My technique to check my camera sensor for the presence of dust.
1. A white paper sheet.
1. Set the white sheet on a table.
2. Mark a black spot(with a marker) on the sheet.
3. Fix the camera on the tripod.
4. Set the camera to it's minimum aperture(for mine f22).
5. Focus on the black dot and take the picture.
6. Observe the picture, if it contains random black dots other than the dot on the sheet, it means that there is dust on the camera sensor or the mirror and it needs cleaning.
Indeed, dust on the mirror will not show up in the image you take, as the mirror gets out of the way when the picture is taken.
Also, the dot on the paper is not really necessary. Taking a picture of a bright object (white computer screen, for example) at maximum aperture, and then pushing the contrast to the maximum will tell you if you have dust on the sensor.
Will you see dust on the mirror as a dust spot on your image? I thought spots on the image was only caused by sensor dust. I use a nifty little sensor loupe with 7x magnification and led's to light up the sensor. All dust shows up quite nicely. The trick is to clean it without damaging the sensor.
Your tip will only get the job done halfway. If there is dust on the lens, it will show up when the camera's set to its minimum aperture. My camera has a mirror-lock-up feature, which lifts the mirror and opens the camera shutter. By looking carefully, I can see if there is dust on the sensor.
I am even lazier, I get the editors telling me when I have these problems lol. No really, I try to keep my equipment as clean as possible but sometimes due to exposed weather etc it is not always possible until you see the images and notice the problem, clone tool for me but thanks for the advice on how to check.
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