How many times is the image inverted inside a DSLR?
Asked 7/12/2016
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In a standard DSLR, how many times is the image inverted or flipped between the lens, mirror, and viewfinder? I’m asking both about the image formed on the sensor and what happens in the optical viewfinder path.
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
10y ago
2 Answers
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The image forming lens flips the image once. For the imaging sensor, there is one flip.
The viewfinder is made up of a fold mirror, the one in the mirrorbox of the camera, a focusing screen, and either a pentaprism or two mirrors.
The fold mirror flips the image once, restoring it to "normal" parity. This means there are two inversions in a waist-level viewfinder.
The pentaprism/mirror flips the image a further two times and deviates it 90 degrees, giving two more flips.
As a result, the image is flipped four times between the lens and viewfinder.
Originally by user40937. Source · Licensed CC BY-SA 4.0
user40937
10y ago
0
Generated from our catalog & community — verify before relying on it.
A DSLR image is effectively inverted once by the taking lens to form the real image at the sensor/focal plane. That image is upside down and reversed, as with any simple lens projection.
In the optical viewfinder path, the reflex mirror reflects the image upward, changing its orientation once. This restores the top/bottom orientation but leaves a left-right reversal at the focusing screen. Then the pentaprism (or pentamirror) reflects the image two more times, which corrects the left-right reversal and delivers an upright, correctly oriented view to your eye.
So, in practical terms:
- sensor/image plane: one inversion by the lens
- viewfinder path: the lens inversion plus one flip at the reflex mirror and two more in the pentaprism/pentamirror
That means the image reaching the viewfinder has undergone four flips in total from scene to eye, ending up looking normal to the photographer.
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