Motion Tracking AVCHD footage

Artistic Support Compositing and Post Processing
1

I am using the Sony HDR-CX160 camera to test blender’s camera tracker. I am filming in 1080i/30fps. Interlace is all over the place. When I take photos, the exif data gives me focal length for each shot, whether I am zoom full out or full in.

What I want to know is how do I get the video’s focal length information. I use a formula to convert the sensor size to millimeters "(sensor mm width = 25.4 * manufactures sensor in fractions). In simpler terms: .25 * 25.4 = 6.35mm.

Here is the HDR-CX160 camera specs

The sensor size is 1/4" (4.5mm)
Focal length: 2.1mm - 63mm

Focal Length (35mm equivalent) : Movie Mode: 29.8mm - 894mm (16:9), 36.5mm - 1095mm (4:3); Photo Mode: 32.5mm - 975mm (16:9), 29.8mm - 894mm (4:3)

Aperture: f1.8- 3.4

Which focal length do I use with blenders tracker data when I am zoomed all the way out? Should it be 2.1 or 29.8?

2

I use a formula to convert the sensor size to millimeters “(sensor mm width = 25.4 * manufactures sensor in fractions). In simpler terms: .25 * 25.4 = 6.35mm.
This is not correct. When it says 1/4” this does not mean the sensor is 1/4 inch (6.3mm). Use the table linked in the post above

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big thanks

4

I got this formula from a website that showed you howto convert sensor width in fractions to mm. It says to convert 1/4 to a decimal(.25) and multiply by 25.4 which gave me 6.35. Since my camera doesn’t use the full sensor in movie mode, I guess 4.5mm is what it is using. The above response say it would be better to use the 35mm equivalent which I will try.

If you have a formula that I can use or refer to, please advise. I would still need to know how to get the other focal lengths when I use the zoom lens between 29.4mm to 898mm. They are no numeric display on the camera when zooming.

thanks

5

It says to convert 1/4 to a decimal(.25) and multiply by 25.4 which gave me 6.35
1 inch = 25.4mm therefore your equation assumes 1/4" is 1/4 (0.25) inches (0.25 x 25.4) which is incorrect for sensor width.

6

Now I see what I was doing wrong. Camera sensors is of a different scale unit. Got it.
Thanks

7

The problem here is not that camera sensors are a different scale unit, it’s that sensor sizes are determined by a very archaic convention that can be very misleading when you try to use your listed specifications in the camera tracker…

Lets use your camera as an example:
sensor size 1/4" -> which is indeed 6.35mm… but that is a diagonal measurement and a bogus one at that.

http://www.dpreview.com/learn/?/Glossary/Camera_System/sensor_sizes_01.htm shows us that a 1/4" sensor type will typically have an actual sensor diagonal that is ~2/3 of the size of the sensor type and that would be ~4.23mm (varies by manufacturer…yours says it is 4.5mm so we will use that)

But you need the width and not the diagonal measurement. assuming that the sensor is a 4x3 aspect ratio (most, if not all, are) some simple geometry will give you a width of 3.6mm

Finally, since the lens has an actual fl of 2.1mm, we can do a little fudging in blender to find that your effective sensor width is probably only ~2.53mm when shooting a movie in 16:9 mode

sw 2.53 at fl 2.1 will solve the same as sw 36 at fl 29.8 so all in all it’s just easier to use the 35mm equivalent in the first place… :wink:

sensorwidthdiagram.png582×720 24.6 KB

8

Dude you are righteous. I see why now my solves were just plain wrong. Thanks for the blender illustration and the web link.