Many believe that leaving the camera switched on when changing lenses causes the electronic “charge” to act like a magnet. Leaving it to attract dust that way. I am not so sure about this as the mirror and shutter should be closed anyway. This would prevent any dust being attracted to the sensor at this time.
The most obvious reason is probably due to lack of care and attention. Taking the lens off in a clean, wind-free environment is your best bet to keeping things dust-free inside the camera. Any dust that gets in (and it will) during lens changes will eventually find its way onto your sensor over time. This happens the more you use your camera, maybe more so with longer exposures…who knows!
Above all else, if you have a sensor and want it cleaned, take it to a pro. I was quoted about £60 by a dealer for the task which is a lot cheaper than replacing the sensor!
So, what happened to me?
I once shot a small but beautiful wedding at the Ritz Carlton in Marbella, Spain (Villa Padierna). Lovely clear blue skies, a lovely couple and tons of great shooting possibilities.
Processing was a breeze with bright bold colours and no more spots of dust than normal. I tend to think that I can cope with “healing out” or “cloning” a few noticeable spots away. That is rather than being without the camera for weeks on end whilst it is being cleaned.
However, I had another wedding a few days later and I was shooting a preliminary “pre-wedding” meal near the beach! I hadn’t used the camera or changed lenses since the previous wedding. I couldn’t for the life of me understand where all this dust had come from.
This is where the dust came from
Update: Since writing this I now realise where the dust had come from. I had cleaned the sensor with a blower brush that had been loose in my bag. It had been in hot “sweaty” Spanish weather and constantly inflating/deflating due to being loose and therefore attracting dust inside. That dust had then become sticky due to the moisture which I then sprayed all over the sensor. Beware and look after your blowers!!!
I looked at the blue sky on the preview screen and it looked like hundreds, no thousands of flies were the coast of Spain! I used my backup camera but also shot a few with the dirty 5D at wide aperture to try the dust.
After painfully processing these images, I decided to have a closer look at the sensor…what a mess! I had another big wedding coming up and needed this camera as my main and had no time to send it away or order cleaning kit (impossible to get in Spain).
Now, was thinking it was just dust and I knew nothing about cleaning camera sensors. I bought a brand new blower brush which was blown and double checked for oil-free cleanliness before use. Then I decided to give just one, gentle sweep across the sensor to try and pull some of it away!
All I managed to do was smear the greasy specks right across the sensor making it look like I had just baked a cake on it!