# Forum

### Author Topic: Focal Length, principal point and radial lens distortion  (Read 2354 times)

#### samdesi

• Newbie
• Posts: 9
##### Focal Length, principal point and radial lens distortion
« on: April 07, 2020, 01:33:57 PM »
Hi,

I exported the camera calibration and exterior orientation to use in ERDAS. I need the values in mm and I know that from the xml (exported from Metashape) the focal length, principal point (x,y) are in pixels.
Looking at previous post I found the formula

F(mm) = F(pixels) * SensorWidth(mm) / ImageWidth (pixel)
My data are the following

Image dimensions:
W 4000
H 6000

sensor size 23.44
Focal length 18.41
Principal point
x 1997.39
y 2997.48

I need to find out:
The pixel size in microns and from the camera model website I gather that is 3.89 microns

the focal length in mm, doing the formula I obtain
18.41*23.44/4000=0.1078

I was wondering how do I calculate the principal point in mm from my initial values?
Second question on the principal pont. Where is the PP in reference to? is it referring to the centre of the image/pixel array?

I have checked the manual but I couldn't find the formulas for those.
Many thanks.

Samantha

#### samdesi

• Newbie
• Posts: 9
##### Re: Focal Length, principal point and radial lens distortion
« Reply #1 on: April 07, 2020, 02:43:15 PM »
Hi I found from the tool > camera calibration menu that the focal length in mm should be 18 mm. Therefore the last parameters I need are   Principal point in (mm)

I know the formula is PPmm= Principal_point (px) * sensor_pixel_size (mm/px)
but I am not sure how to calculate sensor pixel size in mm/px?
ps I include a screenshot of my parameters

Many thanks,
Samantha

#### JMR

• Sr. Member
• Posts: 465
##### Re: Focal Length, principal point and radial lens distortion
« Reply #2 on: April 07, 2020, 05:06:42 PM »
Quote
the focal length in mm, doing the formula I obtain
18.41*23.44/4000=0.1078
? it does not make sense, your focal length in mm can't be sub mm for a sensor this size

The camera calibration dialog gives you several formats when you try to save parameters. The USGS format for example, will give you values in mm for F, and principal point. as well as pixel size.

#### samdesi

• Newbie
• Posts: 9
##### Re: Focal Length, principal point and radial lens distortion
« Reply #3 on: April 07, 2020, 05:15:31 PM »
JMR,

Hi, thanks for your reply. Yes in fact I thought that calculation gave an impossible result (In my second post then I say that's 18mm).

I managed to see in the calibration in mm with export>cameras>infoproject formal. Thanks for the suggestion of using also USGB!