f-stops, DR and zebras

[Egg Born Son]

I had a notion today while I was arranging lights for a scene on a personal project and I'm looking for a wiser head to steer me right.

I don't have a light meter (or more than a notional idea of how to use one properly) so I use my zebras exclusively to determine exposure of skin and highlights. Having two zebras on my HVX I usually set them at 70-80% and 100%. Being an HVX actually hitting 100% indoors is a pipedream, it never happens. So today while moving lights around to set up the scene, making endless adjustments trying to punch some light into the dark corners (to try and give the HVX something to see), I started wondering about what else I could use the second zebra for...which lead to me to thinking idly about DR.

I haven't really put much thought into DR before so I'm making a lot of assumptions here. But if understand correctly, the primary purpose of fill lights and background lighting is to increase the detail in the shadows. My assumption is that the theory behind this nugget of advice is that doing so reduces the physical DR of the scene. I believe my HVX is supposed to have 9-9.5 stops of DR (call it 9) and if the physical lighting can be limited to 9 stops between the brightest highlight and the darkest shadow then it will resolve the most information it is capable of. I'm guessing this is one of the purposes of a light meter.

So if all my assumptions above are in the ballpark, my question is: Can I use my zebras to accurately measure the DR?

If the zebra percentage value is accurate then it should be simple maths...100% divide by 9 stops would suggest that each ~11% should equate to 1 stop. So if I set the second zebra to 11% the entire frame should blow if all the shadows are 9 stops below the highlights (at 100%). While not forgetting to expose skin at around 70% give or take. Correct? Or is there some logarithmic vs linear scale conflict going on in this equation? Or have I simply proved that a little knowledge is a dangerous thing and all my assumptions are bunk?

Thanks in advance guys.

 

[stefanhuberfilms]

Very interesting thought! Wonder how many people are able to give you an answer on that one, i can't!

Do you still use your HVX for projects, or just for personal use?

 

[Egg Born Son]

This is a passion, not looking to ruin it by making it a job. I'm a weekend warrior so it's my only camera. It has features that other cameras just still don't offer and has a unique and very cinematic look out of the box, especially with a lens adapter. Every stills photographer that sees footage out of it is blown away and it's not my camerawork. From time to time I get DSLR envy on portability, guerilla invisibility, cheap support gear (can't believe they whinge about price while paying 25% of what I do), ISO, large sensor and interchangeable lenses. But CCD, real variable framerate, 4:2:2, professional synced audio options as well as DVCPRO and P2 etc bring me back to a state of satisfaction time and time again. It's like a car model of a specific year that is still sought after over recent models years on. Sometimes the planets align and they just get it right.

I'm considering getting a GH2 to complement it and give me a portable b-cam, a scouting and continuity tool, BTS and production stills. Just waiting for the pricedrop that was supposed to come with the introduction of the GH3. Doesn't seem to have filtered through to second hand prices yet and I'm not prepared to absorb someone else's depreciation as most sellers seem to expect. I can wait.

My only gripe is the lowlight performance (although 1080pN would be nice too). In a small room the amount of tungsten you need to get off wide-open apertures is almost unbearable. Got some fluoros now to take a little heat out of the equation but in summer even a/c can't offer much help. I've been dragging preproduction on this project for months now, waiting for a night cool enough to shoot. So far the HPX170 is the only upgrade path I've identified which is only half a stop better but better noise and lighter weight. But I'm good for now. Maybe something better will come along in the near future but I plan on putting some miles on this thing! As long as bluray is the standard and I'm a small time operator why would I need more? If I get to do something big I can rent.

 

[cpc]

You are misinterpreting the values.
Zebras percentage has nothing to do with stops. Zebras only mark certain signal coding values. The actual distribution of DR in these coding values depends on the camera transfer curve. If you have a good knowledge of this curve, you can map zebras % to exposure stops. The only case when zebras % and stops will have a linear relationship is if your camera is using a truly logarithmic transfer curve.
For a typical power gamma transfer curve (like standard Rec 709) there is less coding space in the shadows, and more in the mids and highlights. A typical power curve with a knee (highlights roll-off) will also compress highlights, so you will have lots of coding space in the mids, and less in shadows and highlights.
So, to answer your question, you can judge scene DR distribution with zebras, but only if you know your transfer curve, and it is never as simple a relationship as you describe it.

Fill light's primary technical function is indeed contrast control, but it also has a lot of creative implications as a mean to manipulate mood and key.

