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I am going to set the blender camera as the real surveillance camera, which the focal lens and fov of real camera is 4mm and 69.9 degree.

Firstly, I manually set the focal len of blender to 4mm, the blender automatically set the fov to 140.53 in the perspective view, I changed the sensor width and height, it has the influence but still doesn't simulate the focal len and fov as the real camera I used.

Secondly I followed this post - How to simulate a real camera with blender, but in the panoramic view, I added a new camera, even the parameters (focal len, fov and sensor width height) are changed as shown the post, nothing happens.

I wondered why in the perspective view both focal len and fov are automatically changed? and why the change in the panoramic view doesn't change at all.

The blender version I am using is 2.7.9

This is the picture of focal len at 4mm below

enter image description here

This is the picture of focal len at 12mm below

enter image description here

Increasing the focal len will make the objects in far distance look bigger?

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    $\begingroup$ Forgot to set the sensor? If you set the Sensor Size to ~5.6 as well, FOV is ~69.984. $\endgroup$ – brockmann Aug 21 '17 at 9:38
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    $\begingroup$ Set the Focal Length first, then change the type to FOV, set your Sensor Size and you'll see that the FOV value changes accordingly. Understanding Focal Length and Field of View, TLDR: The focal length of a lens defines the lens’s angular field of view. What camera you are trying to emulate? Adding the type, manufacturer and lens of the camera might be useful. $\endgroup$ – brockmann Aug 21 '17 at 10:17
  • $\begingroup$ @brockmann Thanks, I found the sensor size on the camera specification is 1/2.8', so it is estimated 8.9mm to input to blender, the factory told me the choice of focal lens is 4mm, 8mm, 12mm, the higher the focal lens, the further you can see in the picture. However when setting focal lens parameter in the blender, the small focal lens I set, the further the image will show, completely different from the factory told me, why $\endgroup$ – user824624 Aug 21 '17 at 13:43
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    $\begingroup$ Typically 1/2.8” refers to a sensor size of about 0.19” x 0.14” (4.71mm x 3.54mm) $\endgroup$ – brockmann Aug 21 '17 at 14:57
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The field of view is determined largely by two factors: the lens length and the sensor size (we're going to leave focus distance out of this calculation to keep it simple). So changing the lens size will always change the field of view, unless the sensor size is changed accordingly to compensate.

Increasing the length on the lens will make for a smaller field of view (like a telephoto in a camera). A shorter lens would have the opposite effect, the field of view would be much wider (like in a wide angle lens). How much will depend on the sensor size.

In my opinion, the simplest answer for what you need would be that if you know the field of view you really don't need anything else. Just set it to 69.9 and don't worry about the sensor or lens size.

Knowing the sensor size from the manufacturer might not be the answer either, we don't know many things: how much of the sensor is actually used? Are the measurements of the sensor (and field of view) you are given for the width, height or diagonal. What is the aspect ratio of the generated image? Also there are many things in the real world that do not meet the specifications of the manufacturers (is the 4mm lens really 4mm?)

If you want to make a simulation (for depth of field reasons maybe) that uses a 4mm lens covering a field of vision of 69.9 degree, then move the sensor width until you get to the numbers you want (5.591mm)

enter image description here

Notice that the field of view you just set up is only good for the Camera view.

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