HVX ISO lower than I thought

T

toddeastman

Active member
I tested the HVX for sensitivity and got a disappointing result. 160 ISO. Here are my settings:
720p/24p
180 deg shutter
24fps
4.2mm lens
f 5.6
HD Norm Gamma
Setup P2 = 0%
No ND
Pointed at an 18% Grey card at 50 IRE.

Took a meter reading and matched the f-stop and got a 160 ISO. Is this right? Not very sensitive at all! What could I be missing? I would have at least thought it would fall around 320. :(

Incidentally, the different Gamma react in this way, all lighting factors constant pointed at the same 18% grey card:
HD Norm = 50 IRE
Low = 50
SD Norm = 53
High = 53
B Press = 51
Cine D = 42
Cine V = 45
 
I've run that exact same test and gotten 320, and that's the result we got at the six-cam comparison too.

I don't know what you could have done wrong, the testing methodology sounds appropriate, but the result is way off. I've tested at all stops and it's a consistent 320 from 2.8 up to 9.6.
 
I'm not sure an 18% grey card should be put at 50 IRE.
Ansel Adams - Zone 6 is about the same as a 70% flesh tone so 1 stop darker might not go all the way to 50%.
Much more accurate would be to use a chip chart and put it where a correct video exposure should lie.
I haven't gotton to this yet myself.

You could also try a bright white card & put it at 90% asnd see if that is a different exposure than grey card @ 50%.

Just some thoughts - I haven't tried it yet.
 
the gray depends a lot of the gamma curve used.
for example, with the film rec curve of the varicam, the gray point is around 30-35IRE
And ansel adams (great theory) it's not aplicable when you are working with modified gamma curves.
 
Another question.....
The manual says "When you select CINE-LIKE gamma, we recommend to set the lens aperture lower than normal image level (approximately 1/2) to enjoy the full benefit of the function."
Does that mean overexpose or underexpose by a 1/2 stop? It's written wierd. Sounds like underexpose it by a half but I would like to be sure. Then again a lower f-stop lets more light in! Grrrr!

Thanks!
 
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Yes Gamma will affect where the grey is a great deal. That's why you should use a chip chart which has a range form black to white.
Alternatively put a person in a well lit typical scene. See what f-stop you get in HVX and compare to an incident light meter.
It should vary a bit with the gammas.
Also be careful because the f-stop readouts on the HVX have alot of slop in them.
They only show half-stops instead of 1/3 stops. This kind of sucks because the camera actually has a number of shades of exposure in between those half stops, so you can't really use the numbers to acurately reset a shot after you for some reason changed the setting (either by accident or intentionally.) Just putting it at say 5.6 puts it only within a half stop range.

This is something I'd like to see corrected. Was the DVX the same?
I remember the old XL-1 showed 1/3 stop increments.
 
The HVX and the DVX allow you to adjust the iris in 1/6-stop increments.

The display lists 1/2-stop markings. So you could be at f/4.0, or f/4.0 + 1/6, or f/4.0 + 1/3, and the display will still show f/4.0. When you hit F/4.0 + 3/6 (i.e., 1/2) then the number will change to f/4.8 (being halfway between 4.0 and 5.6).
 
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Did you set your light meter at 24fps. I've done this test as well and got a 320. Does your light meter have shutterangle settings? that could mess up your reading. You might need to get your meter calibrated.
 
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