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I dont think is in the curtains that open and close when you press the shutter. Is it inside the lense? Or is just seen in illustrations that show how a camera works? It has nothing to do with "aperture"?

The best way I could find to explain my question is this: The pinhole I am talking about is that orifice where the light goes thru, and an inverted image (upside-down) is created. This orifice must be present in any camera in order for a defined image to form and register on the film or sensor. But I dont think is the aperture, because the pinhole has to be a very small, "punctual" hole? And is the same orifice existing in the first invented cameras , over a hundred years ago. (camera obscura comes to mind)I just would like to know where is this orifice located in modern SLRs

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I'm not entirely sure that I know what you're trying to ask. What do you mean by pin-hole here? –  John Cavan Oct 26 '13 at 5:35
    
I think what you mean is in fact the aperture. Blades inside the lens that close to limit the light entering. –  MikeW Oct 26 '13 at 7:29
    
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This is a hard question to ask if you don't already know the answer, but a good one. I think the answers to the question I've linked to above cover what you mean. (It's not exactly the aperture.) If you're looking for something else, please let us know. –  mattdm Oct 26 '13 at 10:53
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Two site tips for you: you can (and should!) use the edit link under the question to add details and otherwise improve it. (You don't need to write "edit" or "update", just make changes, because the history is saved. Also, you can reference another user by putting an @ in front of their username, either entirely or just in part — the software figures out what you mean. Like @angelrojas. –  mattdm Oct 26 '13 at 14:22

1 Answer 1

I assume you're referring to the centre of projection otherwise known as the "no parallax point" and frequently erroneously referred to as the "nodal point".

It is the point you need to know to use the pinhole projection model.

It's important to note that it's not the point where the light rays actually cross, it's the point where the lens behaves as if they cross. It's thus a virtual point that's usually somewhere inside the lens but can be in front of the lens.

When focussed to infinity the point will lie at distance in front of the sensor plane equal to the lens's true focal length (the stated focal length is probably rounded). It's rarely stated in the datasheet for a lens, but you can determine it experimentally, by following the instructions that come with "VR" tripod heads:

http://reallyrightstuff.com/websiteinfo.aspx?fc=87

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Can you elaborate a little more on the important difference? What does it mean to "behave as if" they cross but not actually cross, and why is the difference important? –  mattdm Oct 26 '13 at 14:24
    
Many lens designs are not symmetrical and use a retrofocus design, which is how wide and telephoto lenses are kept to a practical size. (pretty sure I got that right) These designs move the point where the light rays cross versus where a "pure" lens design would have it, which is what Matt talks about above. However, the "nodal point" which you want (for panoramas?) is not based on the glass moving the focal point around but instead it is based on the real focal length of the lens. And, like Matt said, it can even be out in front and outside the lens itself. –  Patrick Hughes Nov 10 '13 at 6:28
    
You are mixing two different things. The no-parallax point is the entrance pupil of the lens. The point on the axis lying at one focal length from the sensor when the lens is focused to infinity is both the image-side principal point and the image-side nodal point. –  Edgar Bonet Nov 13 '13 at 9:59

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