Please check my maths

G McMahon · November 13, 2006

Hello all,

Working with some specs I have determined.

To shoot at T2.8 with the aghvx 200 I will need be working with 4.8fc.

Minimun subject luminance:

3 lux @ T1.6, gain +12db, 1/24 shutter

T1.6, +12 db gain = 4 stops

3 lux X 2 (power of 4) = 48 lux = 4.8fc

Is that correct? I am trying to work out the amount of lights I need beforehand. I won't have time to put my hands on the camera to manually check.

Is T2.8 a good stop to work with on that camera?

Graeme

Vedran Rapo · November 13, 2006

Hello all,

Working with some specs I have determined.

To shoot at T2.8 with the aghvx 200 I will need be working with 4.8fc.

Minimun subject luminance:

3 lux @ T1.6, gain +12db, 1/24 shutter

T1.6, +12 db gain = 4 stops

3 lux X 2 (power of 4) = 48 lux = 4.8fc

Is that correct? I am trying to work out the amount of lights I need beforehand. I won't have time to put my hands on the camera to manually check.

Is T2.8 a good stop to work with on that camera?

Graeme

sorry i cant help you, and i know you have no time to explain what is that ... i am student, and have strong will to learn :)

so when you have some time.... i would be very thankfull if you could find a little time to explain it :) thx

Matt Irwin · November 13, 2006

Hello all,

Working with some specs I have determined.

To shoot at T2.8 with the aghvx 200 I will need be working with 4.8fc.

Minimun subject luminance:

3 lux @ T1.6, gain +12db, 1/24 shutter

T1.6, +12 db gain = 4 stops

3 lux X 2 (power of 4) = 48 lux = 4.8fc

Is that correct? I am trying to work out the amount of lights I need beforehand. I won't have time to put my hands on the camera to manually check.

Is T2.8 a good stop to work with on that camera?

Graeme

Hi Greame,

Not sure I can help you with your math, but I have shot with the HVX in low fc.

What you've worked out may be true for minimum illumination, but in oder to get decent images from the HVX, you need to give it plenty of light.

I wouldn't push gain unless it was absolutely necessary. The camera has a bit of a noise issue between (roughly) 10-25 IRE, and that's at 0db gain.

I had one scene that was to be played in slhouette and with the gear we had available, the best I could get was 5 fc on the backlight. Shot using Cine-D, which is normally noisy as hell, but since shadows were reading uniformly zero it wasn't a big issue. After boosting highlights during the grade, it looked good... but not great.

Based on that shoot, I'd recommend sticking with Cine-V or Black Press gamma settings since they are much less noisy than any of the others. And give the chip as much light as possible. Working with minimum fc was not the best idea for this camera (not that I had a choice in this case ;) ).

Bill Totolo · November 13, 2006

Hello all,

Working with some specs I have determined.

To shoot at T2.8 with the aghvx 200 I will need be working with 4.8fc.

Minimun subject luminance:

3 lux @ T1.6, gain +12db, 1/24 shutter

T1.6, +12 db gain = 4 stops

3 lux X 2 (power of 4) = 48 lux = 4.8fc

Is that correct? I am trying to work out the amount of lights I need beforehand. I won't have time to put my hands on the camera to manually check.

Is T2.8 a good stop to work with on that camera?

Graeme

If you want to shoot at a T2.8 you first need to determine the ASA of the camera.

The formula then works like this:

10,000/ASA=F.C.'s required to acheive a T2.8

You can see from the formula that you need to determine one of the variables ahead of time, either the ASA

or the available amount of footcandles.

Example for ASA 200 stock:

10,000/200=50 footcandles to acheive a t2.8

Example for ASA 500 stock:

10,000/500=20 footcandles

This assumes a 180 degree shutter.

I honestly don't know how much gain increases a camera's ASA. As for the HVX, I haven't rated it but I think ASA 200 is pretty safe. So for a std. exposure you may think of using 50 footcandles and factor from there based on your required shutter speed, filter pack, etc.

If you wanted to key at 4.8 footcandles you would need to acheive an ASA of 2083.33

10,000/2083.33=4.8 footcandles

Now you need to study how many foot candles your lights will put out at the distances appropriate to your shoot and understand the inverse square law so you can predict the fall off.

There's software for all this stuff. Check the download section of this website on the home page.

Laurent Andrieux · November 13, 2006

Hello all,

Working with some specs I have determined.

To shoot at T2.8 with the aghvx 200 I will need be working with 4.8fc.

Minimun subject luminance:

3 lux @ T1.6, gain +12db, 1/24 shutter

T1.6, +12 db gain = 4 stops

3 lux X 2 (power of 4) = 48 lux = 4.8fc

Is that correct? I am trying to work out the amount of lights I need beforehand. I won't have time to put my hands on the camera to manually check.

Is T2.8 a good stop to work with on that camera?

Graeme

Do you mean that according to the manufacturer's specifications, this camera requires only 3 lux @T 1.6, with +12 dB and shutter @ 1/24 ?

