How accurate is a DSLR viewfinder?

I use the D7100 and i have noticed there are times when a subject seems out of focus through the viewfinder but actually turns out to be in focus

using a micro lens, the Nikon 85mm, part of the rear of the subject looked out of focus through the viewfinder yet appeared in focus when using "depth of field preview" button

and turned out fine when examining the pic later

is that common?

Might be you need to adjust the dioptre control (next to the viewfinder) to suit your eyes.

That is quite a powerful name for someone who would ask this! I'll tell you what I know on the subject and then you can reiterate it and make use of your namesake!

All cameras are different, different SLR's (be they digital, 35mm, medium format etc) have different viewfinder designs and each will show differing degrees of accurate reproduction of the scene as well as what is recorded to the film or sensor.

You seem primarily to be concerned with focus so I'll concentrate on that, however, there are other ways that an SLR viewfinder's accuracy is in question such as the fact that consumer grade SLR's usually only show 92 to 98% of the total image recorded whereas professional viewfinders show 100% of the image coverage.

--Most DSLR's feature a knob beside the viewfinder that allows for diopter adjustment. This allows the shooter to adjust the camera's viewfinder for his/her own eyesight. If your diopter is out of adjustment, the viewfinder will look blurry to you. The D7100's diopter adjustment knobs, like most Nikon DSLR's is on the upper right of the viewfinder eyecup. If you hadn't paid any attention to that previously, it's time to revisit your instruction manual on how to adjust it.

--Consumer grade DSLR's use mirrors to create the viewing prism which is unlike consumer grade film SLR's and some professional DSLR's that use a big hunk of nice, expensive optical glass cut into a prism with silver adhered to the necessary sides as the viewfinder. Cheap, low quality mirrors diffuse light more than glass ones and make the view from a consumer grade DSLR like the D7100 not nearly as pleasant, clear and free of eye-strain as any common 35mm SLR or professional DSLR. The thinking is that everyone is so obsessed with automation and number of features vs. Cost, that they don't bother to notice quality being taken out of their cameras for something more necessary in increasing rare moments of manual focusing. The in-focus indicator light in your viewfinder can sometimes make-up for the lack of reliability of cheap equipment!

--You might want to try using a different focusing screen if you've adjusted the diopter and still don't like the quality of focus in the viewfinder. I wear glasses and prefer what is called a split screen. This essentially contains a line in the center of the image and in order to focus, you have to line up the two halves of the image that you see. On that note, and this is unlikely but you could also check to be sure that your current screen is firmly seated in the camera. If it is not firmly seated, this could cause things too appear out of focus in the viewfinder. Might be worth taking the camera to a proper camera shop for them to look at this for you. Of course trying to find a proper camera shop instead of a general electronics store nowadays can be a chore outside of major cites!

--Finally, if all of the above is in check, maybe it's time for an eye exam! Pit your vision against the optical quality of Nikon's viewfinder!

Best of luck!

What you saw was the way it's supposed to work. The normal viewfinder view is with the lens wide open, which results in shallow depth of field. DOF preview closes the diaphragm to the set f-stop so you can see what the actual picture will look like. Closing the diaphragm increases the depth of field, which is usually desirable with close-up lenses; it's hard to get the whole subject in focus otherwise.