Thanks useful information: 
Dust in/on camera equipment can impact the resultant photos.
Do you see small dark spots in your images that seem to show up in every image? If you see them consistently in the same location (the size and darkness of the spots can vary depending on the aperture chosen for that photo), you are seeing dust particles that are on your camera’s sensor.
What is sensor dust?
If you have a DSLR or other interchangeable lens camera, you will at some point have to deal with sensor dust, it is part and parcel of our gear. Dust is a normal fact of life and it is all around us, even at our homes that we try to keep clean at all times. Ever seen a sunbeam shining across a room through a small gap in curtains. The air is full of small dust particles. This dust that is all around us, can get into and onto our camera gear. All lenses 'breathe' one way or another or else the internal elements would not be able to move for autofocus and zoom functions. If you use more than one lens, the dust is also able to get into the camera body during the process of changing lenses.
Once the dust is in the camera body, it will either fall on the bottom of the camera (gravity) or move around until it lands somewhere. Some dust particles land on the mirror inside the camera and others might end up getting stuck on the camera sensor and yet others get stuck to the lens. So, as you can see, there are three areas where dust can settle that can affect your images:
1. The camera mirror – when dust ends up being on the camera mirror, you will not see it in your images, but you will see dust particles when you look through the viewfinder. This one is just annoying and it can be easily cleaned either with a small brush or a blower. Be careful touching the mirror in any way.
This one is not a big deal – if you see some dust inside the viewfinder but you do not see it in your images, do not worry about it too much and only clean the mirror if it is too annoying for you. The second and third are the ones that can spoil your images and have a negative effect on affect image quality.
2. The lens exterior, front and/or its back element – while very small dust particles will not affect image quality, the larger ones and dirt/grease will decrease contrast and might even possibly degrade image quality. Always make sure that both the front and the rear elements are clean and dust/dirt free, using a blower or a microfibre cloth is one of the best ways to clean your lens. If there is a lot of dust on the element, be careful using a cloth as you may swipe the dust across the surface of the lens and scratch it.
How does dust, dirt and grease affect the image that comes out of your camera? Dust and dirt on the front element will rarely be visible, unless there is too much of it. Even then, you will not actually see any dots in your image, but rather will notice that your images are a little “hazy” or “cloudy”. It will not show up in a photo as a defined small dark dot. If only a part of the front element is affected, for example an oily finger touched the front element, then it is easily cleaned.
A finger (grease) mark on a front element will appear in your photos like THIS
Dust on the rear element of your lens will result in photos that look like THIS
3. The camera sensor – the worst case scenario, because the dust particles will show up in every image, especially when stopped down to small apertures like f/10. Cleaning the camera sensor is not easy and the process requires special tools that need to be used with extreme care. Cleaning your own sensor is possible, with equipment and supplies from places like cameracheckpoint. If you are not confident in doing so yourself, then professional cleaning services exist.
The size and visibility of the dust particles will change as you change lens aperture. But in general they will appear in your photos as defined dark spots. At maximum apertures on fast lenses such as Nikon 50mm f/1.8 you might not even notice the dust particles in your images, which does not mean that they are not there. They will only be visible at smaller apertures, so as you go to f14, f22 etc you will start to see the dakr spots in your photos. As you increase aperture to a larger number, the dust will appear darker and more pronounced and the size of it will also get a little smaller. Dust particles will always appear in the same spots on every photo. Sensor dust can never be seen through the viewfinder, it only shows up in images. Even then, you might need to zoom in to 100% to see it. Larger dust particles and hair can be visible right away without having to zoom in
A photo showing sensor dust is HERE
Those three circled dots are sensor dust. As you can see, the dots are quite obvious and are much smaller than the earlier example of dust on the rear lens element.
How to see sensor dust
If your camera has dust on its sensor, you can quickly spot it by doing the following:
1. Set your camera on Aperture Priority Mode.
2. Set your metering mode to Matrix/Evaluative Metering.
3. Set your camera ISO to the lowest number such as ISO 100 or 200.
4. Turn off Auto ISO.
5. Turn off autofocus and set your lens on manual focus.
6. Set your aperture to the largest number available for your lens (ie f22)
7. If you are outside, point your camera up at the clear blue sky and take a picture. If you are indoors, find plain white paper, or the front of a clean fridge door, zoom in all the way so that the white fits the whole frame, then make sure that the lens is completely out of focus and take a picture.
8. Open up the photo in your photo editing software, maximise it to the screen and then get as close to the monitor as possible so that only the white color is visible in the frame. Make sure that your focus is way off (completely out of focus) – that way only dust particles will be visible. If you see dark spots like in the above example, then your sensor has dust on it.
There is a shot of the sky at f/16 showing dust on a sensor and other interfering particles, HERE
Sensor Dust (what is it?)
The large dark circle is dust on the rear element of the lens, while the dots and hair are both on the camera sensor.
Summary
Based on the above, you can determine where the dust or mark is, and thus clean the appropriate piece of your camera kit.
There you have it, how different dust/particle/grease elements will appear in your photos, based on where the dust/particle/grease is on your gear. If you are confident at cleaning your own gear, then do so, if not there are numerous professional services who will do it for you, and even other photographers who are willing to do it and show you how to do it. Once you learn how to do it yourself, then you can just purchase the cleaning gear from cameracheckpoint and do it yourself.
Note that images in this article are not mine, thus I have linked them into the thread rather than posted them directly to the thread.
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