You would be shocked at how well lenses still perform despite dust inside them. Intuitively, it’s obvious that something right there in front of everything should be a big problem, right? But Newtonian optics is far from intuitive; the math actually says that that sort of thing is effectively invisible, quite possibly literally too small to measure.
If you have morbid curiosity, it shouldn’t be hard to find examples of DSLR images taken in various states of damage. In general, a significant fraction of the cross-section of the lens needs to be affected before it becomes noticeable short of careful inspection.
A noteworthy caveat: misalignment, on the other hand … doesn’t take much for things to get really bad really quickly. When a lens takes a hit, it’s generally misalignment that degrades the image quality, not visible nicks.
Also: dust on the sensor itself (as opposed to in or on a lens) can be apparent as a dark spot basically identical to the spec of dust. Manufacturers have for decades developed increasingly sophisticated means of addressing the problem such that it’s also mostly a non-issue in practice today. About the only time it’s typically noticeable is in images of a large uniform subject at smaller apertures … such as a clear blue sky on a sunny day.
But, in general, if you don’t see anything amiss in your images, there’s (practically by definition) nothing amiss in the camera system.
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