There are three ways dust can hinder your photography.

 1. Dust on the shutter mirror or view finder: You can see this dust when you are viewing through your view finder and not on you pictures.

2. Dust on the lens: You can see this dust when you are composing using the view finder and also on the pictures.

3. Dust on the sensor: You cannot see this dust when you are composing, you can only see this on the pictures.

 1. Dust on viewfinder: The dust can be on your shutter mirror or on the view finder mirror or prism. This dust do not spoil your picture. This is more of a nuisance when you are composing a shot. You can remove this on most cases using a Rocket blower or using a dry swab. Very important: You should not put your camera on sensor cleaning mode when you are handling just this dust.

2. Dust on your lens. This dust can be seen when you are composing and also on your pictures. If the dust is on the external elements, you can use a lenspen to remove it. It does the job pretty well. if the dust is inside the Lens body. This has to go to the service center. Not advisable to open the lens body (I may advise after I have taken a chance with it 😉 )

3. This is the trouble maker, dust on the sensor itself. You cannot see this dust on your pictures but not when you put your eye to the view finder.

Here is how you will make sure you do not have any dust on the sensor. Set your camera to Manual mode or to Aperture priority mode. Set the Aperture to the maximum aperture available on your lens. Set your ISO to the lowest value, so that you are not dealing with noise during this process.

Point your camera on a plain white wall or ceiling or a blue sky. I prefer a plain wall for this. Now you can have your metering + or - 1, this should be alright. You need to be exposing more than a few seconds. Around 5 seconds you can get, that will be perfect.

Set your focus to manual so that the lens is not hunting for focus on a plain surface. Of you can enable AF-off (SHort cut key) if you have it handy.

When you hit the shutter button, keep swaying the camera around the same surface, so that you are not capturing the imperfections on the surface if any. The final image will show you if it had dust on the sensor.

You can focus on your camera to check for images, or move the image to the computer. Push the white and black on light room to identify the dust.

Here are few pictures with dust on it.

Here is my experience with sensor dust and tips on how to clean sensor dust. The way that worked for me.