The mirror flips up out of the way when taking a picture. If a flaw is appearing on pictures it will most likely be on the sensor. It is possible to clean the sensor oneself, but I would not recommend it unless you're comfortable with the risk, and equipped properly for it. A camera store would probably be better equipped to do the job. If you are lucky, perhaps it's dust. Before going further on this, you should go to the camera's menu and find the entry for sensor cleaning. The camera can give the sensor a shake that sometimes will clear it of dust, and that's your first option.
I am assuming that the flaw appears in pictures but not in the viewfinder. If it appears in both, it cannot be either the mirror or the sensor, but must be in the lens. Make sure you are clear about what is appearing when. If it appears in the viewfinder but not the picture, it's in the mirror. If it appears in the picture and live view, but not the viewfinder, it's in the sensor. If it appears in picture, live view, and viewfinder, it's in the lens.
Thank you for your help. It's clearly on the mirror, I can see it through the viewfinder but not in pictures. I looked at the lens and cleaned it. There is a visible smudge on the mirror. Will alcohol clean this on a swab or vinegar? I don't want to put moisture into the camera if I can help it.
The mirror, being a front surface mirror, is very sensitive to scratching, more so than the sensor. If you are adventurous, you might try the cleaner used for sensors and lenses, and a soft swab of the sort often sold for sensors. I would advise taking it somewhere, or leaving it alone. It's awfully easy to scratch those mirrors. Whatever you do, you need as light a touch as possible, and as non-abrasive a substance as possible.
@reddawn, please remember that the SLR uses a front surface mirror. The coating is right on top with no protective coating on it. It is very delicate, and any scratch at all will be permanent and non-restorable. This is not like a regular mirror. I really don't know what is the safest thing to use on this, and it's possible the right swab will do it, but I cannot safely recommend anything but taking it to someone who does know.
There’s a number of articles online and videos on YouTube that show how you can clean the mirror and sensor. As long as you have the right tools, it’s safe enough to do on your own. If you feel unsure about it, it’s completely understandable. Perhaps check out Nikon’s support site and see how you can go about getting your camera serviced?
Comments
I am assuming that the flaw appears in pictures but not in the viewfinder. If it appears in both, it cannot be either the mirror or the sensor, but must be in the lens. Make sure you are clear about what is appearing when. If it appears in the viewfinder but not the picture, it's in the mirror. If it appears in the picture and live view, but not the viewfinder, it's in the sensor. If it appears in picture, live view, and viewfinder, it's in the lens.
If you feel unsure about it, it’s completely understandable. Perhaps check out Nikon’s support site and see how you can go about getting your camera serviced?