much. My first traumatic experience was when I encountered some great scenery enroute to Sailimu Lake in Xinjiang. Clicked a few pics and immediately I could see black spots in the LCD screen itself. Xinjiang can be quite dusty at places and I guess I didn't take enough precaution.

    Like you said, black spots in the same location - even with different lenses ===> dust on sensor. It felt so stupid to sit in the van and clean filters, lenses and sensor when everyone else was happily clicking away

    Bro, I know that this thread has served its purpose. But rather than start another thread, I thought if I can use it to get your opinion or your experience with cleaning agents. Hope you don't mind me hijacking this thread.

    I rely mostly on lens tissues and lens solution and a small blower that I got from CP when I bought my drybox there. Sometimes I feel that its not good enough. Have been looking around for better stuff.

    Did you or anyone use the "Perfect Pixel Camera Parts" PPCP cleaning accessories ?
    Am thinking of getting these. The prices seem quite reasonable.

    PPCP Sensor Swab Kit
    Premium Grade Lens Tissues
    PPCP Sensor/Lens Solution
    7" Jumbo Rocket Blower
    Hoya Microfiber Cloth
    The SpeckGRABBER

    Any feedback most welcome.

    Iam using these + speckgrabber...very good!

    http://www.kinetronics.com/cgi-local...b78+1166864480
    D800|24~120mm VR|AF VR 80~400mm ED |SB900|50mm F1.8|CD 2X2|FL-360|FastPack 350/Sony RX100M3

  10. #10
    Senior Member asterixsg's Avatar
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    Default Re: dust detecting explaination

    Quote Originally Posted by AxeLa View Post

    Iam using these + speckgrabber...very good!

    http://www.kinetronics.com/cgi-local...b78+1166864480
    Thanks bro. I assume this must be available in CP or other similar stores. Will go take a look this week.

    Cheers.
    The trouble with the world is that the stupid are cocksure and the intelligent are full of doubt....

  11. #11
    Senior Member AxeLa's Avatar
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    Thumbs up Re: dust detecting explaination

    Quote Originally Posted by asterixsg View Post
    Thanks bro. I assume this must be available in CP or other similar stores. Will go take a look this week.

    Cheers.
    Make sure u bargain...got mine in Marina...
    D800|24~120mm VR|AF VR 80~400mm ED |SB900|50mm F1.8|CD 2X2|FL-360|FastPack 350/Sony RX100M3

  12. #12
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    Default Re: dust detecting explaination

    How To Create A Test Image
    To Check For Dust


    To make a test image, you can use about any lens you want but an non-wide angle lens works better then a wide angle just because of the normal falloff found in most wide angle lenses. You want as much of an even exposure as possible from corner to center. The second consideration is minimum aperture opening; we suggest a lens that will stop down to f/22 or greater.

    We have found that a great subject to shoot for the test, is your monitor. Seeing that you already have to use a computer to view your test, there is no need to go elsewhere to make the test. You already have a willing and qualified subject right in front of you, why not use it.

    Prepare your monitor for shooting the test:
    Create a new image in Photoshop
    Fill it with white (most any solid color will do, but we prefer a lighter one)
    Zoom in until it fills your screen



    Set the camera to the following:
    *Mode - Aperture Priority
    *Setting - Aperture to minimum f/22-f/45
    *Lens - Manual Focus set to closest focus setting (if shooting the blue sky, then infinity)
    *Features - Turn "OFF" all special function like "sharpening"
    *Take Picture - shoot camera facing your monitor. Depending how bright your monitor is,
    your exposure may be a couple seconds. During this exposure, move your camera back and fourth being careful to not to point the lens outside of your white box. Moving the camera during the exposure insures that you are not taking a picture of dirt on your monitor. This should be done within a matter of an inch or two from your monitor.
    *Photoshop - Take the image into Photoshop and do a <ctrl>+<shift>+<L> for "auto level" You can lighten or darken if needed.
    *Inspect Image - You can now see where you do or do not have dust. Remember that what you are looking at is an image that is flipped 180 (top to bottom) from when you're looking straight in on your sensor. What shows on the bottom of the image will be towards the top of the camera and visa versa...


    Before converting in Photoshop


    Converting in Photoshop


    After Photoshop and image with dust seen



    Extracted from http://www.cleaningdigitalcameras.com/howto.html

  13. #13
    Senior Member AxeLa's Avatar
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    Talking Re: dust detecting explaination

    D800|24~120mm VR|AF VR 80~400mm ED |SB900|50mm F1.8|CD 2X2|FL-360|FastPack 350/Sony RX100M3

  14. #14

    Default Re: dust detecting explaination

    Quote Originally Posted by michhy View Post
    can we use the same settings to do assessment on lens dust/smudge/scratches?
    No, most of the time dust in the lens will not appear in pictures. Smudges will cause flares if it happens to be in the path of a light source.

  15. #15
    Senior Member asterixsg's Avatar
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    Default Re: dust detecting explaination

    glay78,
    Bro. Thanks a lot for the explanation and the link.
    Gonna try this out tonight...

    Sorry for this stupid question, but why is there a bright circle in the centre of the images ?

    Before cleaning


    After cleaning
    The trouble with the world is that the stupid are cocksure and the intelligent are full of doubt....

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    Default Re: dust detecting explaination

    Quote Originally Posted by asterixsg View Post
    glay78,
    Bro. Thanks a lot for the explanation and the link.
    Gonna try this out tonight...

    Sorry for this stupid question, but why is there a bright circle in the centre of the images ?

    Before cleaning


    After cleaning
    I don't know man
    I guess its due to the auto levelling and it wasnt well levelled??

  17. #17

    Default Re: dust detecting explaination

    Quote Originally Posted by asterixsg View Post
    glay78,
    Bro. Thanks a lot for the explanation and the link.
    Gonna try this out tonight...

    Sorry for this stupid question, but why is there a bright circle in the centre of the images ?

    Before cleaning
    http://www.cleaningdigitalcameras.co...ean_Before.jpg

    After cleaning
    http://www.cleaningdigitalcameras.co...lean_After.jpg
    That's probably because of the hotspot of the lens.. even though the brightness is quite even, but when you bring up the levels like that, slight variation in brightness becomes greatly accentuated. It probably has to do with the aperture also. Maybe you can try open up one stop and see if the circle gets bigger?

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