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Old Aug 5, 2009, 5:16 PM   #1
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Default Minor disaster with my D40

My wife and I recently took a vacation, driving through Canada and ending in Skagway Alaska where my daughter and her husband are working. Along the way we were at a beautiful lake, so I set the camera on a tripod and set it up so that my remote control would activate the two-second delay shutter release. I had the camera all set, but the second I pressed the button on the remote a gust of wind blew the tripod over and broke the mount on my 18-55 mm kit lens. Luckily I had a Sigma 18-200 mm in my bag, and luckily the D40 body survived the incident. All of my vacation pictures turned out very well (at least for me) but I have a question.

I normally shoot with an aperture of approximately f/11. Yesterday I took some photos at f/18 that revealed quite a few dust spots, too many to really clear up with Photoshop. This was after I used a cleaning kit to clean the sensor. I cleaned it again. Doing further testing today I have discovered that if I keep the aperture at f/11-f/13 or wider I don't have any problems with dust spots. If I go any smaller the spots begin to appear. Since this is an aperture related phenomenon, does that mean I have dust inside of the lens? Or is this still a problem with dust on the sensor?
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Old Aug 5, 2009, 6:17 PM   #2
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I normally shoot with an aperture of approximately f/11. Yesterday I took some photos at f/18 that revealed quite a few dust spots, too many to really clear up with Photoshop. This was after I used a cleaning kit to clean the sensor. I cleaned it again. Doing further testing today I have discovered that if I keep the aperture at f/11-f/13 or wider I don't have any problems with dust spots. If I go any smaller the spots begin to appear. Since this is an aperture related phenomenon, does that mean I have dust inside of the lens? Or is this still a problem with dust on the sensor?

Yes you do have dust on your sensor. The more you stop down, the more the dust will become apparent, as your DOF increases with smaller apertures. Larger apertures show less dust spots because of the more limited DOF.
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Old Aug 5, 2009, 10:06 PM   #3
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Yes, I understand DOF. But the sensor is located behind the lens and as I see it should not be affected by aperture setting.
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Old Aug 5, 2009, 11:06 PM   #4
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Yes, I understand DOF. But the sensor is located behind the lens and as I see it should not be affected by aperture setting.

This is because the dust is not really on the sensor, but actually on the front most surface of the front most filter in the "stack" that sits over the sensor. That means that the dust sits out in front of the sensor. So to make it cast a visible shadow, you need to have as close to a point source for the light as possible. A tiny aperture achieves that "point source" effect. It's like casting a shadow on the ground when you're outside. With a clear sky, the sun forms something close to a point source, and thus, you see sharp shadows. When the sky is totally cloudy, the light comes from a wide range of angles, so you don't get distinct shadows.
Same thing here. You'll always see sensor dust the most clearly when shooting at very small apertures (high f/numbers).
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Old Aug 5, 2009, 11:35 PM   #5
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OK, Thanks.
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Old Aug 6, 2009, 10:50 AM   #6
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This brings up a question. Did you clean the high pass filter, or the sensor by raising the mirror from the menu? And did you use a special cleaning kit or perhaps a lens cloth?

You need to raise the mirror and use a blower, or a special senor cleaning kit - or send it to be serviced.

Garry
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Old Aug 6, 2009, 11:12 AM   #7
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I use the menu option to raise the mirror. I used a "dry" method cleaning tool designed specifically for cleaning sensors (or rather the high pass filter covering the sensor).

I live in a relatively small town (approximately 55,000 population) and the only store we have that claims to be full service photo store is named Inkley's dba Ritz Camera. I have yet to have them be able to help me with anything. They recommend a "hurricane" blower but they don't have any and claim they can't get any. They say they will send it in for cleaning, $65. But the way they do business I'm afraid they will be out of business before I ever got the camera back. So I'm not sure what I'm going to do at this point other than limit my f-stop to f/11.
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Old Aug 8, 2009, 12:27 PM   #8
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This link may help you. I bought a cleaning kit from them when I had my Pentax and used it many times. It works well and they have lots of information on the site. So far with my D80, I've only had to blow off the sensor, but eventualy, I will need to clean it. I think it uses a different cleaning solution than the Pentax sensor did - Bruce

http://www.micro-tools.com/store/Sea...CategoryCode=6

Click on there free guide to cleaning Digital SLR Sensors also.
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Old Aug 8, 2009, 5:27 PM   #9
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Thank you for the link. I will probably order something from them. I went to my pitiful local store yesterday and they said, "Oh yes, we have wet/dry cleaning kit right here. $21.95." So I bought it, got home and realized it was just a refill kit for a full cleaning system. It has some pre-moistened wet swabs and dry swabs. Frustrated, I decided to keep it and I have got things working pretty well right now. But I need a full cleaning kit, and I'm not going to get it from them.

Interestingly, I read an article yesterday that suggested that when shooting digital with basically any camera one should not use an f-stop smaller than about f/11 because smaller f-stops tend to cause light diffraction. In that range my pictures have no hint of dust. And I really cannot see any real improvement using smaller f-stops. Actually, the f/18 images seem a little less clear in my experiments today. But maybe that's because I don't have real high quality glass on my camera.
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Old Aug 8, 2009, 6:00 PM   #10
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Interestingly, I read an article yesterday that suggested that when shooting digital with basically any camera one should not use an f-stop smaller than about f/11 because smaller f-stops tend to cause light diffraction. In that range my pictures have no hint of dust. And I really cannot see any real improvement using smaller f-stops. Actually, the f/18 images seem a little less clear in my experiments today. But maybe that's because I don't have real high quality glass on my camera.
This is correct. F-stops are an inverse relationship...the higher the number the smaller the f-stop. So f-18 is smaller than f/11 and you're test is correct. Lower f-stop numbers are larger apertures.
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