Diogo its used by amateurs as a crutch not professional photographers
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----- Reply message -----
From: "Diogo Girondi" <[email protected]>
To: "Nuke user discussion" <[email protected]>
Subject: [Nuke-users] Arri Alexa sensor size - camera tracking question
Date: Tue, May 29, 2012 13:47
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.
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----- 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
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Darren Coombes
Compositor
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. If you received this in error, please contact the sender and delete
the material from any computer or network. No warranty is given by the sender
that any attachments to this email are free from viruses or other defects.
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