Hi, thank you for contacting JustAnswer.com. My name is Russell. I will do my best to provide the right answer to your question.
I find your concern quite commendable. Dots on pictures can be surprisingly disruptive of an otherwise beautiful image caught properly. They are distracting.
In this case, the ultimate judge of whether it will affect photograph quality is simple: Take a picture of a flat white area (a well-lit lightly-colored wall, for instance), and see if any such dot comes out on the image, that is not present on the wall itself.
I suspect that in this case, you have a bad pixel on the LCD screen, which would not affect photographs. Photographs in a digital camera are processed internally, and the LCD screen defect would definitely not be stored onto the eventual 'taken' picture.
As for a spot of dust on the focusing screen as opposed to a bad pixel on the LCD screen, that would not get onto photographs in the least for sure, either.
The only hazard is, you might have a bad pixel on the image sensor. That's what that test photograph of a blank white wall would determine. Since the dot is black, if it were a dot or bad pixel on the image sensor, it would surely show up in such a photo if that is what it were.
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