Night scene

[Matt Steffanina]

Hi,

I have to shot an action sequence on an outside night.

I want to create something similar to the one described in the photograph below: a zenith base lighting simulating the moonlight.

The director has asked us that the scene has to distill "rhythm and frenzy" so it is imperative to achieve that effect that the action sequences of movies like "Save Private Ryan" or "Gladiator" (Staccato) have while recording the Cowboys running and fighting each other.

The area that we must cover with the light is approximately 300m x 50m.

The camera to be used to record this sequence is an ALEXA MINI at 400 ISO, and to be able to achieve that Staccato effect we will shutter at 22.5o, and the optics used are Hawk C Series with T: 4 diaphragm, the color temperature in camera is 5500K.

 

To illuminate the scene we have the devices that appear in the photograph above.
• Fireplaces of 10 x 5 meters.
• Each chimney contains 5 HMI lamps of 1250W each.
• All covered by a full gridcloth fabric
• According to the manufacturer, the chimney lamp assembly responds to the photometric curve on the left.
• The crane can reach each device up to a maximum height of about 40 meters.

 

I want the light generated by the chimneys to be 1 stop below my base exposure. So can anyone tell how many devices i need to cover the area and the height?

Thanks

[David Mullen ASC]

22 degree shutter angle is rather extreme -- "Saving Private Ryan", "Gladiator", etc. all used a 45-degree shutter angle and that's pretty heavy, plus a 2-stop light-loss. If you are setting the Alexa to 400 ISO, you'd be working at an effective 100 ISO, at T4 -- which is a ton of light, more than what you're describing, which is essentially a 6K through full grid cloth.  And that's for a 45-degree shutter angle, half of that again would mean a 3-stop loss, aka 50 ISO. At T4. For a football stadium.  At that point, you'd need Muscos or BeBee Night Lights, all hard. Or a number of 100K Soft Suns, etc.

The old rule is 100 foot-candles for f/2.8 at 100 ASA (24 fps / 180 degree shutter).  So at T4, you'd need 200 foot-candles at 40' away for 45-degrees and 400 foot-candles for 22-degrees.  Now since you want things one-stop underexposed, you can cut that in half but that's still a lot of light.

 

[David Mullen ASC]

If my calculations are correct, 6K at 30' at full flood is about 500 fc.  Light loss for Full Grid Cloth is about 2 1/2 stops, so let's say you end up with 80 foot-candles. So maybe you lower your diffused 6K to about 25' high to get 100 fc.  So for a 10m long soft light, let's say separated by 10m to the next one (which will cause some dark areas) you'd need 15 of them in a row in one direction and five in the other to cover that 300mm x 50mm field -- that's 75 cranes!  So you have to go higher and less diffused to reduce that number.  And maybe you don't need the edges so well light so more like 13 x 3, more like 40 cranes?  Still seems like a lot.

I'd consider a row on one side of hard HMI's for more output and shoot in profile into backlight, and have a second row on the fill side with much dimmer light.

These HMI's have to be flicker-free because with short shutter angles, you run the risk of triggering the camera and getting in sync with the low part of the sine wave of pulsing light, and getting less exposure.