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How many footcandles for 2.8 with HVX-200?


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#1 Tim O'Connor

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Posted 18 February 2007 - 10:29 AM

Yesterday I was shooting 720P 24N, Cine D, 180 shutter film cam and I
used an open face Omni to bring an actors's face so that it had zebras at
70
IREs and then I dialed the iris down so that it still was at 2.8 but most of
the zebras were gone except for on the tip of the nose and a touch on
a chekbone.

I've found that I need a lot of light with the HVX-200 but when I
measured
the footcandles (with a Spectra Pro analog, an old one to be sure but one
that was checked on a Spectrometer last week and checked out okay)
i was getting above 400 footcandles depending on how much within 2.8
I adjusted to raise or lower the IREs. I say how much within 2.8 because
you can iris down or up and it will change but still read 2.8 until it jumps
to say 2.4.

Four hundred plus footcandles is a lot of light! That's what you see in an
an early 70s t.v. show with slow film stocks when a burglar is in the
bushes or two people are
talking in bed. Okay maybe two hundred footcandles but still in a lot of those dark night scenes
there's such brightness that the burglar could read a
newspaper.

This makes the ASA about 25. I've heard people say that the ASA for
the HVX-200 is rated 320 but it's not linear so at the open end of the lens
it's about 160 ASA. Well, a 2.8 at 160 and 1/50th exposure is 64
footcandles according to a table on the Panavision website. If anything
then, it's not going to be more footcandles when going for 70 IREs at
2.8.

Does anybody know what's going on here? Also, I've been using the side
dial to adjust the iris and it drops from 2.4 to open with nothing in
between. I thought that you could get a 1.7 but I haven't seen it.

Oh, by the way. I wasn't using any ND or other filters.

Thanks.
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#2 David Mullen ASC

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Posted 18 February 2007 - 10:38 AM

Perhaps your camera's shutter speed changed to compensate, or you were wrong about the ND's, or the gain level went into minus db, or you misread the meter.

Basically you must have done something wrong because the camera is not 25 ASA. I'd start from scratch and double-check all the settings.
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#3 Tim O'Connor

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Posted 18 February 2007 - 02:44 PM

Perhaps your camera's shutter speed changed to compensate, or you were wrong about the ND's, or the gain level went into minus db, or you misread the meter.

Basically you must have done something wrong because the camera is not 25 ASA. I'd start from scratch and double-check all the settings.


Thanks. I was pretty thorough I thought but still I'm going to redo that test and doublecheck
everything because the results don't make sense. I'll post what I find out.
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#4 David Mullen ASC

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Posted 18 February 2007 - 05:44 PM

Sometimes you don't realize when a camera is automatically switching gain or shutter speed to compensate for a locked-in f-stop.
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#5 Tim O'Connor

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Posted 18 February 2007 - 07:28 PM

Sometimes you don't realize when a camera is automatically switching gain or shutter speed to compensate for a locked-in f-stop.


Yeah, sounds quite possible. I don't know the camera inside and out although I really like it a
lot. Given what people have said about not ever using gain with it (although of course if I
saw a U.F.O. I would) I had set the gain switches to 0 and I was in manual mode but something was
obviously going on. So far my shoots have gone pretty well. I shot some stuff Friday on a snow
covered beach and I'm astonished at how beautiful it looks.

I'll be pretty busy editing tonight and tomorrow but I'l looking forward to retesting and spending
some time with the scene files and settings and generally experimenting. First though, I'm going to
figure out that xfootcandles equals 2.8 equation!

Thanks!
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#6 Sam Wells

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Posted 18 February 2007 - 10:47 PM

That scenario sounds more like 40 footcandles to me, did you add a zero when reading the meter's scale here ? Wrong slide ?

40 fc would mean an effective EI of 250 if I did the math right, more in line with that camera's sensitivity as described here.

-Sam Wells
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#7 Michael Nash

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Posted 18 February 2007 - 11:44 PM

Also, I've been using the side
dial to adjust the iris and it drops from 2.4 to open with nothing in
between. I thought that you could get a 1.7 but I haven't seen it.


At the long end of the zoom range you'll probably drop a little sensitivity. "Open" at the long end may be 2.0, but 1.7 at shorter focal lengths. You'd have to test it to be sure.
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#8 David Mullen ASC

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Posted 19 February 2007 - 02:53 AM

That scenario sounds more like 40 footcandles to me, did you add a zero when reading the meter's scale here ? Wrong slide ?

40 fc would mean an effective EI of 250 if I did the math right, more in line with that camera's sensitivity as described here.

-Sam Wells


Yes, maybe the display said "40.0" if it were a digital display...
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#9 Tim O'Connor

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Posted 19 February 2007 - 09:58 AM

Yes, maybe the display said "40.0" if it were a digital display...


Oh gosh, I just realized something. I remember that I had been measuring by a window
right before this measurement for the 2.8 and my Spectra analog meter, which
has a footcandle scale from 0-250, takes slides to read for higher levels or ASA
speeds as you probably know. I had put in the x10 slide for the footcandle scale because
the meter
was pinning in my earlier measurements and I knew that but somehow the math I did
in my head was I'm realizing now x100! That makes perfect sense. If I'm off by a factor of
ten, then yes I would have been getting 40 footcandles and at one point, when I got
what looked like just above 600 footcandles to my multiplication deficient mind, I would have
been getting 64 footcandles, which at 2.8 corresponds to an ASA of 160 as the HVX-200 book
says.

This sounds like such a dumb mistake. Of course there's a big difference between 40 and 400
footcandles but for the sake of discussion, I might have been fooled a bit because the actress
was
sitting on a bench in the center of a stage and the only light was the open face 650w which I
ordinarily wouldn't use to key without bouncing or diffusing it so it was a hard, shiny light
that gave a bright look to her light skin in that desert of darkness.

Of course, I also was fooled because evidently I totally spaced out. Thanks for all your help
everybody; you knew that there was a mistake in there somewhere. I'm going to keep testing
too.




At the long end of the zoom range you'll probably drop a little sensitivity. "Open" at the long end may be 2.0, but 1.7 at shorter focal lengths. You'd have to test it to be sure.

Yes, I was at the long end. Good point. I'll test that too to see if I can find where it comes in
but I'm sure you're exactly right.

That scenario sounds more like 40 footcandles to me, did you add a zero when reading the meter's scale here ? Wrong slide ?

40 fc would mean an effective EI of 250 if I did the math right, more in line with that camera's sensitivity as described here.

-Sam Wells


Wrong slide indeed. Good thing I don't work for NASA!
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