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HD light metering -- need advice

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Chris Taylor
HD light metering -- need advice
on Oct 20, 2009 at 9:03:14 pm

Hi, I'm looking for a potentially better way to find the proper light value when shooting HD through a lens adapter/lens package. It seems like I'm missing something by following the f/stop my meter is telling me in accordance to the speed of my lens.

Currently, I'll look at my "target" f/stop according to the particular lens I'm using with a few other factors thrown in like adapter light loss, -3dB gain, light superiority, etc. Then, naturally, I'll choose a source that will provide that value.

However, it seems when I go from, for example, an f/1.4 50mm to an f/3.8 zoom, I'm needing to push way more light into the frame than my video monitor is letting on (not that I trust my monitor completely). Let's say I'm over-correcting for my rig's light loss by 2 stops, for my key I would look for about an f2.8 on my meter with my 50mm (f/1.4) wide open. But then on my zoom (f/3.8) I would need almost an f8? That's almost 3 stops from one lens to the next which seems a little extreme. Again, I know you can't trust the monitor, but it doesn't look that much darker to me when switching lenses in the same lighting conditions.

There maybe other factors in there that I'm not considering or maybe I'm getting this system confused with a t-stop exposure. The settings on my camera aren't changing at all and of course I shoot wide open on the lens themselves all the time.

Until I can afford to buy a waveform, this is my practice. I 'm sure there are plenty of people on here who can steer me the right way.

Thanks.

Chris




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Michael Palmer
Re: HD light metering -- need advice
on Oct 20, 2009 at 9:47:01 pm

Your 1.4 50mm prime is way faster than any zoom that you put on the adapter. The 50mm has less glass elements allowing more light to past. Try sticking with just the primes. I recently picked up 6 Nikon primes for my Red Rock Encore and all are a f2 are better except the 20mm @ 2.8.

I rely on my histogram for determining my stop.

Good Luck
Michael Palmer


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Chris Taylor
Re: HD light metering -- need advice
on Oct 20, 2009 at 10:01:58 pm

[Michael Palmer] "Your 1.4 50mm prime is way faster than any zoom that you put on the adapter"

Oh I defeinitely know how slow my zooms are. I only have a couple primes under f2 but I'm more curious about my current application with metering.

Chris





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Todd Terry
Re: HD light metering -- need advice
on Oct 20, 2009 at 10:29:16 pm

What Michael said.

In the video world, rely on histogram, viewfinder zebras, and a good monitor to determine exposure... do not rely on your light meter to expose. Unlike the film world, you can instantly see if your exposure is good... so go by that. If film cameras had an absolute exposure-accurate viewfinder assist, cinematographers would be using those for primary exposure calculations, not the meter.

The meter has it's place, but not for primarily determining exposure for video. I have three meters that have not been out of their cases since our last real film shoot.

And yes, primes are going to always be so much faster than zooms, as you know. It's the simple optics of light going through four pieces of glass rather than 16 or 18. My primes are all T1.3 (super fast), whereas my fastest zoom is T3.4 (slow as molasses). With my matched primes, my f-stop setting stays pretty much the same when I switch lenses, but if I change to another prime that is not part of the matched set, I often have the need to tweak the exposure as there are other factors involved. In the video world your eyeballs are a much better tool for exposure than a meter.


T2

__________________________________
Todd Terry
Creative Director
Fantastic Plastic Entertainment, Inc.
fantasticplastic.com






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Rick Wise
Re: HD light metering -- need advice
on Oct 21, 2009 at 12:16:20 am

One of the reasons it is nearly impossible to use a light meter to determine exposure with video is that it's almost impossible to determine the "true" ASA of any video camera. And, the relative ASA in low light will be different from the relative ASA in bright light. You can try to reverse engineer this: with a waveform monitor, set the iris to the correct exposure. Then with your meter measure the light and find the ASA that gives you the same f/stop. But, because the relative ASA changes in low light, this is mostly a waste of time. The only use I find for light meters on a video shoot is to get consistency.

Zebras are you invaluable friend when it comes to judging exposure with video. Especially when you have no monitors, no waveform, no engineer. Set the zebras to 70. Then place a "normal" Caucasian in front of the camera. Adjust iris until there is just a whisper of zebra on the hot spot (usually the nose or forehead, sometimes a part of the cheek). And you are set. If your subject is particularly dark-skinned, borrow a Caucasian to set iris.

Shooting landscapes you may be better off setting the zebras to 100. When parts of clouds show some zebra you are probably OK. But here it's more difficult. Due to the narrow range of exposure with most video cameras, usually you have to sacrifice either shadow or highlights to get a decent exposure. Depends on the framing, the look, the time of day, etc.

Rick Wise
director of photography
San Francisco Bay Area
and part-time instructor lighting and camera
grad school, SF Academy of Art University/Film and Video
http://www.RickWiseDP.com
http://www.linkedin.com/in/rwise
email: Rick@RickWiseDP.com


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