dust detecting explaination


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htthach

New Member
Feb 26, 2006
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dover
#1
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 :embrass:
 

michhy

New Member
Oct 21, 2005
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#2
can we use the same settings to do assessment on lens dust/smudge/scratches?
 

Mar 4, 2006
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Singapore
#4
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.
 

htthach

New Member
Feb 26, 2006
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dover
#5
thanks
that's sound like a great explaination to me
 

asterixsg

Senior Member
May 22, 2006
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Somewhere in this creation
#6
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...
 

htthach

New Member
Feb 26, 2006
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dover
#7
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).
 

asterixsg

Senior Member
May 22, 2006
2,060
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Somewhere in this creation
#8
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 :embrass: 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 :cry:

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.
 

AxeLa

Senior Member
#9
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 :embrass: 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 :cry:

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-loca...ts.html?L+scstore+hnzy5945ff780b78+1166864480
 

glay78

New Member
Sep 28, 2005
332
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Northeast
#12
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
 

lsisaxon

Senior Member
Nov 29, 2004
11,941
0
0
#14
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.
 

asterixsg

Senior Member
May 22, 2006
2,060
0
0
Somewhere in this creation
#15
glay78,
Bro. Thanks a lot for the explanation and the link. :thumbsup: :thumbsup:
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
 

glay78

New Member
Sep 28, 2005
332
0
0
Northeast
#16
glay78,
Bro. Thanks a lot for the explanation and the link. :thumbsup: :thumbsup:
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 :dunno:
I guess its due to the auto levelling and it wasnt well levelled??
 

lsisaxon

Senior Member
Nov 29, 2004
11,941
0
0
#17
glay78,
Bro. Thanks a lot for the explanation and the link. :thumbsup: :thumbsup:
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.com/med/CCD_Clean_Before.jpg

After cleaning
http://www.cleaningdigitalcameras.com/med/CCD_Clean_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? :dunno:
 

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