I'm not familiar with Nikon lenses, but Canon ones even if designed for APS-C sensors are still named disregarding any compensation for crop factors. Personally I never heard of lenses that state their focal lengths compensated by a certain crop factor since that tends to vary from camera to camera and it's not a characteristic of the lens it self.
Canon EF-S lenses are a good example of this, they are designed to only mount on EF mounts of bodies with APS-C sensors (1.6x crop factor) but their focal lengths are still stated "1:1" taking a full frame if you will as base, so you still have to multiply their focal lengths by 1.6. My EF-S 10-22mm for instance when mounted on my 7D is becomes a 16-35mm and not the 10-22mm written on the lens. On Tue, May 29, 2012 at 5:28 PM, Howard Jones <[email protected]>wrote: > The crop factor was based on the original question. Not my maths. A 100 mm > lens on cropped back will give an equivalent lens angle (not actual) of a > longer lens, with the original question this was 1.5. > > You are both agreeing with by stating the film back size gives a different > fov (but not focal length - agreed) > > However, > 1. If you can't get a 100mm lens that is designed for a crop film back, ie > compensates for the sensor size, then you are correct that in this case the > focal length is the same, regardless of sensor size, however the fov is > different. > > However if you can buy a lens that has compensated for the sensor size and > gives the same fov as a 35mm lens then there is a discrepancy. > > I'm under the (mis)understanding that the latter is available, however I > can quite believe I'm mistaken. > > 2. We are all assuming nuke behaves correctly in this fashion. In the back > of my mind, if you put the focal length into the camera tracker and then > change the film back, it changes the focal length, but you can then change > it back. > > I can't check this last point until tomorrow, but if it's the case then > what does this mean for the solve? If its not the case then it's because > I'm getting on and the mind plays tricks. > > Cheers > H > > ------------------------------ > * From: * Alex Fry <[email protected]>; > * To: * Nuke user discussion <[email protected]>; > * Subject: * Re: [Nuke-users] Arri Alexa sensor size - camera tracking > question > * Sent: * Tue, May 29, 2012 8:49:44 AM > > 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]> > 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], http://forums.thefoundry.co.uk/ > http://support.thefoundry.co.uk/cgi-bin/mailman/listinfo/nuke-users > > _______________________________________________ > Nuke-users mailing list > [email protected], 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. > 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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