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Lecture 15: Cameras and Lenses (10)
yirongzhen

I thought focal length is a property of the lens. Then, to change the focal length, we need to change lens?

ellenluo

Focal length isn't always fixed for a lens! Zoom lenses allow a photographer to change the focal length without changing out the lens itself.

julialuo

If the focal length needs to be much smaller for a smaller sensor size (e.g. phone) to get the same FOV, does this affect the quality of the image? I'm guessing that larger sensors cost more money so there should be some sort of trade off with image quality?

zehric

It seems like from the later slides, and just intuitively, that smaller sensors do result in lower quality images. I think this intuitively makes sense because you capture less light onto a smaller sensor if the density of samples on the sensor is the same and thus will have a smaller number of pixels.

wangcynthia

Are we defining focal length as the distance between the lens sand the focal point (where all the rays converge) when the lens is focused at infinity?

mishywangiepie

@wangcynthia I believe focal length is the distance between the lens and the sensor, not where the rays are focused at.

go-lauren

To summarize what is said in lecture, FOV is affected by both the size of the sensor and the focal length. FOV is proportional to the size of the sensor, which makes sense intuitively because a bigger sensor should capture a larger portion of the scene, and inversely proportional to focal length.

Michael-hsiu

Is increasing the field of view equivalent to reducing the sampling rate on the sensor to get more of the scene onto the sensor? I wonder if there are differences in processing speed when a camera is taken depending on the FOV.

mnicoletti15

Confused about focal length does, could anyone help me out?

bbtong

@mnicoletti15, could you describe further? At a high level, focal length (f) is the distance between a (usually glass) camera lens and a sensor. In DSLR cameras, when you see those massive, long-range telephoto lenses, they have a very narrow FOV but a very long zoom range as the result of having a long focal length, which contributes to the huge size of the lens. Meanwhile, on smartphones, the sensor and lens are basically sandwiched on top of each other. They have to use a series of lenses with very, very small (sub-mm) size focal lengths. This results in a wide FoV due to a small focal length, but it is adjustable by the many lenses.

edricx

@mnicoletti15 Check out this lens focal length simulator from Nikon: https://imaging.nikon.com/lineup/lens/simulator/. At a basic level focal length just determines how "zoomed in" a photo will be (bigger focal length in mm = more zoomed in). Focal length in mm does not directly equate to field of view in degrees, because the size of the sensor (the h variable) also comes into play when determining the field of view. It is correct to say that field of view in degrees is inversely proportional to the focal length in mm. You can change the sensor size on the Nikon lens simulator by switching between the "FX format" and "DX format" buttons next to "Select Body". Nikon's DX format cameras have a smaller sensor (smaller h), so if you keep the focal length the same, you'll see that the photo becomes more zoomed in when you switch from FX to DX.

kavimehta

Focal length is the distance from the lens that the rays converge (i.e. when the rays come into focus)

Staffxiaxiling

Just to clarify since there seems to be some conflicting comments about focal length, it is the distance from the lens of the point where any parallel ray, aka one that is perpendicular to the lens, will converge.

It is not necessary also the distance from the sensor to the lens as described by this diagram.

mkeshavarzi

"Put a sensor at the focal length, behind, or in front?" Some good explenation in this article: https://physics.stackexchange.com/questions/51522/put-a-sensor-at-the-focal-length-behind-or-in-front

xiaoyankang

"In practice, the vertex of this triangle is rarely located at the mechanical front of the lens, from which working distance is measured, and should only be used as an approximation unless the entrance pupil location is known." https://www.edmundoptics.com/resources/application-notes/imaging/understanding-focal-length-and-field-of-view/

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