MITMAS_531F09_lec06_notes - MAS.963: Computational Camera...

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Unformatted text preview: MAS.963: Computational Camera and Photography Fall 2009 Class 6: Cameras for HCI Prof. Ramesh Raskar October 16, 2009 Scribe: Anonymous MIT student Class 6: Cameras for HCI Exam: Light Fields continued A light field can be represented in 4 dimensions: The sensor with x and y, and the Lens with and . Light fields are a complete representation of the the light rays captured by the lens. Therefore, parameters like focus, zoom and aperture size can be changed after the photo is taken. Light Field with Pinhole Mask Subject Lens Pinhole Mask Sensor Figure 1: Light Field Camera with Pinhole Mask (not to scale) Figure 1 shows an example of a light field camera with a pinhole mask in front of the sensor. If we look at the sensor in 1D and assume an total x-resolution of 900 pixels and a resolution of 9, the local x resolution is 100. The disadvantages are therefore a loss in resolution and a loss of light, as most of the light gets blocked by the mask. Therefore, while this model is very clean on a theoretical level, it is inecient to apply in the real world. How does the spacing of the mask from the sensor effect the captured light field regions? Figure 2 depicts the effects of the spacing between the sensor and the mask. When the mask is correctly spaced, the blobs on the sensor barely touch each other. When it is spaced too far away, artifacts appear due to overlapping blobs. If the mask is too close, empty space between the blobs results in wasted pixels. 2-1 Pinhole Mask Sensor a Pinhole Mask Sensor b Pinhole Mask Sensor c Figure 2: Effect of distance between mask and sensor: a) correct spacing between sensor and mask. b) mask is too close, sensor pixels are wasted. c) mask is too far away, blobs overlap. f-number The f-number is the ratio of the focal length over the lens diameter. A larger lens diameter results in a smaller f-number. Example: If the lens has a diameter of 25 mm and the focal length is 50 mm, the F-number is 2. If the lens diameter is 12.5 mm, the f-number is 4 When the f-number is decreasing by factor 2, the area is decreasing by factor 4, therefore only 1/4 of the light reaches the sensor. Going down by one f-stop means going down from 2 to 2.8, after that 4.2 (3*square(2)) and 5.6 (4*square(2)) same angle for same f-number: cone of light for each pixel We want a ratio of focal length to lens diameter, which is equal equal to the mask distance to blob-size. If that is matched, the blobs will barely touch each other, if not, there will be overlap or lost pixels (see 2). Pinhole Photography Problems Because of the pinhole, very little light reaches the sensor, therefore long exposure. Also, the image is blurred because of diffraction: single point in the world maps to a blurred spot on the sensor. Analogy to water hose: when the size of the opening in the water hose becomes comparable to the size of the water molecule, it will start to spray....
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This note was uploaded on 02/07/2012 for the course MAS. 131 taught by Professor Rameshraskar during the Fall '09 term at MIT.

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MITMAS_531F09_lec06_notes - MAS.963: Computational Camera...

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