Answers to this other Question on photo stackexchange have prompted me to seek clarification on this.

consider for example the following images:

Original scene

enter image description here

How a mirror "sees" it (flipped horizontally hence inverted)

enter image description here

How a lens "sees" it (flipped horizontally and vertically hence NOT inverted)

enter image description here

The mirror image is flipped once, hence it is inverted, but the lens image is flipped twice (horizontally and vertically) hence it is not inverted.

This is all relevant when trying to understand the path of the image through an SLR camera. Using an image from one of the answers to the aforementioned question we have this:

enter image description here

Now, if you use my interpretations to understand the inversions, then you have an odd number of inversions in total. So how then is the image at the viewfinder not inverted? Certainly, something I said is not right. I will leave it to you good people to identify my error and correct it for me.


1 Answer 1


The answer is that the pentaprism is actually a roof pentaprism. The image is laterally-inverted (left-right inverted) because the image actually bounces an additional time due to the roof of the pentaprism.

Pentaprism diagram from Wikipedia: Single-lens reflex camera, CC-BY-3.0

  • 2
    \$\begingroup\$ Well that's a pretty significant distinction that's not part of the answer to the other question. \$\endgroup\$
    – Octopus
    Jul 12, 2016 at 22:29
  • \$\begingroup\$ Agreed that it's a significant distinction, but it the details of the reflection actually was mentioned in at least one of the answers. Albeit, not very obviously. I was actually writing an answer to that question when you asked this one! \$\endgroup\$
    – scottbb
    Jul 12, 2016 at 22:31
  • \$\begingroup\$ I am confused. Not able to understand how the 'F' formed as shown in the bottom of roof penta prism. Could someone clarify? \$\endgroup\$
    – matco
    Nov 12, 2016 at 23:05
  • \$\begingroup\$ @matco it doesn't matter how that F forms. It serves as an arbitrary starting point to show how the rays of its corners go through the prism which is what the image is trying to illustrate. \$\endgroup\$
    – null
    Nov 13, 2016 at 9:43

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