About roof pentaprism

Mathew Collins · November 12, 2016

Hi,

I am not sure this question is qualified in this forum.

This is a question about roof pentaprism.

I have found two pictures for the image formed by the prism.

1. https://en.wikipedia.org/wiki/File:Roof_pentaprism_example.jpg

2. https://en.wikipedia.org/wiki/File:SLR_Pentaprism.svg

About Picture 1

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I could match the 'F'(lower side) in the picture with image formed by the mirror. But is that top 'F' correct?

About Picture 2

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I am not able to find how the image 'F' formed at the bottom of the prism?

There are some links.

http://photo.stackexchange.com/questions/80067/how-many-times-is-the-image-inverted-inside-a-dslr-camera

http://photo.stackexchange.com/questions/80085/mirror-vs-lens-do-either-or-both-invert-the-image?noredirect=1&lq=1

Could some one explain this? Just confused about the working of pentaprism.

-Mathew

David Mullen ASC · November 13, 2016

Doesn't the diagram in the #2 Wiki entry explain it to you? The big "F" at the base of the pentaprism isn't a formed image, it's the incoming image and you follow the light rays to the image coming out the other end.

Mathew Collins · November 13, 2016

>The big "F" at the base of the pentaprism isn't a formed image, it's the incoming image and you follow the light rays to the image coming out the other end.

My question is about the 'F' at the base. But I am not able imagine how the 'F' is ended up in this position.

Would the inverted image of 'F' formed by the lens, and reflected thru mirror end up like this?

Edited November 13, 2016 by Mathew Collins

David Mullen ASC · November 13, 2016

The "F" is not really at the base, it's the image of the "F" coming through the base. You follow the light rays bouncing off the inner surfaces to see how they ended up being flipped and inverted by the time they come out the other side.

The image of the "F" comes into the base upside-down because that's how to appears after it has passed through the camera lens and then reflected off of a mirror, so the prism is correcting that to appear as it did before the camera lens and mirror.

Mathew Collins · November 14, 2016

In the following pic, which side is the viewfinder located, left/ right side? or from which side I have to view the final 'F' thru viewfinder?

https://en.wikipedia.org/wiki/File:SLR_Pentaprism.svg

I understand that

1. Inverted image is formed by the lens.

2. Inverted image is reflected in the mirror.

3. ' F' formed in the ground glass from the image in the mirror

4. Corrected image is visible while looking thru viewfinder.

Is there any side changes( left to right, right to left) happening during these steps?

Edited November 14, 2016 by Mathew Collins

Mark Dunn · November 14, 2016

The eyepiece is on the right.

David's image only shows two reflections on the pentaprism, but it in fact reflects the image three times (It's correct in the Wikipedia image, if a little hard to follow) , so the right-way-up image from the mirror goes from flipped (left-right) to correct(first reflection) to flipped (second) to correct (third).