Why not use Sensors without Optics?! • 1 Pinhole: Rays of light come straight The Art, Science and Algorithms from each point of an object ! of Photography! – no distortion.! – straight lines are still Lenses & Depth of Field (DOF)! straight! Optics I! – infinite DOF! CSCI 4900/6900! Maria Hybinette! • Larger Pinhole! – Fuzzy image, due to geometric blur! 1! !"#$%%&&&'#()*+*),#!-./*.'0(1%10%23/4)5*6789'!:;,2! <

Larger Pinhole! Smaller Pinhole!

• Diffraction limit, smaller apertures means more diffraction ! • Small hole does not create a bright dot but a diffused circular disk, called an Airy’s disc, surrounded by concentric circular rings !

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Pinhole Size Summary! Replacing pinhole with a lens!

• Large pinhole gives geometric blur (2mm)! • Optimal pinhole gives little light (0.35mm)! – Maximum sharpness, aperture is proportional to its distance from the image plane.! • Small pinhole gives diffraction blur (0.07mm)!

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• 1 Pinhole: Rays of light come straight from each point of an • Parallel rays converge to a point located at object ! focal length f from lens! – no distortion, ! – straight lines are still straight! – infinite DOF! • Rays going though center of lens are not •! Larger Pinhole fuzziness! deviated – so points are shown at the same • Lens: Need to collect rays emanating from a ‘near’ point in the scene through different pinholes, so that they so converge at a point at the perspective! sensor.!

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Gauss’ ray tracing construction! Same Lens: Changing Focus Distance!

• Focus distance (from f f camera to the object in scene). !! – To focus on objects at different focus distances, move the sensor relative to the • Rays coming from points on a plane parallel to lens.! Scene Inside Camera the lens are focused on another plane parallel to the lens!

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Depth of field! Depth of field (Depth of focus*)! • Range of distance of subject that is sharp! • Range of distance of subject that is – Shallow / deep ! sharp! – Short / long! Control parameters:! – Small / large! • – Focal length (f)! • Control parameters:! – Focal length*! – Camera to subject distance (s)! – Camera to subject distance! – Aperture (f-number) (a)! – Aperture (f-number)! – Format size (as related to circle of – Format size (circle of confusion)”! confusion, C)!

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!"#$%%11! 65'&/Q/#64/)'0(1%&/Q/%R6#:!70?7S6,4< 12! !"#$%%65'&/Q/#64/)'0(1%&/Q/%R6#:!70?7S6,4< Focal Length! Same focus distance: Focal Length!

• A measure on how • Tree is in focus at the lens focal length when it is strongly of how strongly a placed ‘infinitely’ away.! lens focuses or converges • Weaker lenses (lower optical power) have longer focal light.! lengths (assuming the lens is • Categories (35 mm M6).3(6465mm)! N051

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Same focus distance: Focal Length! FOV and Focal Length!

• Weaker lenses (lower • Wide Angle – decreases size, long DOF! optical power) have longer focal lengths • Telephone – magnified, narrow, short DOF! (assuming the lens is built with one lens)! • To stay in focus needs to move sensor further back! • If sensor size is constant, the field of FOV = 2 arctan( h / 2 f )! view becomes smaller!

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Focal Length & DOF! Focal Length & Field of View!

• Same distance same setting.! • Wide Angle – decreases size, long DOF! • Telephone – magnified, narrow, short DOF!

500mm 1000mm FOV is measured diagonally on a 35mm

17! 18N66Hyperfocal Distance!

• Larger distance the longer DOF! • Point of focus everything from half that • Smaller, shorter DOF! distance (a set distance) to infinity is in focus! • Where the focus occurs with relation to the – Aperture size (f-stop)! hyper focal distance! – Focal Length of lens!

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Changing focal length versus Summary: Hyper Focal Distance! changing the view point!

• Largest possible DOF for a given f-number! – Half the hyperfocal distance to infinite and and !"#$%%&&&'-03:3A6'*0;%&):*!\O]/OJPE^/--2.< beyond!!

• Moving back while changing focal length lets you keep objects at one depth the same size! • In cinematography, this is called the dolly zoom, or “vertigo effect”, after Alfred Hitchock’s movie!

21! 22! • http://www.kevinwilley.com/GIF!

Focal length on portraits! Summary!

• Standard portrait lens (85 mm)! • Pinhole cameras compute correct linear perspective! – But too dark! – Diffraction limited! • Lenses gather more light ! – But only one plane of scene in focus! – Focus by moving the sensor or lens! • Focal length determines field of view! – From wide angle to telephoto! – Depends on sensor size (shortly)! Wide Angle! Normal! Telephoto!

23! 24! Changing focal length versus changing the view point! Changing the sensor size!

• If the sensor is smaller, the field of view is smaller too!

E/46<)51,6W< • Smaller sensors .:0(-<:6,,/51< either have fewer • Changing the focal length let us move back from the pixels, or noisier subject, while maintaining the size of the image! pixels (closer to • But moving back changes perspective relationships! together)!

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Slide Credits/Resources!

• Prof. Fredo Durand & Prof. Marc Levoy! • Videos: http://snodart.com/tutorials.php! • Depth of field, Focal Length:! – http://en.wikipedia.org/wiki/Depth_of_field! – http://www.cambridgeincolour.com/tutorials/depth-of-field.htm! – http://en.wikipedia.org/wiki/Focal_length! – http://www.kevinwilley.com/l3_topic02.htm! • London, Stone, Upton “Photography” Book! • Applets (next week):! – http://www-graphics.stanford.edu/courses/cs178-10/applets/#lens! – http://www-graphics.stanford.edu/courses/cs178-10/applets/zoom.html! • Various:! – http://www.stsite.com/camera/cam02.php! – http://super.nova.org/DPR/LensPortrait/! – http://super.nova.org/DPR/! – http://www.cambridgeincolour.com/tutorials/camera-lenses.htm!

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