How many times is the image flipped (inverted) inside a standard Digital Single Lens Reflex camera?

  • 2
    \$\begingroup\$ Is your question about the number of times the image is inverted in the lens path, or the number of times the image is inverted in the body of the SLR, or both? \$\endgroup\$
    – scottbb
    Commented Jul 12, 2016 at 14:57

2 Answers 2


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.

  • \$\begingroup\$ Doesn't the lens flip top for bottom AND right for left? So technically it does not invert the image, not the same way a mirror does. \$\endgroup\$
    – Octopus
    Commented Jul 12, 2016 at 20:56
  • \$\begingroup\$ @Octopus the lens flips it up down and left right. A mirror and lens invert the image in the same way if used under the same restrictions. \$\endgroup\$ Commented Jul 12, 2016 at 21:03
  • \$\begingroup\$ "A mirror and lens invert the image in the same way"... in the same way as what? Not the same as each other. \$\endgroup\$
    – Octopus
    Commented Jul 12, 2016 at 21:04
  • \$\begingroup\$ @Octopus they invert the image in the same way as each other when used in the same way. If the object is outside the front focal point of a lens and outside the front focal point of a mirror, they have the same effect on parity and handedness. The front focal point of a plane mirror is at infinity and the object will rarely be beyond it. \$\endgroup\$ Commented Jul 12, 2016 at 21:24
  • 1
    \$\begingroup\$ If you want to use them in the same way then consider as a planar lens a plate of glass, you view the image from the opposite side with the glass but on the same side with the mirror, there is one extra axis of inversion! \$\endgroup\$
    – Octopus
    Commented Jul 12, 2016 at 22:52

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The image forming rays from the camera’s lens are intercepted by a hinged first surface (reflex) mirror set at a 45⁰ angle. This first reflection rights the upside down image, but, being a mirror image, it is reversed right for left. This image is projected onto a viewing screen. The bottom side of this screen has been roughened by scrubbing it with fine sandpaper or a chemical etch to give it a “tooth”. Thus the bottom surface is nontransparent. This surface allows the image forming rays to be scrutinized with amazing clarity. The image however is flawed; it is has a vignette (dimmed edges), and it is reversed left to right. Atop the ground glass view screen is a Fresnel lens. This is a thin lens consisting of concentric rings which are each a segment of a lens. The purpose of the Fresnel is to mitigate the vignette.

The position of the viewing screen is convenient if the camera is held at waist level. Photographers prefer to work with the camera held at eye-level. To accomplish this, a five-sided glass prism called a pentaprism is mounted atop the view screen. The pentaprism reflects the image of the view screen twice. This action flips the image left to right and redirects the image 90⁰ so that it can be viewed from eye level. A viewfinder eyepiece allows the image on the view screen to be seen by the photographer.

Note the word “reflex” is Latin for “to bend back”.

  • \$\begingroup\$ I think the OP was asking about inversions inside the lens, but I'll vote up for thinking outside (or maybe inside) the box. \$\endgroup\$
    – Caleb
    Commented Jul 12, 2016 at 19:28
  • \$\begingroup\$ @ Caleb Thanks for the up-volt. I tried to use words, to tell the tale. If the OP reads my post, he can count how many times the image forming rays are inverted or flipped. \$\endgroup\$ Commented Jul 12, 2016 at 19:56
  • \$\begingroup\$ A standard pentaprism inverts the image twice (by 2 reflecting surfaces) , but SLRs usually contain a roofed pentaprism which inverts the image one more time left to right, for a total of 6 inversions (if you count 2 for the lens assembly). Thanks to @scotbb who answered this other Q. \$\endgroup\$
    – Octopus
    Commented Jul 12, 2016 at 23:00

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