shooting mode, aspect ratio and focal length

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davetronic

Well-known member
are there any variances in focal length when switching a shot from 480 to 720?

here's the scenario: a lock-down with a fixed camera position shooting 480 in 4:3 then doing another shot at 720 (obviously 16:9) without being able to change the camera position. is matching the early shot's frame as easy as cropping the edges or does the focal length change slightly too?
 
Theoretically you will have a slightly shallower depth of field on the 16x9 clip since you are using more of the chip therby having a larger image plane. But the difference will be VERY slight and most likely not noticable. As for focal length, it seems to stay the same. Cropping the 16x9 yields the same frame line as shooting in 4x3. Why not just extract a 4x3 image from the 16x9 footage rather than changing the camera?

filmstox.
 
well, it was a 4 camera shoot (3 DVX's + the HVX) so we had initially set the HVX to match the DVX's. Then there was the slow-motion shot, which required the format change and change in aspect ratio... but you're right, we could have just framed the initial SD shot in squeeze mode and just cropped later to match the DVX - as long as there wouldn't be a change in focal length... i was told it did appear to change (i was not on that particular shot and haven't seen the footage), thus my 'theoretical' question.
 
I did a quick check before my last post, I framed up on a 4x3 reference and switched between tape mode (DV 4x3) and 720 with the 4x3 safety marker turned on, I did not notice any change in the field of view.

Hope that helps,
Filmstox
 
there would be no change of field of view - the same chip is used to acquire at the exact same resolution and then processed into the final resolution
 
davetronic said:
are there any variances in focal length when switching a shot from 480 to 720?
Click to expand...
No, focal length is a property of the lens, so regardless of what mode you put it in the focal length will stay the same. Which means the depth of field and perspective will all stay exactly the same.

here's the scenario: a lock-down with a fixed camera position shooting 480 in 4:3 then doing another shot at 720 (obviously 16:9) without being able to change the camera position. is matching the early shot's frame as easy as cropping the edges or does the focal length change slightly too?
Click to expand...
Okay, so you're asking about field of view -- yes there will be a change in the field of view, going from 4:3 to 16:9. The 4:3 is made by a center extraction, so you'd just chop the sides off the 16:9 to get an equivalent field of view.
 
filmstox said:
Theoretically you will have a slightly shallower depth of field on the 16x9 clip since you are using more of the chip therby having a larger image plane.
Click to expand...

Common misperception, but simply not true. The imaging size of the chip is irrelevant in the depth of field. What the imaging size affects is the field of view, but depth of field is solely a property of the lens.

With a wider imaging chip you have a wider field of view, so you normally would use a more telephoto lens in order to achieve a comparable framing. It is the more-telephoto nature of the lens that creates the shallower-DOF look, not the size of the imager.
 
I think some of this discussion is about the confusing notion of actual and effective focal lengths. By definition, the field of view of the lens is (roughly) the angle subtended at the diagonal of the format to the limits of the scene captured. This means that if the length of the diagonal of the format remains the same - you can have a hairline letterbox image shape or a round image - the angle of view remains the same. If the diagonal length on the format changes then the field of view changes and with it, an effective change in the focal length. This is what happens when you use a lens designed, say for 35mm, on a 1/3" chip as in the Canon XLs: the very small image diagonal gives a big increase in the effective focal length of the lens you mount, but of course the lens itself hasn't changed.

And, as Barry says, the change in effective focal length leads to a change in depth of field because dof is fundamentally decided by image magnification and aperture. But there's something else you may wish to keep in mind: depth of field in SD looks greater for the same image magnification and aperture than HD because the lower definition hides image blur more than the higher res of HD.
 
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