I noticed that when looking at small items in close distance, minimum focusing distance for eye seems to be about 15cm (6") - closer than that, vision promptly gets blurrier instead of seeing more details.
So how can I see a sharp image on focusing screen through viewfinder, which is perhaps a couple of centimeters (less than an inch) from my eye? Am I really looking at the image projected on focusing screen, or are there some optical tricks used to create a larger false image further away?
Inside the viewfinder is a convex lens which enables your eye to focus on the screen. This works in exactly the same way as convex lens used in a pair of glasses to correct long sightedness.
The power of this lens is usually adjustable via a small dial next to the eyepiece to account for differences in people's eyesight. Some people who wear glasses have the option of removing their glasses when shooting and relying on the viewfinder lens, whilst some leave the adjustment in the default position for someone with normal vision and wear their glasses whilst shooting.
In the latter scenario the image seen is the result of four separate optical instruments - the taking lens, viewfinder lens, eyeglasses and eye lens!
Now that I fully understand I'll answer with some pictures.
This:
From here. Is a basic cross section of travelling light. 5 is the focusing screen (notice how it's the same distance as the sensor 4), 7 is the pentaprism and 8 is the eyepiece (basically what Matt Grum is referring to).
Here:
From here. Shows a bit more clearly the lenses or eyepieces involved. This is what makes us able to focus at a distance within 1cm.
A quick Google Images of viewfinder construction and DSLR construction gave me those images.
If I understand you correctly, the ability to focus on an object is generally affected by the viewer's personal eye sight condition. Some people may not be able to focus 6" or closer.
But to further the answer, you are able to see this image in focus due to the distance your eye is from the view finder. you're essentially recreating an extra point of focus in the direction of light towards your eye. This is how lenses work. By using a macro lens, you would be able to focus much closer to your subject. This is down to the way the lens is built and the distance the series of glass is situated. By keeping your eye a certain distance from the view finder you could say you are changing the ability of the lens. Of course this would never affect the image produced.
So in answer, I suppose you could call it an optical trick but really that's just how lenses work. Some more technical reading can be done here.
Hope that helps.
