Wataru said:
If you close the eyepiece shutter prior to unscrewing the eyepiece will come off easily. The eyepiece shutter locks the eyepiece in place. Nikon should perhaps document that better.

You need to remove the eyepiece if you are attaching the magnifying viewer, for example.

Yup, noticed the same thing on my D4

The manual p. 39

A Diopter-Adjustment Viewfinder Lenses
Corrective lenses (available separately; 0392)
can be used to further adjust viewfinder
diopter. Before attaching a diopter-adjustment
viewfinder lens, remove the DK-17 viewfinder
eyepiece by closing the viewfinder shutter to
release the eyepiece lock (q) and then
unscrewing the eyepiece as shown at right (w).

No mention of this being required before attaching the right-angle viewfinder, which I use often. But, we learn....

Nikonjeon said:
Thanks for your reply TaoTeJared! If I use Hoodman eyepiece suggested by ericbowles, this reflection will go away?

Probably some of it. The reflection could be a multi issue thing caused by the lens, filters, lack of lens hood, dirty (oil on) viewfinder, reflection off of glasses, etc.

Whenever you point a lens at a light source, you will get this. Since the APS-C sensor mirror is smaller (as well as the eyepiece) it is much less noticeable as it comes in from the "cropped" areas that DX doesn't cover.

Thanks for your reply TaoTeJared! If I use Hoodman eyepiece suggested by ericbowles, this reflection will go away?

That is due to the larger eyepiece. If you wear glasses you will notice it more. It is more of stray light coming in from the eyepiece than the lens.

Thanks for all your replies! This reflection is not the lens glare. It comes from a shinny parts sitting at the bottom of the inside of the view finder close to the eye. I don' t know why they design like this, or only my camera has this issue?

ericbowles said:
...note that the existing eyepiece is hard to remove. I had to gently use pliers to remove the eyepiece. And the replacement eyepiece tended to work its way loose, so I applied a single drop of blue Loctite to hold it in place....

If you close the eyepiece shutter prior to unscrewing the eyepiece will come off easily. The eyepiece shutter locks the eyepiece in place. Nikon should perhaps document that better.

You need to remove the eyepiece if you are attaching the magnifying viewer, for example.

I noticed the same "issue" and added the Hoodman eyepiece.
http://www.hoodmanusa.com/products.asp?dept=1061

If you go this route, note that the existing eyepiece is hard to remove. I had to gently use pliers to remove the eyepiece. And the replacement eyepiece tended to work its way loose, so I applied a single drop of blue Loctite to hold it in place.

Please tell us what lens you are using, where some of your photos are posted which show the problem, and also what your interests are. This problem is almost an impossible situation. Makes me wonder....we need more info....

I don't quite understand your post Nikonjeon - do you mean do you mean you are seeing the reflection with your eye to the viewfinder but it doesn't show on the image, that you are getting light entering through the viewfinder and showing in the images, or that you see lens flare through your viewfinder when looking into a bright light? The last two are normal with almost any camera and the first doesn't matter.

Hello, I've got a new D800 and found light reflection inside the viewfinder when I pointing to a bright light. I never had this problem with my D7000, so I am wondering if this is normal for D800. Thanks for your reply.