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Micro Lenses and CCD's

Published : 22nd Sept. 2005


Let me throw another wrench into this non-linear response theory about HD CCDs. Last week, I was shooting a sunrise locked off with a Panasonic Varicam at 12 fps on the camera's intervalometer. At the end of the shot, I wanted to pan up off the horizon into the sky and capture a low flying jumbo jet at 50 fps. Since I didn't have anything but an SD monitor, the camera's Y get (nice tool Panasonic), and zebras to judge exposure consistency from the 12fps shot into the 50fps shot it was kind of touch and go. I was on a Canon HD zoom with an f-stop of approximately 4 1/4 (4.5). Theoretically, opening up the f-stop to 2 should have given me the same zebra pattern in the same place in the sky. It didn't. In fact, I was more than 1 stop underexposed at 50 fps and had to wait 10 minutes while the sky brightened to get approximately the same exposure as I had at 12fps. I had similar exposure "inconsistencies" over the course of shooting when changing "shutter angle" on the camera. I understand that f-stops on an HD zoom lens are not the same as T-stops on a 35mm lens and that this is especially true at the long end of the zoom but the inconsistencies seem to me to be more than simply a problem with markings on the barrel.

Steven Gruen
Steadicam/Lighting Cameraman
Paris, France



Some of the drop off that you are seeing is due to the lens not necessarily all the cameras fault.

B. Sean Fairburn
DP, LA


What is the correlation between frame rate and exposure on the Varicam? With a film camera, of course, 12fps/180 degree shutter the frame is exposed for 1/24th of a second. From what I have read about the Varicam, everything is recorded at a consistent 60fps (do they mean fields?). Frame rates slower than 60fps are recorded on multiple frames.

-Alan Hereford * Cinematographer
Marin Co., CA USA


I'm not sure how the drop off is due to the lens if the only variable in the shot I described was frame rate?

Perhaps someone from Panasonic could help us with the answer. I assumed, perhaps wrongly, that the correlation is the same as in film (i.e. double the frame rate 1/2 the exposure). I made the same assumption for the shutter variations which would what labeling shutter speeds in degrees (actually you have the choice between degrees and frequencies) as is done on the Varicam would lead one to believe. The point to my post is that getting a consistent response in the field is not as simple as making the above assumptions perhaps because the variables are not mechanical but electronic. This is not a major problem but you do need to be aware of it.

As I understand it, yes, everything is recorded at a consistent 60 fields per second and then those fields meant to be read are flagged and the FRC reads only those fields. So when I'm shooting 12fps the rate at which the CCD is sampling (refreshing? what's the word?) is 12fps and these 12 fields are spread out over 60 recorded?? How are these theoretically identical fields grouped, read then?

Steven Gruen
Steadicam/Lighting Cameraman
Paris, France


Many, many zoom lenses exhibit "drop off" at the telephoto and near-telephoto range. The greater the zoom ratio the more difficult for lens designers to prevent this. In effect, the f-stop number goes down (say, from 1.7 to 2.6) as the focal length increases, it's just simple mathematics. Many of us refer to this lens condition as "ramping" (not to be confused with "speed ramping" during a shot) and it's always an issue if you're shooting wide open with a zoom, or more open than the maximum effective aperture when zoomed in all the way.

It's fairly basic stuff, about the only shooters I've found unaware of ramping are video news guys who spend all their time in "auto iris" mode ;-0

Some camera houses such as Panavision have even gone into a lens and installed a mechanical "stop" to prevent the lens from zooming in beyond the point of consistent exposure, and yes, this "upgrade" does produce even and flat field illumination throughout the range, at the expense of reduced zoom ratio.

Jim Furrer, Director of Photography
Dark Street Films
USA based

http://home.attbi.com/~darkstreetfilms


This is also true of many zoom lenses designed for still photography, where it is necessary to "stop down" to zoom all the way in.

Brian Heller
IA 600 DP


Basic stuff, maybe. I wasn't in auto-iris (never am, I leave that to those "video news guys", pooh) and I was a long way from the long end of the zoom, approximately - 18mm. The zoom, a 4.7 x 11, goes all the way to 52mm. What I saw wasn't drop-off, it was beyond that.

Steven Gruen
Steadicam/Lighting Cameraman
Paris, France

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