“Dusting is a good example of the futility of trying to put things right. As soon as you dust, the fact of your next dusting has already been established.”
George Carlin
The Problem
The CCD sensor carries an electromagnetic charge. Particles of dust that get inside the body of the camera are attracted to and stick to the sensor. The dust particles obscure the image. This is most notable at the bottom of the censor (gravity) which is at the top of the image – in the sky. It is most notable when the image is fairly uniform (like the sky) and when the light reaching the censor is unidirectional – if you use a small aperture.
Where does it come from?
- When you change lenses dust enters the camera.
- Wear and tear / Lubricant within the camera creates it’s own dust.
It is a unique problem of dSLR cameras and although possible is not common with compact cameras.
Dust spots show up on a blue sky.
Strategies
Clean the sensor
You need to gain access to the sensor. You need to remove the lens, open the shutter and lift the mirror so that you can see the sensor. Usually this can be achieved by a special menu item. You will need magnification. Strategies to remove dust include
- Blowing compressed air
- Sucking
- Brushing (electro-statically charged brush)
- Wet swabbing.
I take my camera to a mechanic who performs this sensor clean for me. He prefers sucking and wet swabbing. He uses Windex on his lint free swabs. It is possible to do this yourself quite cheaply. I recommend you read several blogs before you try it. I have included references at the end of this article.
Clean the Image
This relies on spot healing tool brush that is available in both Photoshop and light room. This is really only an intelligent version of the clone tool. However it does give superior results to the clone tool. Just place the brush (at an appropriate diameter and hardness) over the dust artifact and it seemingly disappears. The trick is to find all the spots. It may look different once printed or projected.
Dust artifact detection
Lightroom has a special dust detection filter for highlighting where dust spots are located on an image. You can mimic this filter in photoshop by using a levels adjustment layer, or by creating a negative of the image. Look for dust marks in both positive and negative layers and remove with the spot healing tool.
Reference Image
You know you can spend a lot of time removing the same dust spots in the same locations on several images. Another strategy is to locate the dust with a reference image. You will need to use your camera. With a small aperture take a photograph of the ceiling or a blank white wall. Put the image into your editing program. Use contrast enhancement to make the dust more obvious. If you overlay this reference image over your image it can show you where to look for the dust. It is also useful to show you where to clean your sensor.
Dust mask from the reference image.
Nikon developed a program to remove dust automatically from the image by using the reference image. It was slow and not always 100%. This technology has not been replicated by the editing packages and so is not widely available. I only add this as a footnote.
lasso tool and curves adjustment
Dust marks are usually small and correcting a few marks does not impair image quality. As the number of marks increases, it becomes impractical to clean the images and it’s time to clean the sensor. Occasionally you may want to preserve the detail where a dust mark appears. In this instance I use a techniques that I often use with raindrops. I trace carefully around the dust mark with the lasso tool. Feather the edge carefully and then attempt to correct by using a curves layer to increase contrast, lightness, saturation and sharpness. This can be tricky, but can give a satisfactory result if you are patient.
References:
https://www.howtogeek.com/162413/how-to-cheaply-and-safely-clean-your-cameras-dslr-sensor/
I acknowledge the image from the same site.
Camera Clips 