camera aperture

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Hi all,

I can't figure out how to set the aperture in houdini. I've got footage captured with a 35mm film camera, telecined to pal D1 (720x576). The lens is 24mm.

I've read the tutorial called “real world camera matching” on odforce, but it seem to deal more with film resolution and not video rez. Jason Iversen had a comment on the page which stated “ And be aware that if you are working at video resolutions that you are doing the camera matching using footage that has not been optically scaled in telecine.” - So if it has not been optically scaled in tc, what should I put in the aperture..? Sorry if I missed something here, I've searched both sidefx and odforce forums but can't find anything that I can understand. Thanks in advance.

Peter
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wow thats a real tricky one.

I assume, even though it is 720x576 you would have to punch in the original film width and lens settings for correct perspective. However, i guess it would also have to be correctly scaled up so that you are working with that exact image in the background (and not using any black bars that are created when tc'd).

bit baffeling, i've often wondered what would happen in that kind of situation.

im sorry i cant offer any real advice, but i would be really interested in your findings of how to solve the issue.

J
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Hey, i also forgot to mention that ive had similar apeture headaches.

If u search for my old posts on this forum and odforce, ive had some really great advice from people over time. especially Mario Marengo who was really kind to take the time to explain things to me.

http://odforce.net/forum/index.php?showtopic=2749&hl= [odforce.net]

hope this serves to help somehow
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Hey JBG,

Thanks for your reply, I found your thread on odforce a few days ago, but didn't really help, sort of just confused my even more. I'm starting to feel real dumb, I've done this countless times in brand X & Y, and never had a problem getting a “pretty good” match. And for this job, it's all locked off plates! Should be a breeze!

I must be missing something, because it can't be this difficult. I know the lens, I know my resolution… but what the heck is my horiziontal aperture? The information in the manual (search for aperture) is not for feeble minded ppl like me, it has stuff like “fovx = 2*atan( ( apx / 2 ) / focal ) = 2*atan( ( cam1/aperture / 2 ) / cam1/focal ) = 2*atan( ( 41.4214 / 2 ) / 50 ) = 45˚” - huh?

I just need to roughly match my cam to the real cam on set, doesn't even have to be that precise.. I'll tweak it by eye later… but right now I can't proceed as not knowing what number to put into the aperture has stalled me as it has such a big influence in the overall setup.

Cheers,

Peter
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Hey Peter

Yeah, its bar far the worst bit, its a real headache.

The horizontal apeture should be the size of the film back or ccd. In your case this is 35mm film, i dont think this will change even if telecined - though i may be wrong, id be interested to know.

The thing is that the ‘apeture’ information given in manuals or online for camcorders and the like is quite often never clear whether this is in the x direction, or in the diagonal.

The formula is basically trigonometry, and if u look at that thread i gave u again from Marengo, he manages to break it down really clearly.

To try and get my head round it, and to have constant reference, i wrote up some notes on what he said to make it clear for myself. ill attach them for you and future queries.

one thing i didnt realise (as a beginner) was that you could literally tap in the formulas into houdini. So if u dont know the apeture in x, literally type in the formula to work it out, with the variables u do know (using attributes Houdini will understand, ie fovx for field of view in x direction)

If anyone notices anything wrong with it, might be worth letting me know.

Cheers

J

Could work out how to attach a pdf, so ive writted in out below:


UNDERSTANDING THE CAMERA/LENS RELATIONSHIP
- MADE EASY (AS POSSIBLE!!) -


Definitions:
FOVX = field of view in X direction example = 39.6(degrees)
FOVY = field of view in Y direction example = 26.9 (degrees)
APX = aperture in X example = 36 (mm)
APY = aperture in Y example = 24 (mm)
FOCAL = focal length example = 50mm
RESX = pixel resolution in X example = 3600 (pixels) (thus scanned at 100px per mm)
RESY = pixel resolution in Y example = 2400 (pixels)
ASP = pixel aspect example = 1

Three Key Relationships:
tan(FOVX/2) = (APX/2) / FOCAL tan(FOVY/2) = (APY/2) / FOCAL APX/APY = (RESX * ASP) / RESY

These formulas can be used inside Houdini, or manually solved to determine any camera parameter. If you only know a few parameters – such as resolution or focal length, then the rest, such as aperture and field of view can be determined in this way

So….
To find the FOVX = 2 * atan( (APX/2) / FOCAL ) APY or = (RESY*APX) / (RESX*ASP) FOVY or = 2*atan( (APY/2) / FOCAL )
Example (using above example values):
APX = 2*FOCAL*tan( FOVX / 2 )
= 2*50*tan( 39.6 / 2 )
= 36 (mm) = 2*atan(0.36)
= 39.6 (degrees)
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Hey JBG,
The horizontal apeture should be the size of the film back or ccd. In your case this is 35mm film, i dont think this will change even if telecined - though i may be wrong, id be interested to know.

This is what I tried first, but it was way off. The closest aperture for my shots, are around 18, which I kinda manually got by trying numbers for too many hours.

I just don't get it, maybe it's just me ops:

I'll email support for this, can't wait any longer, I've got 8 shots to setup and 3 days lost coz can't figure this out.

Thanks mate, appreciate your help.

Cheers,

Peter
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