Film back and focal length

D

devilry

Member
I'm doing some visual effects shots with the dvx. And its crucial to find out what the exact film back and focal length is of the camera. I'm not using the zoom functions or anything, i'm just using the normal/widest angle. How can i find out what the exact film back and focal length is on the camera?

thanks.
 
No one knows? I'd really appreciate some info, i can't find any exact information on it through google. I just got my new camera today and going to be doing some visual effects tests tomorrow and i need to know the exact focal length and film back, so the vfx will look perfect.
 
The focal length that is expressed on the lens barrel is accurate. The location of the focal plane is about the back of the hot shoe (should be close enough for nearly any application).
 
A filmback is the size of the CCD's on the camera. From what i found, the size of a 1/3" ccd would be 4.8mm x 3.6mm. That sound about right?

I think i found out what the focal length was for the camera as well, i was using a 4.5mm focal length.
 
I wasnt sure, i got confused with some of the information. Like on the manual it says DVX is 4.5 to 45mm. 35mm equivalent = 32.5 to 325mm. So i wasnt sure if i should put in 4.5 or 32.5. But i understand it all now.
 
I had another question actually about the film back, the size of the CCD's. When you shoot in widescreen (squeeze mode), does the size of the ccd's change?
 
devilry said:
I wasnt sure, i got confused with some of the information. Like on the manual it says DVX is 4.5 to 45mm. 35mm equivalent = 32.5 to 325mm. So i wasnt sure if i should put in 4.5 or 32.5. But i understand it all now.
Click to expand...

You could use either—ether combo will give you the same angle of view, which is the only imporant measurement unless you're trying to simulate the depth of field characteristics of the lens.

devilry said:
I had another question actually about the film back, the size of the CCD's. When you shoot in widescreen (squeeze mode), does the size of the ccd's change?
Click to expand...

No, you just lop off the top and bottom. Since most aspect ratios leave horizontal alone and vary in vertical, it's best to express lens AOV as a horizontal measurement, i.e. hAOV.

-Stu
 
Thanks for the info prolost, it helped out. Reason i was wondering about the size of the ccd's is that i need to know EXACTLY what the size will be if im shooting in squeeze mode. Because the visual effects program im using needs to know exactly what the size of the filmback is in order to properly work with the visual effects. So, if there's no change, great, if there is change than i'll need to find out what that is exactly. Like you said the top and bottom get chopped off, wouldnt that change the size?
 
The vertical size, yes. Sorry, I answered your question literally (the CCD does not change size), but the filmback of course does—only in the vertical direction.

What software are you using?

-Stu
 
Well the software im using isnt really a vfx program, its actually a matchmoving program called Boujou. It takes the footage you shot and tries to 'solve' it and figure out where the camera is positioned in a virtual enviroment. So you can add in visual effects seamlessly. In order to do that, you need to start off with the correct information about the camera and how the footage was shot, to get an accurate 'solution'. Which is why i was asking what the exact film back and focal length was for the dvx-100a.

If the vertical direction does change, how can i figure out the exact size? I would like to do some tests and see which film back size would be the most accurate.
 
I figured it was something like Boujou or Syntheyes. These programs can guess the lens info in a pinch. It's better to give it a starting point from which to work, but not strictly necessary.

But if you feed it your working resolution and pixel aspect ratio, you should then only need to provide one dimension for the filmback, which should be the horizontal.

You may want to take a look at this book to learn more about matchmoving.

-Stu
 
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