Forum

Author Topic: Focal Length in Camera Calibration Parameters  (Read 6735 times)

• Newbie
• Posts: 3
Focal Length in Camera Calibration Parameters
« on: November 17, 2015, 12:36:42 AM »
Hello,

i'm trying to found focal length of my camera. But i can't see the focal length in the camera calibration parameters. I'm saved the calibration report in every format. (photomodeler, agisoft, calcam).  This is an action cam. (SJ4000). My model is referenced and this is an aerial photogrammetry job. My calibration reports at attachment. Thanks.

Paulo

• Hero Member
• Posts: 1105
Re: Focal Length in Camera Calibration Parameters
« Reply #1 on: November 17, 2015, 03:14:06 AM »
The calcam and Agisoft calibration give the focal length F or Fy , Fy in number of pixels in this case it gives F as 2321.97343 pixels. So actual value in mm will be given by F (no.pixel) * pixel size (mm).

To get the pixel size we must know the sensor size. Looking up th SJ4000 it seems it has a 2/3 inch sensor which equals 8.80 mm width by 6.60 mm height.

So pixel size is 8.80mm/4032 or 0.0021825 mm,

So F(mm) = 2321.97343 * 0.0021825 = 5.067707010975 mm

Note that this cam has fisheye lens and you should choose Fisheye as cam,era type in calibration and not frame....
Best Regards,
Paul Pelletier,
Surveyor

Paulo

• Hero Member
• Posts: 1105
Re: Focal Length in Camera Calibration Parameters
« Reply #2 on: November 17, 2015, 03:37:10 AM »
Sorry it seems after further research that the SJ4000 has an Aptina 1/3-Inch CMOS Digital Image Sensor
AR0330. So in this case the sensor size 1/3 Inch represents:

Type   Aspect Ratio   Dia. (mm)   Diagonal   Width   Height
1/3.6"   4:3   7.056   5.000   4.000   3.000
1/3.2"   4:3   7.938   5.680   4.536   3.416
1/3"   4:3   8.467   6.000   4.800   3.600
1/2.7"   4:3   9.407   6.721   5.371   4.035
1/2.5"   4:3   10.160   7.182   5.760   4.290
1/2.3"   4:3   11.044   7.70   6.16   4.62
Best Regards,
Paul Pelletier,
Surveyor

Paulo

• Hero Member
• Posts: 1105
Re: Focal Length in Camera Calibration Parameters
« Reply #3 on: November 17, 2015, 03:58:45 AM »
Continuing from last post:

sensor width is 4.8 mm for Cmos 1/3 Inch sensor, thus pixel size is 4.8 / 4032 = 0.0011905 mm

and Focal length (mm) is 2321.97343 * 0.0011905 = 2.764254 mm which is more like a fisheye
Best Regards,
Paul Pelletier,
Surveyor

• Newbie
• Posts: 3
Re: Focal Length in Camera Calibration Parameters
« Reply #4 on: November 17, 2015, 04:32:18 PM »
Firstly thanks for your reply. You said pixel size is 0.0011905 mm, but sensor's catalog (AR0330 Data Sheet)  says 2.2 micron x 2.2 micron, in mm is 0.0022. Which is true in your opinion? If true one is 0.0011905 mm my flying height must be 116.34 m else 62.95 m, in 5 cm GSD.

Paulo

• Hero Member
• Posts: 1105
Re: Focal Length in Camera Calibration Parameters
« Reply #5 on: November 17, 2015, 04:53:36 PM »
Yes, i did see this but this corresponds to a 2048(H) x 1536(V) (4:3, still
mode) size array according to same sheet. But your images have 4032 (H) by 3024 (V) explaining the smaller pixel size (almost half as resolution is almost doubled). It seems the original images of 3 MPixels are somehow sub sampled to arrive at 12 Mpixels.
Best Regards,
Paul Pelletier,
Surveyor