Some years ago all of the pictures taken on my Pentax K10 contained small fuzzy dots; and the number of dots grew over time. I was told by the knowledgeable camera store guy that I had a fungal grown in the camera!! This made sense to me as I had spent time trekking in the Himalayan monsoon, and then a few months later, spent a couple of weeks in Vietnam, also in a wet season. I had the camera cleaned, but the spots appeared again a few months later. So in frustration I placed the camera in storage and was about to give up on photography.
Instead I bought a new camera - a Canon 5D 11. Much better than a K10.
I have since realised that the spots were dirt on the sensor, and I have had it happen a few times to me with my current of cameras ( I have since sold the 5D), but I now know how to clean the sensor and it is quite easy.
There are numerous youtube clips so have a look at them, and make sure you know how to set your camera mirror to lock up mode.
Ensure you have a full battery before starting,
- remove the lens
- turn on,
-set mirror lock up,
- shine a headtorch or light onto your sensor - you may be able to see that dirt/dust/oil spot
- now clean the sensor using the paddle as per instructions below.
Ebay is awash with small paddles that can be used to wipe across the sensor - search for sensor cleaning. I preferred to purchase a name brand cleaning fluid though - since its going to wipe across your sensor its important to trust it wont cause damage. The Eclipse optic cleaning fluid, manufactured in the US by Photosol is a popular one and was recommended by Vanbar in Melbourne. It cost me Aus $39 which is pretty cheap considering you only use 2 drops per clean.
Put 2 drops on the paddle, and then wipe across the sensor. Turn the paddle over and wipe back the other way. All done. Now throw the paddle away - dont use it again.
Test for spots on your sensor. I do this by setting the exposure to about 1 second, point to the sky and click. Do this a few times, then open the image in LR (or whatever you use) and check to see spots are gone. If they are still there, repeat. And remember, the dirt will appear on the photo in the opposite quadrant to where they are on the sensor: ie: tope left area on photo, then look to the bottom right of sensor.
Having mastered this process I no longer worry too much about heading off into the dirty world with my pristine gear ;-)
I hope this helps someone.