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I saw some speck on the screen but it did not show in the photo that i took. so i decided to clean it on the mirror and sensor. but after i clean it, the dust did remove but other dot came out and finally the very big speck come out on the sensor. i tried to clean it on both sensor and mirror but it was not remove. i can see it in the view finder but it does not appear in the photo. what should i do? will if affect to my photo next time? enter image description here

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When you look through the viewfinder you are looking through a focusing screen, a mirror and out the lens. The sensor is covered by the shutter and mirror. So any speck you see through the viewfinder is NOT on the sensor. It appears that the dust is on your focusing screen. See this answer for a link how you might clean it. But it will not show up in any photos.

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Anything that shows up in the viewfinder but not in the photo is somewhere in the light path between the mirror and the exit pupil of the viewfinder. If using a quality bulb air blower doesn't clean it you need to either just live with it or send it to a professional service center for cleaning or repair. Be sure to use a blower that doesn't spray dust into your camera's light box. Good blowers have a one-way intake valve with a dust screen to prevent sucking dust into the blower along with the air.

The mirror and the focusing screen in the roof of your camera's light box are two of the most easily damaged components in the entire camera. Even the filters that cover the imaging sensor are more durable and easier to clean without damaging them than the mirror and focusing screen.

I often recommend that people can learn how to clean their own sensors. I rarely, if ever, recommend that someone who has not had proper training attempt to clean the mirror or focusing screen.

Any damage to the mirror or focusing screen can also affect the accuracy of the camera's light meter, which is located in the prism housing. The light measured by the meter is affected by the reduction of the mirror's reflectivity and the brightness of the focusing screen. When the coating on the mirror is damaged it affects how much light the mirror reflects, especially in the semi-translucent middle of the mirror which is the area critical for most metering modes. When the non-smooth surface on the bottom of the focusing screen is damaged it affects how much light passes through the screen.

Cameras that have various optional focusing screens that can be swaped often have a menu option to designate which particular screen is installed so that the camera can compensate for the varying amounts of light allowed to pass through the different screens.

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The image sensor has a protective cover glass. The reflex mirror is “first surface” meaning the reflective aluminum coat is deposited directly; it is not protected by glass. The aluminum is soft and easily scratched so approach cleaning with extreme caution. Many first surface mirrors are over coated with a hard protective layer of quartz but you must approach this task as if the aluminum is unprotected. Best to leave everting as is. If you can’t stand the spots, then take or send the camera to be cleaned by the manufacturer or an authorized repair service. If you are a risk taker, you can attempt to clean.

If you must undertake this project: Modify a Q-tip by overlaying the cotton tip with a scrap of used and well washed Tee shirt. First try only distilled water and rub ever so gently in a circular motion. If the distilled water will not budge the dirt, shift to a commercial lens cleaning agent. If the spot continues to remain then attain the highest grade ethyl alcohol you can find. I use EverClear, 99% ethyl alcohol sold in liquor stores in some locations. Vodka is about 50% ethyl alcohol but may contain impurities. You need high purity so no residue remains after it evaporates. I have used Vodka with no ill effects. Try a 10% solution made with distilled water and ethyl alcohol. If the spot remains then up the percentage.

Remember, spots on the mirror and ground glass viewing system do not image, they are just annoyances. If you are a risk taker than proceed. If you are not, then send the camera out or live with the spots.

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    Remember that the spot on the mirror may be where the mirror coating has flaked off due to the previous cleaning, so don't be too aggressive in trying to remove it lest you make it bigger. – Caleb Sep 1 '16 at 21:08

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