View Full Version : Figuring out Camera Film Back.

03 March 2012, 06:14 PM
Hi guys,

Very new to camera tracking - and am trying to track my first proper shot after watching the escape studios videos about using PF Track.

I currently have a legria hf r26 camcoder.

It has a 1/4.85 CMOS however I cannot find any information abou the width and height of this cmos when i use it at 16:9.

Have been looking here and at a wikipedia link however I can only guestimate and I do not believe it is correct.

I have a couple of questions, first of all can anyone inform me of the film back size, and also does aspect ratio effect it or do I not need to take that into consideration?

Also - is this really important like I believe it is - or does the HD 1080p preset 0.58 inch x 0.32 inch suffice for most things?

Thanks for your time.

03 March 2012, 07:58 AM

Finding out the film back is pretty difficult, I can only guess this as well. However it may be best to go with the preset and see what it gives you. Or even your closest estimate.

The film back is pretty important, if its wrong then the field of view can be wrong which can causes a whole lot of annoyances with your matchmove.

About your aspect ratio, I don't know about anyone else but I've found if the footage is in 16:9, if you input just the width of the film back then PFtrack will fill in the height. It always want to be in the right aspect ratio and changing from 4:3 to 16:9 only effects the height of the film back. At least that's what I've figured

03 March 2012, 04:31 PM
The filmback size is only handy if you want the listed focal length on your 3D camera to match the focal length on the lens you were using to shoot the footage. As long as the aspect ratio of the filmback is the same as your footage then it doesn't need to be accurate. The field-of-view is the important setting you need and most matchmoving applications now days will solve the field-of-view reasonably well.

If your still really keen to figure out the filmback size it is possible, but it requires a bit of maths and python in order to work it out...

03 March 2012, 09:18 AM
Thanks both for your responses, im letting the film back at the setting for HD 1080p at the moment (0.58" & 0.32") and seems to be giving an efficent solution.

Another question I would like to ask, just to make sure im getting this right, is using the estimate focal node in pf track 2011, it gives me a focal length of 21.007 and fov of 38 & 22.

Now, looking at the camera specifications, It states the focal length as 3.0 - 60.00 mm. equivalent to a 35mm focal length of 41.2-824mm.

Is the value of 21 as a focal length possible or should it be in the range of 41.2-824mm?

Thanks again for your help, really apperciate it!

03 March 2012, 10:56 AM
I'd go with the 21mm, I was taught that even though its equivalent to a 35mm you should stick with the camera's focal length. Don't know why its just something i was told and hasn't caused any problems yet

03 March 2012, 03:06 PM
It should only be in the range of 41.2-824mm if you've set the filmback up as a 35mm filmback ( (36mm x 24mm). Otherwise use the 21mm, when it comes to solving the 3D camera most matchmoving applications are able to give a reasonable estimate of the FOV (which combined with the filmback size you've used will give you a focal length).

A 50mm lens is a 50mm lens, no matter what type of camera you put it on. When people refer to 35mm equivalent, all they're referring to is that it gives the equivalent FOV on a 35mm still camera. The lens itself will still give the same projection - but depending on the size of the film negative or digital sensor - will result in part of that image getting cropped (or vignetting if the image projection from the lens is smaller than the sensor).

04 April 2012, 10:33 AM
Thanks for the information.
and thanks for the link, earlyworm

04 April 2012, 03:22 PM
Hey, just wanted to say thanks for the help with information, really has helped and have solved afew shots that I have intergrated some basic cg into, thanks again!

Have one more question though if possible, managed to get a canon 1100d.
Which is listed as having a having a film back size of
IMAGE SENSOR: 22.2 x 14.7mm CMOS
It only shots in 720p (1280 x 720)
However when trying to input the correct film back into PF track with a pixel aspect ratio of 1.0,
It seems to auto scale the width to be incorrect, so after inputting 14.7 into height it puts the width to 26.133 - 26.133 x 14.7

Where am I going wrong here? Thanks again for your help guys.

04 April 2012, 04:52 AM
just use the "22.2" and the "14.7" will be reduced (ignore the 14.7) , because it's 16:9 not 4:3
I've known that a few days ago. Hope that's correct :p

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