Why does a camera viewfinder look sharp even though it’s very close to my eye?
Asked 10/23/2012
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When I look at something very close to my eye, it becomes blurry once it’s closer than about 15 cm. But a camera viewfinder sits only a couple of centimeters from my eye and still appears sharp. Am I actually focusing on the focusing screen inside the camera, or does the viewfinder use optics to make that image appear farther away? How does the diopter adjustment fit into this?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
13y ago
2 Answers
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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!
Originally by user1375. Source · Licensed CC BY-SA 4.0
user1375
13y ago
0
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You are not focusing on the focusing screen as if it were physically only a few centimeters away. The viewfinder includes eyepiece optics (a small convex lens system) that make the focusing screen’s image appear at a more comfortable virtual distance for your eye.
In a DSLR, the scene is formed on the focusing screen, then redirected by the pentaprism/pentamirror to the eyepiece. The eyepiece acts like a magnifier/corrective lens, so your eye can focus on that image even though the camera is close to your face.
That’s also why most cameras have a diopter adjustment next to the eyepiece: it changes the optical power of the viewfinder to match your eyesight. Some people can use the viewfinder without glasses by adjusting the diopter; others keep it neutral and wear glasses.
So yes, the image originates at the focusing screen, but optical elements in the viewfinder create a virtual image your eye can focus on easily.
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