Filmback Width & Height - how to calculate?

#1

Hello guys, I just bought camera Canon Vixia HF S20 to learn Matchmoving. I so like this camera.

Yesterday I’ve shot test footage (1920x1080x25fps Squared pixel) and imported that footage into Boujou and it asking me about FilmBack Width and Height.

I have no idea what is Filmback and where I can see it. Can u please help me?
From specifications it says:
Lens: f=6.4-64 mm, F/1.8-3.0
35 mm equivalent.
Movies: 43.5 -435mm
Photos: (L, M, S): 39.9-399mm
Photos: (LW,MW): 43.5-435mm

On camera lens I see: 6.4-64mm, 1:1.8
58mm filter diameter.

I need to know exactly how much filmback on this lens.

When I import footage to Boujou I see default settings:

Can u guys help me to calculate filmback?

Thank you in advance CGsociety :thumbsup:

#2

I am not an expert at Boujou, I mainly use PFtrack, but the film back is the size of your sensor. If you don’t know what it is for sure don’t worry about it too much. Film back is only important when coupled with your lens focal length and then it is only used to determine your angle of view. So you have two problems here, you don’t know exactly what your lens focal length is unless you are zoomed all the way in or out, and you don’t know exactly what your film back (sensor size) is for certain. The two problems kinda cancel each other out.

Maybe someone with more Boujou experience can help here, in PFtrack if you don’t know what to put there you should just leave it alone and the solver will figure out a good angle of view and focal length most of the time. This information can help a solution, but misinformation would do more harm than good.

#3

Hi,

First of all if you want to dive into this I recommend reading up on some matchmoving principles, it will make things easier to understand. There;s a great book by Tim Dobbert that will go into all specifics.
But yes, focal lenght and filmback are interrelated and will get you the best result when starting to solve a scene.

from your post I see this:
Lens: f=6.4-64 mm, F/1.8-3.0
35 mm equivalent.

So this camera has a 6.4-64mm lens, compared to a 35mm film camera. The only thing you need to know now on what setting you made the recording. If you don’t know, set it to fixed unknown in Boujou and let the software figure it out. Unless you used the zoom :wink:

For the filmback try to find the CCD chipsize in the manual, most of the time it’s something like a 16:9 1/3" CCD and set in mm or inches. Just convert the values if neccessary and fill in the numbers.
Be sure to set the pixel aspect ratio to 1 as well.

Leave all the rest alone for now, and try to see if you can get a proper solve. Test the solve with some dummy objects and export it into a 3D app. From there, see if still fits. If not, it could be the solve or the location of the 3D objects not completely lining up.

If all fails, just set it to HD and a pixel aspect ratio of 1, and let Boujou figure something out for you. But the more information you can give the solver, the better the endresult will be.

rob

#4

It is possible to calculate the size of the filmback (sensor size is a more accurate term for digital cameras) using some of the information you have - the fact that you know the equivalent focal length for a 35mm camera is a big help.

http://www.earlyworm.org/2008/08/filmbacks.html

The 1/3 inch thing is a left over from the days when cameras weren’t digital - please ignore it for matchmoving purposes.

http://provideocoalition.com/index.php/sony/story/sony_publishes_guide_to_sensor_sizes/

Here’s how I’d figure it out…

First up in Maya (you can also do this with regular python or even with google calculator) open up the script editor and create a python tab. Enter the following bit of code…

import math as m

def fov(focal, back):


	fov = (0.5 * back) / (focal)
	fov = 2 * m.atan(fov)
	fov = 57.2957795 * fov

	return fov

Highlight that bit of code and press enter on the numpad to execute it. Now below that piece of code enter…

fov (43.5, 36)

The 43.5 is the equivliant focal length of your camera, the 36 is the width (in mm) of a 35mm still camera. Highlight that bit of code and hit enter (again on the numpad). The result it returns is the FOV of a lens on a particular size of filmback.

# Result: 44.9588687839 #

So we now know that the FOV of your camera is 44.95 degrees. Cool - now we can work out how big the sensor is. To do that - enter the following code into the script editor…

import math as m

def back(fov, focal):

	back = fov / 57.2957795
	back = m.tan(back / 2)
	back = focal * (back * 2)

	return back

Again highlight and hit enter. Now type in…


back (44.9588687839, 6.4)

The 44.95 is the FOV we got from the above calculation, the 6.4 is the actual focal length of the lens (not the equivalant).

The result it returns is the width of the sensor in mm’s.

# Result: 5.29539148438 #

Take that result divide it by 16 and then times that result by 9 to get the height of the sensor…

5.29 / 16 * 9 = height of sensor.

Keep in mind this calculation doesn’t take into account lens distortion - so it might be slightly off - you can account for lens distortion by using trignometry and shooting a grid and measuring the FOV of the camera. But it’ll take a while to explain.

I hope that helps.

#5

Hi Will,

Thanks for that reply and the links. Very useful information. Nice blog too btw :wink:

I agree on the lens distortion thingy, this is -besides the solving bit- also important for the compositing part. That’s why I recommended the book by Tim Dobbert as it covers all the basics and more.

Also the training at Gnomon, c m i v f x (cannot type or it becomes ******) or FXHPD is a very good way to get into the matchmoving thing quickly.

rob

#6

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