The general formula is :

T²/t = E.S/250 Where T is the T stop, t the exposure time, E the required illumination in Lux and S the ASA sensitivity.

--> MIND that + 12 dB is not equivalent to a 4 stops gain but only a 2 stops gain ! It's 6 dB for 1 stop since the dB definition is GdB = 20.Log(N) where N is the arithmetical factor, N being 2 for one stop (you double the signal value each time you open up one stop). Therefore the gain will be 20.0.3 = 6 dB since Log2 = 0.3

Therefore, if I follow you, let's find out if we get rid of this gain as well as of this shutter speed :

from 3 lux @ T 1.6 G= 12 dB, 1/24, I'd say :

6 lux T 1.6 G = 12 dB, 1/48 (sorry, I like to get rid of this one first), then :

24 lux T 1.6 G = 0 dB

I'd then find a sensitivity for this camera, at its nominal settings of 1.6².48.250/24 = 1280 ASA !!! That's a lot ! I would suspect this kind of camera more likely beetween 800 and 1000, but, hey, why not, let's start from this point( it's only one or two third of a stop more sensitive anyway).

Of course, if you keep the shutter @ 1/24, it should be 2560.

If you want to work at T 2.8, with no electronic gain, at 1/48 s (that is very close to 1/50 or even 1/60 at 30 fps), you then need 2.8².48.250/1280 = 73.5 Lux, that is about 7 fc, to about 9 fc if you consider 1/60 s.

I know also a rule of thumb, that is called "1.2.4" : you need 1000 lux at 200 ISO for a stop of 4, or 100 Iso require 2000 lux @ T 4, therefore, we see that 100 ISo require 1000 lux @ T 2.8, therefore, 200 ISO requires only 500 Lux, 400 ISO 250 Lux, 800 ISO 125 LUX, and 1600 ISO 62.5 Lux, all for a stop of T 2.8.

That coroborates the value of about 70 LUX ie 7 fc for a sensitivity about half way beetween 800 and 1600 ISO.

and only about 35 LUX ie 3.5 fc if you stick to the 1/24 s shutter speed.

But I can't be more precise since I don't know your nominal speed. 1/48 ? 1/50 ? 1/60 ?

G McMahon · November 14, 2006

Do you mean that according to the manufacturer's specifications, this camera requires only 3 lux @T 1.6, with +12 dB and shutter @ 1/24 ?

yes, that is what I mean.

MIND that + 12 dB is not equivalent to a 4 stops gain but only a 2 stops gain

I include the two stops from 1.6 to 2.8.

from 3 lux @ T 1.6 G= 12 dB, 1/24, I'd say :

6 lux T 1.6 G = 12 dB, 1/48 (sorry, I like to get rid of this one first), then :

24 lux T 1.6 G = 0 dB

so I should factor that it is 1/48 sec, not 1/24 sec. Like a film camera, if your shooting at 24fps your meter should be at 1/48?

I'd then find a sensitivity for this camera, at its nominal settings of 1.6².48.250/24 = 1280 ASA !!! That's a lot ! I would suspect this kind of camera more likely beetween 800 and 1000, but, hey, why not, let's start from this point( it's only one or two third of a stop more sensitive anyway).

Of course, if you keep the shutter @ 1/24, it should be 2560.

cant you just use your T1.6 is 24lux to determine the lux at T2.8 (2 x stops = 96lux)

Vedran, I am shooting a green screen shoot, low budget. I find you have to work hard to justify expense of equipment. I know its not the best way to have to work but I find if have just say to a producer I need this they believe I am being indulgent. By working out my minimum foot candles I can get an idea of what fixtures I shall need. I determined the stop of T2.8 as the zoom on that camera will not affect the shooting stop at T2.8. If I shot at T2 if I zoomed in to get on the long end of the lens it would automatically close me down to T2.8 (price of a zoom lens). Lux was the unit of measure the camera manual uses. I wanted to convert it to foot candles as I am more versed with that (probably like talking metres and feet). I used no formulae, just counted stops.

I am trying not to shoot at bare minimum light, I am reading so much on the subject of shooting green screen with this camera but there is so much documentation, and some that conflicts others. Its like trying to choose which religion to follow in one week.

Thank you all,

Graeme

Laurent Andrieux · November 14, 2006

so I should factor that it is 1/48 sec, not 1/24 sec. Like a film camera, if your shooting at 24fps your meter should be at 1/48?

Well... Mind that the shutter speed in a video camera is actually the exposing time, not the frame rate.

The frame rate will always be of 30 fps if shooting NTSC, 25 fps if shooting PAL.

At these nominal settings, the exposure time is to be considered respectively of 1/60 and 1/50 when shooting interlaced.

If you shoot with the shutter speed at 1/24 s it means the frame rate is still the niminal one, but the exposure time of each frame becomes 1/24 s instead of resp. 1/48 if 24 fps, 1/50 if 25 fps or 1/60 if 30 fps.