What is the area of a lens's image circle that falls outside the sensor or film called?
Asked 1/21/2017
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A camera lens projects a circular image, while the sensor or film is usually rectangular and only records part of that projection. Is there a standard term for the portion of the projected image that exists outside the capture area? I’ve seen "image circle" used for the whole projected circle, but I’m specifically asking about the part that does not land on the sensor/film.
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
9y ago
2 Answers
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The camera lens projects a circular image that is larger than the image sensor. Not all that circular image will yield a satisfactory image The part of the image circle that yields a satisfactory image is call the "circle of good definition". The diameter of the circle of good definition must be equal to or grater that the diagonal measure of the imagining sensor. The area outside the circle of good definition is called the "circle of illumination".
Excerpt - Textbook "Photographic Lenses" C.B. Neblette Morgan & Morgan Fountain Press, London 1965 - page 127 (image sensor substituted for the word "film".
Originally by user44949. Source · Licensed CC BY-SA 4.0
user44949
9y ago
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The standard term is usually image circle for the entire circular projection made by the lens. There isn’t a widely used separate term just for the part outside the sensor or film; people generally describe it as the unused portion of the image circle.
In older lens terminology, you may also see:
- circle of good definition: the part of the image circle that has acceptable image quality
- circle of illumination: the larger illuminated area, including regions outside the best-quality portion
For most photography discussions, image circle is the clearest and most common term. The sensor or film simply crops a rectangular area from that circle.
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