 

[Egg Born Son]

Thanks for the responses. Hoped I'd found my magic bullet but I did suspect nothing is ever that easy. I just did the experiment and its a moot point anyway. The zebras don't go lower than 50%. I'll just have to keep approximating by eye. Short lived thread. Ah well, it was worth a try. :tongue:

 

[stefanhuberfilms]

The HVX is still a really reliable and great camera, i second everything you said, the display and viewfinder suck though... BTW, did you see my low noise loglike picture profile i posted in the HVX forum some months ago? Maybe this will solve a tiny bit of the noise issues you got with the HVX... I don't know if a hpx170 will give you a much better image than your hvx, i guess you would be happier with a ac130 or 160, for the lowlight and overall image, but not for the codec ;-)

 

[j1clark@ucsd.edu]

I had a notion today while I was arranging lights for a scene on a personal project and I'm looking for a wiser head to steer me right.

I don't have a light meter (or more than a notional idea of how to use one properly) so I use my zebras exclusively to determine exposure of skin and highlights. Having two zebras on my HVX I usually set them at 70-80% and 100%. Being an HVX actually hitting 100% indoors is a pipedream, it never happens. So today while moving lights around to set up the scene, making endless adjustments trying to punch some light into the dark corners (to try and give the HVX something to see), I started wondering about what else I could use the second zebra for...which lead to me to thinking idly about DR.

I've only used 'zebra' indicators on cameras that have no variability, and for situations of 'last gasp avoiding blowout'.

If you want to get more precise 'correct lighting', given your camera is the HVX200, I'd recommend using Adobe's On-location, while no longer updated, allows a firewire based camera to have 'waveform' monitor scopes, and so you can 'easily' see what your lighting does relative to the camera.

Such a setup is not very portable... but for fixed setups, I think that's the way to go for 'video cameras'.

I use a couple of light meters one spot one incident, to assess lighting. But that because I've set up a test scene a number of times and measured light, and what my GH-1 does in response...

As it is, I've come to the conclusion that I can get a dynamic range of about 1.75 stops above middle grey, and about 2 stops under, before 'bad things happen' to the image.

If it so happens that the 'dark area' falls below that 2 stop difference, then additional light is required to avoid 'bad dark things', and likewise for greater than 1.75 stops, some sort of screening to mitigate blow out...

I have noticed that some camera manufacturers will state their 'camera' is better set up for a 'middle grey' at 40% IRE rather than 50% that one might expect for a 'middle grey'... that would probably lead to 1.5-2 stops for 'darker than', and perhaps 2-2.5 stops for 'brighter than'...

As for shooting with no meter or no waveform monitor... uh... no... Since a meter is cheaper than a new camera with builtin metering... I'd recommend that approach...

 

[Egg Born Son]

The HVX is still a really reliable and great camera, i second everything you said, the display and viewfinder suck though... BTW, did you see my low noise loglike picture profile i posted in the HVX forum some months ago? Maybe this will solve a tiny bit of the noise issues you got with the HVX... I don't know if a hpx170 will give you a much better image than your hvx, i guess you would be happier with a ac130 or 160, for the lowlight and overall image, but not for the codec ;-)

The noise issue doesn't bother me too much. Perhaps as a result of the pixel shifting it's quite organic looking on the HVX, it has character somehow. It doesn't look particularly digital, almost like grain. My main complaint is the amount of light required to expose. And the inability to stop down indoors. Throwing 2000W on an establishing shot indoors, f4 is pretty dark with the lens adapter. I have to use wide angle lenses at infinity if I want deep focus. Still, limitations force lessons and inspire creative solutions. Maybe it will bother me more down the track.

Agree about the display and viewfinder. Atm I'm using a basic external monitor but I'm looking to upgrade to one with more features after this project. I'll look into getting one with waveform as jclark suggests. I was waiting on acquiring some rack gear from work, in particular a broadcast waveform monitor I've got my eye on and professional calibrated monitor for colour correction with a bunch of diagnostic functions. They have a bunch of other goodies in the transmission tower but I have to wait for the next upgrade and that could be in months or in years. I got all my analog audio gear from there after they decommissioned a bunch of analog racks after the digital upgrade. I kept some, sold some but still threw out hundreds of thousands worth of gear. Crazy. Unfortunately I have to wait, they'll know who it was if I just pinch it!

 

[stefanhuberfilms]

+1 on the organic look. i also loved shooting on my HVX-LEX combo, but since i own a fs700 now i just use the HVX for eng style work and jobs where is no time to change lenses...