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I know how lenses work, and I know what a focal point is, but I don't get what Blender means by "focal length".

I can set the distance to the focal point of a camera in the depth of field panel, while changing the focal length in the lens panel changes my FOV, (larger focal length -> smaller FOV) but what quantity is this change based on? I need to set the FOV with great accuracy, based on distances calculated from the geometry in my scene, so I would like to know what the meaning is of the distance that I have to fill in in the lens panel.

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Focal length is the distance between the lens and the focal point:

enter image description here

At focal point the image is sharp and it's the place where the camera sensor is located (or camera film). The Field of View of each lens depends on the sensor size:

enter image description here

The conversion formula is this:

FOV = 2 * arctan(half_the_diagonal_of_sensor / focal_length)

Blender allows you to enter the "zoom" of camera in either Focal length (mm) or in Field of View (°):

enter image description here

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  • $\begingroup$ But then why do I still need to set a depth of field? I thought this was the distance to the focal point.. $\endgroup$ – Gnub Oct 13 '17 at 16:26
  • $\begingroup$ @Gnub Depth of Field is another thing. It's how fast sharpness diminishes on each side of the focused distance. It is driven by the size of aperture opening (how much light is allowed through). It is measured in F-stops or also in millimeters (in Blender). It is this iconic element of the camera lens assembly: d2rormqr1qwzpz.cloudfront.net/uploads/0/1761/… $\endgroup$ – Jaroslav Jerryno Novotny Oct 13 '17 at 16:35
  • $\begingroup$ @Gnub In camera lens assembly there are multiple lenses inside, and with such system the camera can focus over different distances while still maintaining the same field of view (zoom). This is simulated with the Depth of Field Focus distance in Blender. $\endgroup$ – Jaroslav Jerryno Novotny Oct 13 '17 at 16:46
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    $\begingroup$ To understand deph of field do a search for circles of confusion $\endgroup$ – cegaton Oct 13 '17 at 17:31
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It's the length of the lens group of the virtual camera, which will modify the perspective of the image: an orthographic view is the same as an infinite focal length, a classic telephoto is 200 mm, the eye's approximate view is 50 mm, a classic grandangle is 28 mm, under 16 mm we can call it "fish-eye", and so on ...

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