Why doesn’t the focal point being at the sensor collapse the whole image to one point?
Asked 9/24/2020
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I’m trying to reconcile two optics statements:
- The distance from a lens’s principal plane(s) to the corresponding focal point(s) is the focal length (in air).
- When a photographic lens is focused at infinity, the rear nodal point is separated from the sensor/film by the focal length.
What confuses me is this: if the focal point were on the sensor plane, wouldn’t all the light converge to a single point instead of forming an image? I had assumed the image would need to form some distance beyond the focal point.
Am I mixing up focal points, nodal/principal points, and the image plane? A clear explanation of what “focused at infinity” means in this context would help.
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
5y ago
2 Answers
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"because if the focal point, the point as I understand it to be where all the light converges, was on the film plane, an image wouldn't be rendered, it would just be an indistinguishable point of light."
This understanding is incorrect... at all points on an objective lens exists all of the light required to form an mage (a portion of the total). That is why you can have a 200mm f/4 lens (50mm objective element) and a 200mm f/2 lens (100mm objective element). It is more accurate to understand objective lens area (aperture, f#) as being "stacking images."
The point at where all light converges is then where all source points from all areas of the objective element converge as a single point at the image plane. i.e. a point source in the scene converges as a point on the sensor.
This is a related diagram I made regarding DoField/DoFocus, but it shows the concept. The narrow aperture light/paths also exist w/in the wide aperture image; I just didn't include them for clarity/simplicity. Only the blue source is in true focus; and the narrow aperture image is darker (grey) because there are fewer images (light paths) focused/stacked/combined at the image plane.
Originally by user70370. Source · Licensed CC BY-SA 4.0
user70370
5y ago
0
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The key misunderstanding is that a lens does not send all light from the whole scene to one point. Each point in the scene forms its own point on the image plane. The sensor records the full image because different scene points focus to different locations.
When a lens is focused at infinity, incoming rays from a very distant subject point are effectively parallel, and the lens brings that bundle to focus at the image plane. For a simple lens in air, that image plane is one focal length behind the principal plane (often approximated in photography by the rear nodal point). So the sensor being at the focal plane is exactly what gives a sharp image of distant subjects.
The “focal point” in this context means where rays from one object point at infinity converge, not where the entire scene collapses. A ray diagram usually traces one object point at a time, which can make it look like all light meets at one spot.
So the statements are not contradictory: at infinity focus, the image plane is at the focal distance from the lens’s principal/nodal reference, and that is where the image forms.
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