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Nikon d750 dot on image - always same place. Visible speck through viewfinder in the right place , with lens off. I know it can't be any sensor mark seen via viewfinder, none the less I've cleaned the sensor. I think I'm right in thinking dot on mirror can't effect the image. I've carefully wiped mirror. Where is the speck? Thanks. My first dslr. Missing film already, Tom

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  • It sounds like dirt in the viewfinder assembly. Can you confirm this problem speck is not visible in photos ? This YouTube video may help. Same problem can happen with film SLRs by the way.
    – StephenG
    Jun 11 '20 at 21:21
  • Thanks, Steven. Speck visible on images and through viewfinder without lens on.
    – tombox
    Jun 12 '20 at 9:12
  • I confess that's perplexing to me. To get a visible speck in an image would require dirt very close to the sensor itself and I don't see a way to get both a viewfinder speck and a corresponding image spot from just one cause. A greater mind than mine is needed for this problem. :-)
    – StephenG
    Jun 12 '20 at 9:25
  • Perhaps two specks that optically look to be in same place in ration to image but are in separate parts of camera. I could always clean the sensor again. Am I right in thinking the mirror plays no part in this? Not effecting the image but I be seeing something on the mirror in the viewfinder? I will check the video you posted, thanks. T
    – tombox
    Jun 12 '20 at 11:17
  • In both DSLRs and SLRs the mirror sits down to redirect the light to the viewfinder until you shoot at which point it flips up so all light goes to the sensor/film. So you are correct that the mirror cannot affect the image formed by the sensor. There is normally a second smaller mirror used to direct light to the AF system to get focus, but I don't think that would be the path of both sensor and viewfinder at the same time. Here a link to a post on DPReview with a diagram and brief explanation of that secondary mirror.
    – StephenG
    Jun 12 '20 at 16:18

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