On 5/29/12 at 10:28 PM, (Howard Jones) wrote:
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.
well, there might be a case that somebody labels the lens with a
"35mm full frame equivalent" (rather common with point-and shoot
cameras, which say something like 4-40mm, 35-350mm
equivalent)... but that would be not the proper focal length
optically speaking, and they should still list the real focal
length. (I've never seen anything like this for cameras with
interchangable lenses though)
but to clear things up, let's take the simple example of a canon
5D vs 7D (full 135mm frame size vs APS size):
if you buy a 50mm built for the 5D, it will give you a field of
view of a "normal lens" and cover the full sensor size.
if you put the same 50mm on the 7D, it is still a 50mm for all
optical calculations, and it will give you the field of view of
a 50mm on a 7D - which in comparison looks about the same as if
you would put a 75mm lens on the 5D (hence people who were used
to the 5D call it a 1.5 crop factor - but it's really just old
habits - we could just as well call it 0.6x crop factor if you
were used to a 6x6 hasseblad camera where you'd consider 80mm
normal). it's still a 50mm lens though.
if you buy a 50mm designed for the 7D, again it will give you
the same field of view as the other 50mm, and again it will look
like a slight telephoto lens. so if you use the 5D 50mm, or the
7D 50mm doesn't make any difference (for the field of view).
lastly if you put the 50mm designed for the 7D on the 5D, it
will still behave optically like a standard 50mm - ie it gives
you the same magnification per sensor area. however it wont
likely wont cover the full sensor size with light, meaning you
can't really use it for the full frame. so you would have to
crop an APS size piece out of the image if you don't want the
vignetting. one could argue that this cropping makes it an
narrower field of view, ie slight telephoto lens, but this it's
rather because you're not using the full sensor size and not a
change in the optical properties of the lens.
but even if you buy a 50mm lens for a 6x6cm hasselblad it will
give you the same field of view (and magnification) as the 5D
50mm and the 7D 50mm. so a 50mm is a 50mm is a 50mm ;)
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.
as far as i can see it doesn't. it changes field of view (as is
expected). and even if it did it would be rather due to
decisions in UI behavior rather then to the physics behind it.
hope that clears up a few things
++ chris
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