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Is this distortion and...if so...why?


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#1 Tim O'Connor

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Posted 15 November 2007 - 02:19 AM

This is a screen grab from a Final Cut Express timeline. The footage was dubbed in
camera from P2 card to Mini-DV tape and then captured into Final Cut Express. The shot
was obtained with an HVX-200 shooting 720P 24PN, Cine V with stock lens.

In this shot the frame of the archway in the hall seems curved but is not in
actuality. I haven't had this occur before. Does anybody know?


Thanks.

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#2 Saul Rodgar

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Posted 15 November 2007 - 03:34 AM

This is a screen grab from a Final Cut Express timeline. The footage was dubbed in
camera from P2 card to Mini-DV tape and then captured into Final Cut Express. The shot
was obtained with an HVX-200 shooting 720P 24PN, Cine V with stock lens.

In this shot the frame of the archway in the hall seems curved but is not in
actuality. I haven't had this occur before. Does anybody know?


Thanks.



Hmm. Barrel distortion. Your zoom was at its widest angle setting.

From wikipedia:

Barrel distortion, in which image magnification decreases with increasing distance from the optical axis. The apparent effect is that of an image which has been mapped around a sphere. Fisheye lenses, which take hemispherical views, produce this type of distortion as a result of a hemispherical scene being projected onto a flat surface.
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#3 Nick Mulder

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Posted 15 November 2007 - 04:01 AM

why ?

cos things get smaller the further away they are ;)

What we perceive as straight lines in our vision are far from being so - try to dislocate your expectations and actually investigate the data coming through your eyes and you'll see what I mean ... (much easier on drugs)

The camera doesn't have these expectations and once the data is put up against relatively 'straighter' reference (your frame in Final Cut) you get your 'what the ??!' moment .

I will admit though that the fact our film plane/sensors are flat and not spherical like the back of our eye makes it worse.

Edited by Nick Mulder, 15 November 2007 - 04:02 AM.

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#4 Tim O'Connor

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Posted 15 November 2007 - 09:45 AM

I was full wide
and I have been full wide before but maybe in this case more than any other with my using this camera I gave it so
much opportunity for distortion with all those horizontal lines in the right places that it couldn't resist.

It's funny because a post I read this month led me to some reading in which I found several pictures given as
examples of how barrel distortion can bow lines.


Thanks!
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#5 Michael Nash

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Posted 15 November 2007 - 05:04 PM

You just had a good clean subject to reveal the barrel distortion. In a frame with less distinct straight lines near the edge of frame you might not notice it as much.

It's not just the horizontal either; you can see the doorframe on the right bows outward also, and not just because the horizon is slightly tilted.
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#6 Tim O'Connor

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Posted 16 November 2007 - 12:36 PM

You just had a good clean subject to reveal the barrel distortion. In a frame with less distinct straight lines near the edge of frame you might not notice it as much.

It's not just the horizontal either; you can see the doorframe on the right bows outward also, and not just because the horizon is slightly tilted.



Yes, I think whenever before I've been full wide I've never had such a barrel distortion friendly composition as this.

Plus you're right on; the wall is really bowed.

That reminds me; did you ever get a shot of a building from across the street in which
the company sign is really important but when you level your camera/tripod, the building sign is not level in the frame?
Sometimes I think that the sign isn't level but other times I think that the sign is leveled to the different grade of
the land across the street. Then if you attempt to cheat the camera level in order to correct for it, either the buidling
looks off or anybody walking though the shot seems to be leaning one way or the other.

Thanks.
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#7 Freya Black

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Posted 16 November 2007 - 01:48 PM

This is a screen grab from a Final Cut Express timeline. The footage was dubbed in
camera from P2 card to Mini-DV tape and then captured into Final Cut Express. The shot
was obtained with an HVX-200 shooting 720P 24PN, Cine V with stock lens.

In this shot the frame of the archway in the hall seems curved but is not in
actuality. I haven't had this occur before. Does anybody know?


Thanks.


Can anyone tell me if the HVX and DVX cameras share the same lens?

I'm finding there tends to be a lot of barrel distortion on low end camcorders, some worse than others. I'm guessing this is due to the struggle to make such tiny, tiny zooms and that perhaps cameras like the DVX/HVX may suffer from this less as they have larger glass. Certainly I have an old canon video8 camera that seems to not suffer from this that much and it has a larger lens.

I'd be intrested to hear peoples experiences especially at the high end of consumer equipment. I've had good experiences with the Canon XL1s. I seem to remember the XM2 was also good but I only used it briefly. The PD-150 seemed less positive and I've never used a DVX100 or HVX at all.

love

Freya
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#8 Chris Keth

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Posted 16 November 2007 - 02:22 PM

Can anyone tell me if the HVX and DVX cameras share the same lens?

I'm finding there tends to be a lot of barrel distortion on low end camcorders, some worse than others. I'm guessing this is due to the struggle to make such tiny, tiny zooms and that perhaps cameras like the DVX/HVX may suffer from this less as they have larger glass. Certainly I have an old canon video8 camera that seems to not suffer from this that much and it has a larger lens.

I'd be intrested to hear peoples experiences especially at the high end of consumer equipment. I've had good experiences with the Canon XL1s. I seem to remember the XM2 was also good but I only used it briefly. The PD-150 seemed less positive and I've never used a DVX100 or HVX at all.

love

Freya


I don't think it's the same, but the range of focal lengths is very similar.
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#9 John Sprung

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Posted 16 November 2007 - 04:09 PM

Yup, barrel distortion from the lens. Point your camera square on dead center at a framing chart to see exactly how much of it there is.

I remember once on an historic location having a long, cordial, but inconclusive discussion with a director about leveling the camera. The homeowner sat there giggling at us, and then pointed out that the house was neither level nor square.... ;-)



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#10 Michael Nash

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Posted 16 November 2007 - 04:46 PM

That reminds me; did you ever get a shot of a building from across the street in which
the company sign is really important but when you level your camera/tripod, the building sign is not level in the frame?
Sometimes I think that the sign isn't level but other times I think that the sign is leveled to the different grade of
the land across the street. Then if you attempt to cheat the camera level in order to correct for it, either the buidling
looks off or anybody walking though the shot seems to be leaning one way or the other.


That can happen from plain-old perspective, if you're tilted up at all. If the sign is near the side of frame, it naturally looks like it's leaning inward. That's one reason architectural photographers use a view camera, to compenasate for perspective:

Posted Image
(Photo by Julius Shulman; one of my favorites)

In motion pictures we have shift/tilt lenses that can do the same thing.

Sometimes you can disguise a detail that's actually leaning by eliminating any strong horizontal or vertical lines from the frame. Try getting close and shooting up at the sign, so all verticals and horizontals have linear perspective (2-point perspective, in this case). Then level your camera so that it looks "right."

Of course with any lens the way you check for "level" is by comparing against a true vertical line in the center of frame (crosshairs are good for this). That eliminates all influence from perspective and distortion.
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#11 Tim O'Connor

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Posted 16 November 2007 - 05:32 PM

I understand that the lenses are different but don't know the exact details.

I'll do that framing chart test and see what I get. That's funny about the homeowner chuckling
at your sincere attempts to level to a tilted building.

Wow, Michael, that's a lot of good info.!


I learn something new every day on here. Thanks.
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Gamma Ray Digital Inc

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