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Sony VX2100 / PD170 / PDX10 Companion
Topics also include Sony's TRV950, VX2000, PD150 & DSR250 family.


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Old May 4th, 2003, 05:23 PM   #1
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Question just to understand "behavior" of VX2000 in auto mode. In full darkness outside, shot cherry blossoms with just porchlight for light --in medium shot (been reading the Five C's...) exposure was f1.6 with 18 dB gain--in closer zoom exposure changed to f2 still with 18 dB gain-- in closer zoom yet exposure changed to f2.4 but still 18 dB gain (I did not move camera location, just zoomed). With f 1.6 I thought camera would first reduce gain as much as it could and then reduce lens opening--i.e. would have gone from f1.6 at 18 dB to f1.6 at 12 dB gain and then f1.6 at 5 dB or something like that. Something to do with zooming?? In any case all the video was surprisingly good even with 18 dB gain.
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Old May 4th, 2003, 06:03 PM   #2
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This "behavior" is done to protect you against asking too much of the camera, and specifically the lens. With a professional lens that will actually hold the stop you set, as the lens begins running out of light as you extend the focal length by zooming in on your subject, at wide open aperture, you will see an effect know as "portholing" where the image begins to exhibit a vignette look around the edges. The farther you zoom, the more pronounced the effect. Obviously, this is not a good thing, and as someone working with a professional lens, it is expected that you will see this effect, and back off until it goes away.

With the prosumer camera with an electronic lens, the manufacturers have established a look up table that basically says, "too long a focal length for this stop, we'll pick a better stop." So what you are seeing is perfectly normal for lens on this camera.

Now, as to how good the picture is with 18db of gain added, that is another matter. If you look at the picture on a good monitor, look at the low light areas, especially the blacks. This is where you will see the most evidence of the gain boost, and this is what most people would object to. The cameras are amazing, but they still like light.
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Old May 4th, 2003, 09:15 PM   #3
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Porthole

Thank you for answer, Wayne. As I experiment more and more it is very helpful to know what to watch for when I balance things off--for instance, now I can see how bad the portholing gets (now that I know what "portholing" is) if I do a manual large lense opening in low light and high zoom--balance high gain quality vs portholing. BTW, I expect the 18 dB would look poor under close scrutiny, but did not look too bad on consumer TV.
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Old May 7th, 2003, 12:01 PM   #4
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Hi Wayne - good to see you over here.
Dennis - look again at your footage all shot at maximum aperture and maximum gain. You'' notice that the wide-angle shot is probably the best exposure and the full tele shot is in fact a stop under exposed.

The specification for the VX2k's lens points out that at wideangle it's an f 1.6 lens and at telephoto it's f 2.4, so it looses a full stop as you zoom. Other cameras are much worse - the canon GL2 looses over 1.5 stops.

This is not a fault, it's merely design intent. You could well have a 12x zoom that stayed at f1.6 throughout the full zoom range. In fact in the 70s and 80s there were a few Super 8 cameras that had 10x zooms that maintained f1.4 throughout the zoom, but the penality is pretty severe.

Sony reckon we wouldn't want to pay for a physically bigger and costlier lens. It would weigh more and require bigger filters and these disadvantages would not look so good alongside the competition. So we loose a stop and like it.

tom.
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Old May 7th, 2003, 01:05 PM   #5
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Exposure Behavior

Tom, thank you for input in addition to Wayne. I just like to understand the "why" and the tradeoffs behind behavior. So far I really like the VX2000 (upgrade from TRV 9) particularly low light capability and options it gives to adjust "look" of footage.
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