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Author Topic: Videocam lens speed question  (Read 5996 times)
Robert Roaldi
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« on: December 18, 2006, 10:36:04 AM »
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I have been an amateur photographer for 30 years but have no experience at all with videocams of any vintage. Until yesterday, I never even held one.

At an xmas social event yesterday I had trouble taking available light pics with my Canon G3. At ISO 400, I got mostly blurred but some usable shots at 1/25 th. I was avoiding using the flash. A friend was there with a 2nd hand videocam that he bought last week and knows very little about. He also had no auxiliary lighting.

We watched his film on his TV monitor later on. My question is why does his video appear well-exposed? Are the lenses on videocams very fast? Or is my brain being fooled by the motion and inherent low resolution into thinking that the image is ok?

I know that with video the shutter speed is in the 1/60-1/30 th range, so this implies wide apertures under the conditions yesterday. But he was also able to zoom quite close and the exposure was still ok. I never looked up the zoom range on his cam but he zoomed in MUCH tighter than I can with the G3 and at the long end, my effective f-stop did not allow shooting at all. Is it normal to have constant aperture zooms on these things?  Is there an internal ISO boost operating on these videocams? I would guess that their noise requirements are less stringent given the way we watch the films.

If you can point me to a web primer on the subject rather than spell out what is probably obvious (but not to me), that would be fine. Many thanks.
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Chris Sanderson
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« Reply #1 on: December 18, 2006, 11:00:47 AM »
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A good web article on videocam's light sensitivity can be found here.

Basically, the design & build of videocams' chips are often based around the marketability of LUX sensitivity (esp. Sony). Since I do not believe that there is an international  standard for light sensitivity for videocams such as ISO/ASA, this term obfuscates the whole issue in marketing-speak. Suffice it to say that the chips are often built for high sensitivity; they are generally tiny; ISO or 'gain' is automatic and often amplified to the point of terrible signal to noise ratios for the sake of a recognisable picture. With automatic gain, automatic shutter speed/frame rate control for low light, tiny sensors and relatively large lenses, it's small wonder that videocams can record in near-dark and have massive focal length ranges. Hope this helps.

CS
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Christopher Sanderson
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Robert Roaldi
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« Reply #2 on: December 18, 2006, 11:14:00 AM »
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Thanks, the link along with the rest of that site answer my questions. Much appreciated.
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Jonathan Wienke
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« Reply #3 on: December 18, 2006, 02:25:33 PM »
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Video cameras can work at much higher ISO than still cameras for two reasons:

1. Video cameras work with much lower resolution than still cameras. Even the best HD video cameras only record at 1920x1080 resolution (just over 2 megapixels), and standard definition cameras are generally 720x480 (just over 1/3 of a megapixel). If one were to take an unacceptably noisy shot from a DSLR and size it down to these resolutions, it would look reasonably good.

2. In video, each frame is only displayed for a short time, 1/30 of a second or so, before being replaced by the next frame. The noise in successive similar frames tends to average out over time, and is perceptually more difficult to see. You can see this for yourself by playing a DVD on your computer, and periodically pausing playback and looking for image artifacts. You'll find things when the image is paused that you simply won't see during playback.
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Robert Roaldi
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« Reply #4 on: December 19, 2006, 08:49:05 AM »
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Thanks very much.
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