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### #1 Mathew Collins

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Posted 12 November 2016 - 06:22 PM

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.

--------------------

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

--------------------

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

http://photo.stackex...e-a-dslr-camera

http://photo.stackex...redirect=1&lq=1

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

-Mathew

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### #2 David Mullen ASC

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Posted 12 November 2016 - 08:09 PM

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.

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### #3 Mathew Collins

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Posted 13 November 2016 - 12:01 PM

>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 by Mathew Collins, 13 November 2016 - 12:02 PM.

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### #4 David Mullen ASC

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Posted 13 November 2016 - 12:14 PM

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.

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### #5 Mathew Collins

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Posted 14 November 2016 - 06:55 AM

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..._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 by Mathew Collins, 14 November 2016 - 07:05 AM.

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### #6 Mark Dunn

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Posted 14 November 2016 - 08:34 AM

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).

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### #7 Mathew Collins

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Posted 14 November 2016 - 12:01 PM

Thank you.

Still not able to follow the image 'F' formation (bottom) in the ground glass.

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### #8 David Mullen ASC

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Posted 14 November 2016 - 12:41 PM

https://en.wikipedia..._Pentaprism.svg

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### #9 Mathew Collins

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Posted 19 November 2016 - 01:49 AM

Thank you all. Trying to understand it.

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