Focal point

[Helen B]

You will probably know most of this already, but I’ll give some brief explanations for anyone who is interested but is not familiar with the terms.

A lens has two nodal points – the front and rear, also known as first and second. For a simple lens they may be coincident (at the same place). The space between the nodal points is called the nodal space. The nodal points are where the centre of a thin single-element lens would be if it represented the entire compound lens. The front nodal point is where the thin single lens would be with respect to the world in front of the lens (the “object space&#8221 and the rear nodal point is where the thin single lens would be with respect to the image (the “image space&#8221. Because, in terms of the outside world, the lens can be treated like a thin single-element lens positioned at the front nodal point there is a strong tendency to believe that the front nodal point is the correct point of rotation for a stitched panorama. (As an aside, the rear nodal point is the best point of rotation for a panoramic camera with a swinging lens – the camera body stays in one place, only the lens swings.)

The problem is that the front nodal point does not define the complete bundle of rays arriving from a point on an object, passing through the lens and forming an image of that point. It only defines the path of one very special ray from a point, and that ray may not even make it into the lens or onto the image. That isn’t a problem for object points that are in perfect focus, but it is a big problem for object points that are not in perfect focus, because it means that the position of the front nodal point does not define where the actual bundle of rays that are focused elsewhere will cross the film/sensor plane. One can be very tempted to imagine that if two objects are lined up with the front nodal point and the camera+lens is rotated about the front nodal point so the three things (two objects and the front nodal point) stay lined up, the image of one object will always be perfectly superimposed on the other – ie no parallax. Sadly this is not true, because at least one of those objects is not in perfect focus and thus its location at the film/sensor plane is not determined by that special ray that is lined up from one object to the next then to the front nodal point. That ray need not even arrive at the film/sensor plane.

The complete ray bundle coming from an object point and passing all the way through the lens to the image is determined by the entrance pupil. The entrance pupil is not a point, of course, but an area with the shape of the iris. It can actually change for different object angles (ie it can be in different places for an object on the periphery of the field of view and for one on the lens axis). It always acts as the point of perspective in terms of the outside world – it is where the lens ‘sees’ the world from, and where the outside world sees the lens’ aperture to be.

I’m going to stop there for now. Does that make sense, or would you like me to expand on anything? It's a bit rushed and perhaps too brief at the end.

 

[Ron Evers]

I would like to read your CV Helen. Really!

 

[Raincheck]

Here the link to the pano I shot this afternoon without reading Helens reply, didnt get the what ever it called point right but will get it next time, thanks for the comments.
http://www.thephotoforum.com/forum/...ama-canon-100-400mm-c-c-if-you-have-time.html

 

[ann]

she is my hero for sure

 

[tirediron]

Outstanding and informative post, as always! Thanks,Helen!