Having trouble repeating 320 ISO/ASA Experiment

S

sebas1366

Member
I'm new to doing camera experiments and I'm having trouble obtaining the correct ASA for the HVX. I keep getting 160 instead of 320 (the tested ASA in Barry's book). I am using a different gamma curve (curve v) but according to the book that only accounts for a 1/3 stop difference, I briefly tested this and found that to be true in my case also. Can someone check over my experiment real quick. I appologize in advance for the bad grammer and spelling in it. Now that I think about it, it probally has to do with the auto iris (which I did not use). How do you use it to get a correct 50% exposure?

The experiment can be found here: http://dmedia.ucsc.edu/~sburke/ASAEquivalent.htm
 
sebas1366 said:
I'm new to doing camera experiments and I'm having trouble obtaining the correct ASA for the HVX. I keep getting 160 instead of 320 (the tested ASA in Barry's book). I am using a different gamma curve (curve v) but according to the book that only accounts for a 1/3 stop difference, I briefly tested this and found that to be true in my case also. Can someone check over my experiment real quick. I appologize in advance for the bad grammer and spelling in it. Now that I think about it, it probally has to do with the auto iris (which I did not use). How do you use it to get a correct 50% exposure?

The experiment can be found here: http://dmedia.ucsc.edu/~sburke/ASAEquivalent.htm
Click to expand...

I think that it says in Barry's book that ASA for a video camera is
a useful reference but has conditions. It's not a flat linear response
like a film stock at any light level. Apparently, the HVX-200
responds
like it's 320 ASA at higher stops and then like it's got a lower ASA
as you open up the iris.

That's why I think a light meter is useful for measuring light levels
i.e. footcandles and ratios but a waveform monitor is what to
use otherwise.

"zoom in to z55 (max zoom), Set to Cine V (contrast)"

Am I remembering incorrectly or isn't max. zoom "Z99"?
 
55mm=max focal length.
This is a perfect example of how numbers can be manipulated. Unless you really love grain you use high speed film for it's sensitivity. By stating the HVX has an asa rating of 320 you (royal) are implying that it is a sensitive camera when the reverse is true. Cine D has the least contrast by stretching the blacks and compressing the highs resulting in the most noise. My use of the camera (primarily Bpress) totally agrees with your rating of 100-160.
 
I too would agree with your findings. To properly expose a standard gray card (about 42%) the HVX requires about 45 footcandles with a 180 degree shutter at f2.0. That would put the equivalent ASA between 125 and 160.
 
wow maybe I got something right. It's all very exciting to me. When I have the time I'd love to do some more extensive testing. How do you know that the grey card should be exposed to 42% instead of 50%
 
I use a 42% exposure based upon a lot of testing that I have done over the years. The bit of extra headroom that it gives can really help.
 
so when your saying that you expose for 42%. Thats basically the same as saying leave 8% headroom at the top. so your whites are at 92% and you'll have less change of blowing them out?

or are you saying something like for the hvx 50% is really more like 42% when using a grey card
 
I am suggesting that with a standard gray card as your exposure reference, you should set a camera like the HVX to read 42% assuming that it is illuminated with a flat (both spectrally and illumination spread) source. This will give you a neutral exposure starting point. Now if you need more shadow detail or are really worried about the highlights blowing out in a high contrast scene, adjust your exposure up or down as needed. The underlying reason for using 42% as a target is based upon the exposure response of this camera (determined by testing that I have done with it).

The important thing to understand is that a gray card alone (or any reference) is not going to give you the "best" exposure in any given situation. You always should consider the tone of the scene, the contrast ratios of the setup, using these as a bias from your gray card reading. This is where experience and knowing what you want comes into play, that is one of the primary jobs of a DP.
 
Since moving, I can't find my light meter, or I'd recreate the circumstances.

We did this testing at the six-way shootout at DV.com. The gray card was lit flat and even, the camera was in HD NORM gamma IIRC, and Jay took a reading off the spotmeter. We then adjusted the iris on the camera until the waveform monitor showed 55 IRE on the gray card (which is also about what autoexposure would give you) and matched that to the spotmeter. The result was 320. All the cameras were measured exactly the same way. Actually, I'm not 110% sure that it was 55 IRE, as I wasn't on the waveform monitor, Jay and Adam were.

CINE-D renders everything darker; you'd have to open up quite a bit to get 55 IRE on a gray card at that same light level. It probably would render at around 42 in CINE-D.
 
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