It doesn't matter what sensor the lens is designed for, it's focal length remains the same. It is what it is.
What are you basing 27mm on? A crop factor of 1.5? Relative to what? A 35mm still camera? 35mm stills equivalent focal length descriptions should have no place in a discussion about tracking footage from a movie camera, they only exist as a crutch for stills photographers who grew up using their focal lengths as working slang to describe a particular field of view. On 29/05/2012, at 6:23 PM, Howard Jones <[email protected]> wrote: I'd agree with Randy here. If the 18mm prime is not designed for the film back then it would have an equivalent focal length of, in your example of 27mm (though that is just based on your example. ) However if it is a prime lens designed for the chip, then it is an 18mm lens, so no conversion necessary. However again, the solve you would get should work either way, but the relation between the point cloud and camera will vary and will begin to be an issue for objects that should be close to the camera. That is you can double a focal length and move an object twice as far away in nuke and it should look the same, except for objects very close, when distortion artefacts creep in, and I don't mean lens distortion characteristics here before any one jumps in, I mean if you stick a sphere right next to the lens and have a super wide angle, that isn't the same as an object further away with say a standard lens size. (eg 50mm for 35) So you can track and try 2 solves with 2 different lens lengths, and if there are objects close to camera you should see a difference in shape of any test spheres etc, otherwise it should still lock. I couldn't find anything quickly online to confirm your film back sizes, but someone has created a whole set of presets for cameras (Nathan?) which are very useful on nukepedia. H ------------------------------ * From: * [email protected] <[email protected]>; * To: * Nuke user discussion <[email protected]>; * Subject: * Re: [Nuke-users] Arri Alexa sensor size - camera tracking question * Sent: * Tue, May 29, 2012 7:21:12 AM He is right. T-Mobile. America’s First Nationwide 4G Network ----- Reply message ----- From: "Darren Coombes" <[email protected]> To: "Nuke user discussion" <[email protected]> Subject: [Nuke-users] Arri Alexa sensor size - camera tracking question Date: Tue, May 29, 2012 01:01 Thanks for the reply Alex, I'll have a look. We have someone here saying that the 18mm lens would not be an 18mm lens in our tracker settings, I just want to make sure what they are saying is right. Cheers. D. On 29/05/2012, at 4:57 PM, Alex Fry wrote: The "crop factor" is described by the sensor measurements you input. Your 18mm lens is still 18mm, no conversion is necessary, your field of view has changed, but not your focal length.. An 18mm lens has a different field of view if its mounted on micro4/3, DX, super35, 35 stills, 70mm or medium format, but it remains an 18mm lens. Have a play with Nuke's camera node, tweaking the focal length, haperture & vaperture knobs to get a feel for the relationship they have to each other. -Alex On 29/05/2012, at 4:27 PM, Darren Coombes <[email protected]<javascript:return>> wrote: Hi, we have some plates shot with the alexa on a 18mm prime lens. I Need to camera track this material. On the Arri website, it says the sensor size for the alexa is 23.76 x 13.37mm when using the ARRIRAW setting with 1.78:1 aspect ratio. in the camera tracker settings, would I put the film back size as 23.76 x 13.37mm? And then I guess i need to find out the crop factor for that sensor size and use that when entering the focal length of the lens? So if we shot on a 18mm lens, and theoretically the crop factor was 1.5 (not sure what the actual crop factor is yet), you would times 18 x1.5? and that would give the right focal length? Can someone point me in the right direction with this? and is my theory of this right? or am i missing something? Thanks. Darren. * * * * * * *Darren Coombes* * * *Compositor* *www.resolutiondesign.com.au* <http://www.resolutiondesign.com.au/> *resolution: sydney australia* tel: *+61 2 9360 9908* fax: +*61 2 9360 8908* *22* burton st: east sydney: *2010* post office box: *614* darlinghurst: 1300* *abn: *3305 196 1856* *2010 Silver One Show Entertainment Award: Online Branded Entertainment * *2009 Gold Promax/BDA Award: Best Broadcast Design Image Spot * *2009 Winner Creative Review Hotshop Award: Post Production / VFX* *2009 Finalist Creative Review Hotshop Award: Broadcast Design* The information transmitted, together with the attachments, is intended only for the person or entity to which it is addressed and may contain confidential and/or priviliged material which may be subject to copyright. Any review, retransmission or other use of, or taking of any action in reliance upon, this information by persons or entities other than the intended recipient is prohibited. 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No warranty is given by the sender that any attachments to this email are free from viruses or other defects. <Unknown.jpeg> _______________________________________________ Nuke-users mailing list [email protected] <javascript:return>, http://forums.thefoundry.co.uk/ http://support.thefoundry.co.uk/cgi-bin/mailman/listinfo/nuke-users _______________________________________________ Nuke-users mailing list [email protected] <javascript:return>, http://forums.thefoundry.co.uk/ http://support.thefoundry.co.uk/cgi-bin/mailman/listinfo/nuke-users *Darren Coombes* *Compositor* *www.resolutiondesign.com.au* <http://www.resolutiondesign.com.au/> *resolution: sydney australia* tel: *+61 2 9360 9908* fax: +*61 2 9360 8908* *22* burton st: east sydney: *2010* post office box: *614* darlinghurst: 1300* *abn: *3305 196 1856* *2010 Silver One Show Entertainment Award: Online Branded Entertainment * *2009 Gold Promax/BDA Award: Best Broadcast Design Image Spot * *2009 Winner Creative Review Hotshop Award: Post Production / VFX* *2009 Finalist Creative Review Hotshop Award: Broadcast Design* The information transmitted, together with the attachments, is intended only for the person or entity to which it is addressed and may contain confidential and/or priviliged material which may be subject to copyright. 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