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Photo Forum / Digital Photography / DSLR Cameras / August 2005Circles on Canon 350D
Hi, just purchased a Canon 350D. All pictures with blue sky have three small dots on them (View here http://www.duncanwilliams.f2s.com/ ) I have cleaned the lens but no joy, could these be produced by the cmos sensor? It's going back at the weekend but wondered if anyone else had experienced this problem. Duncan > Hi, just purchased a Canon 350D. > All pictures with blue sky have three small dots on them (View here [quoted text clipped - 9 lines] > http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups > ----= East and West-Coast Server Farms - Total Privacy via Encryption =---- Looks like water-marks on the lens, or dust on the sensor.. Either way, not something to return a camera for. Just clean it and proceed. Yhuda > I have cleaned the lens but no joy, could these be produced by the cmos > sensor? They are shadows in the optical path. Either very large, but curiously diffuse, blobs on the sensor, or, perhaps more likely, and judging from near perfection of the circles, their size, focal ratio at the time, crud on the rear element of your lens. Clean it scrupulously. >> I have cleaned the lens but no joy, could these be produced by the cmos >> sensor? [quoted text clipped - 3 lines] >near perfection of the circles, their size, focal ratio at the time, >crud on the rear element of your lens. Clean it scrupulously. It is highly unlikely that dust on the lens could focus even well enough to show such blobs. It is clearly dust on the sensor. The smaller the aperture (larger numerically), the more obvious these become. If you normally shoot mostly available light indoors, you are unlikely to ever see these, as your aperture will be large enough so they vanish totally. It is only when taking outdoors (plenty of light), and at very small apertures to maximize depth of field that you are likely to see these. Every time you change a lens, you risk getting more dust in there, so don't depend on the dealer to clean it -- learn to clean it yourself. And learn to take test shots to determine the possible need for cleaning before going into taking shots like your examples. BTW -- was there a groundhog in the near foreground of one of those shots? You certainly put the maximum image in -- enough so that it took me some time to download the images, even with a T1 line for my net connection. Good Luck, DoN.  SignatureEmail: <dnichols@d-and-d.com> | Voice (all times): (703) 938-4564 (too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html --- Black Holes are where God is dividing by zero --- > It is highly unlikely that dust on the lens could focus even > well enough to show such blobs. It is clearly dust on the sensor. The > smaller the aperture (larger numerically), the more obvious these > become. I suppose it is possible there is some perfectly circular dust "out there"... > > It is highly unlikely that dust on the lens could focus even > > well enough to show such blobs. It is clearly dust on the sensor. The [quoted text clipped - 3 lines] > I suppose it is possible there is some perfectly circular dust "out > there"... The blobs shown in his example shots weren't perfectly circular, just diffuse roundish gray patches, absolutely typical of dust. Shows where inexact info can lead ... Colin D. >> It is highly unlikely that dust on the lens could focus even >> well enough to show such blobs. It is clearly dust on the sensor. The [quoted text clipped - 3 lines] >I suppose it is possible there is some perfectly circular dust "out >there"... Note that the dust is on the front surface of the anti-aliasing filter and the IR filter, so it is a measurable distance in front of the actual sensor. At large apertures, the light from the lens forms a rather broad cone focused on each pixel in the sensor. This cone is enough larger than the dust particles so they will not be seen. (They will still exist, but the percentage change in illumination will be so small as to not be apparent. At extremely small apertures, the light from the lens is very close to a tight collimated beam, and will cast quite sharp shadows of the dust particles onto the sensor. At slightly larger apertures, the light will form a fairly acute cone, which will cause the shadow to compose a noticeable percentage reduction (say perhaps 10-25% in the light reaching a cluster of sensor elements surrounding the dust particle, with no element losing all of its light. This is pretty much what was shown in the impages presented by the original poster. So -- the diameter of the circle is defined by the cone generated by the specific aperture selected, and the percentage of darkening will be defined by how large a percentage of any of these cones is blocked by the dust particle. Enjoy, DoN. SignatureEmail: <dnichols@d-and-d.com> | Voice (all times): (703) 938-4564 (too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html --- Black Holes are where God is dividing by zero --- It`s dust ! Max > Hi, just purchased a Canon 350D. > All pictures with blue sky have three small dots on them (View here [quoted text clipped - 9 lines] > http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups > ----= East and West-Coast Server Farms - Total Privacy via Encryption =---- It's not the lens causing this. You have very small dust particles on the surface of the sensor assembly. In the menu on the camera you will find a function called 'sensor clean'. Select this, - I can't rmember if you also have to fire the shutter - and you will then be able to see the sensor assembly. If your eyesight is good enough you may be able to see the spots on the surface - remember the image is upside down on the sensor, so look at the lower areas. Google for cleaning methods. Take a close shot of a piece of white paper to fill the screen, out of focus (use manual focus and set the lens to infinity), and a small stop, say f/16 or smaller (bigger number) . Now have a look at the result, and you will probably see numerous blobs all over the screen. These are dust blobs. It's normal with dslr's. Generally they are less visible among the detail of an ordinary shot, even less so with large apertures, f/8, f/5.6. Colin D. Many thanks for the info, followed your advice and cleaned the sensor. Being new to digital cameras I'm still learning. anyway, all done and dusted:-) Duncan > It's not the lens causing this. You have very small dust particles on > the surface of the sensor assembly. In the menu on the camera you will [quoted text clipped - 13 lines] > > Colin D. > Many thanks for the info, followed your advice and cleaned the sensor. > Being new to digital cameras I'm still learning. [quoted text clipped - 23 lines] > http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups > ----= East and West-Coast Server Farms - Total Privacy via Encryption =---- Great. Pleased to help. Colin D. >Hi, just purchased a Canon 350D. >All pictures with blue sky have three small dots on them (View here [quoted text clipped - 3 lines] >It's going back at the weekend but wondered if anyone else had experienced >this problem. Just clean your Sensor see below... http://www.pbase.com/copperhill/ccd_cleaning ******************************************************** "A nice man is a man of nasty ideas." _Introductions to History of the Reformation_ Jonathan Swift 1667-1745 >>Hi, just purchased a Canon 350D. >>All pictures with blue sky have three small dots on them (View here [quoted text clipped - 7 lines] > > http://www.pbase.com/copperhill/ccd_cleaning Pretty cool, I see he now has brushes and stuff; they look good, a little pricey, but still nice. --- Slack It's definitely worth it. I bought the whole set and it makes cleaning the sensor (and other parts of the camera) very easy and safe. -- Martin >>>Hi, just purchased a Canon 350D. >>>All pictures with blue sky have three small dots on them (View here [quoted text clipped - 12 lines] > --- > Slack |
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