Metering HMI's During the Day for a Night Shoot

moses_sparks · May 7, 2002

Here's the deal: I have been asked to shoot a scene that includes a

large architectural structure, but also about a full city block of

buildings in the background, and it has to be done at "magic hour",

i.e. twilight.

Buildings must be lit and "glamorous".

Because of the huge area to be covered, I have decided the best way

to light the shot, and the background buildings in particular, is to

hire a couple of movie lighting techs to rake them with HMI's. These

will be balanced to the many existing light sources like

streetlights, sconces, etc. that I've already metered.

The HMI's have to be placed something like 50 feet from the buildings

or they will be in the shot.

We will start setting up in the afternoon so that when it begins to

get dark we can shoot until there's no more ambient light.

Here's my question: Is there a way to accurately meter the output of

the HMI's only while it's still full daylight outside?

An incident reading is going to get the natural AND artifical light,

which won't be accurate for the nighttime conditions, and the ambient

light changes so quickly at that time of day it won't be possible to

meter and adjust at that point.

It's also not possible to have a pre-light day (which is how it would

be done for TV or Feature production) because we have to shut the

road down to traffic, and the powers that be will only allow that for

one day.

I need F11 at 1 second, on ISO 400 film.

I would normally use high powered flash for this type of work, but

the scale is pretty enormous, and I'm concerned about sync problems.

Any ideas?

Thanks!

henry_a · May 7, 2002

No problemo !

Set up one of those monster HMIs anywhere on any night before the picture. Meter it at 50 feet. Or even better light a building with it and after you meter make some Polaroids.

Glad its you and not me doing this. ; >)

mark_hubbard · May 7, 2002

Moses,

This is WAY over my head from a production standpoint, but isn't there a physics problem involved with light fall-off with only one second to perform the "rake"? Or were you going to go for multiple exposures?

Steve Simmons did a post office at night (for natural-looking ambient interior light) and held his precise position for several hours and did a second exposure at dawn to get the light right on the exterior. The double-exposed shot looks relaxed and entirely natural, yet it took incredible control and exposure accuracy to get it just right. (It's included among the illustrations in his book on large format photography.)

Are you doing these shots in large format? Why only one second (why 400 ASA film)? Will there be people in the scene?

Have you considered a digital composite?

I, too, am glad it's you and not me!

Mainly, it's that one-second "rake" that concerns me....

Best wishes,

ellis_vener_photography · May 7, 2002

Flash (I recommend the balcar 6400 w/s or similar Profoto or

Speedotron gear is going to be much more efficient watt second

vs. watt, but you won't be able to see what you are doing. Pocket

Wizard MultiMAx units --a tranceiver (set to receive) on each

pack--will insure you have no sync problems. flash will also set

you multi pop/expose on each frame to get the desired intensity

of light on the subject.<P>Can you do a run through the evening

before? Surely you know a D.P. or assiistant cameraman or top

of the line Grip ( you are going to need several)who can help.

and ofcourse you should have an assistant you trust do the

metering with a spot meter) as the light falls through the dusk

hour.<P>I sure hope you have a really big budget. if so, want to

fly me out as a consultant? (ha!)

art_haykin · May 8, 2002

If you have a full block of background buildings to rake in one second, that's one mile in 12 seconds, or about 5 miles per hour travel time. At 50 feet, you'll some "candlepower,"and a moving power supply, as I see it. A shot like this should require a rehearsed team, and several test runs. Getting it right on the first run seem highly unlikely, if Iunderstand all the requirements correctly. If you get it, please publish it here. Have fun.

miles_feigenbaum___dallas_ · May 8, 2002

Howdy Mo!

Yes, please keep us posted; this sounds like you'll be earning your pesos on this one!!!

your friend in Dallas,

miles

_____133 · May 8, 2002

You did not mention what kind of luminaire you would be using. However, many of the HMI manufacturers produce photometric charts for each type of luminaire. These charts may get you in the ballpark.

jackflesher · May 8, 2002

I've done similar shots to this, but always as double exposures. The first shot to get the twilight right, and then wait a few hours until it's dark, open the camera up and "paint" for the right amount of time (metered just prior) with the HMI's or using flash fill. The big issue with the HMI's is to keep them moving and not overlap them too much while making sure you cover everything; the big issue with flash is to make sure you also don't overlap too much and create hot streaks -- and I found it harder to judge with flash. I've had sucess with both methods, but also more than a few failures -- and my shoots were not on quite as large of scale. Good luck, and keep your fingers crossed!

Cheers,

moses_sparks · May 8, 2002

Hi guys,

The term "rake" is apparently confusing some folks here, the HMI's will not be moving during the exposure, they will simply be positioned to rake along the side of the buildings from a back angle.

I've pretty much worked out the lighting plan, but accurately determining the output of those lights is the sticky wicket. I'm basing the exposure on the existing practicals already in the scene, and that's where the f11 at 1 second exposure comes from. At some point the ambient light will drop to that same level, and that's probably going to be the optimum time to shoot, bracketing like a madman, of course.

Today I have someone measuring the exact distance from the building wall to the spot where the HMI will be placed. Then I will have the Set Lighting boys demo a light for me at that same distance (inside their warehouse) and meter that, and hopefully that will put me in the ballpark.

The ideal thing would be to do a dry run at the actual location, but that just isn't possible.

This shot is being done on the main road that runs through Universal Studios, and it gets an amazing volume of traffic, so they will only let me shut it down on a Sunday evening, before a holiday (May 26th).

Apparently they are reluctant to tell people like Steven Spielberg and Jim Carrey to take a detour while some schmuck takes a picture...go figure!

Ellis, you must have read my mind, I am also exploring the possibility of using flash, because that will definitely give me more control, and I'm checking out the Pocket Wizard Multi Max units to see if they will reliably fire at the distances involved. I also have the issue of syncing two cameras however, and that further complicates things.

I'm shooting with a pair of RZ ProII's, from two different angles, one with a 50mm lens, and the other with a 37mm fisheye. The fisheye view is really what they want, but because of the rapidly changing light and the nightmare logistics, I want backups to choose from. I have to provide a digital file as well as a transparency, so that means we will be able to massage the light a bit in Photoshop.

I have a nice budget, but flying you out here is not really in the mix!

The crew is 8 people including me, my assistant, 2 grip/lighting guys, a laborer to hose down the road (glamour!), 2 security officers for traffic control, and the studio's facility rep to coordinate.

I'm also considering the double exposure approach, but that limits the amount of film I can shoot in the window of time available. The background buildings go dark long before the primary subject does, which catches the sun right up to the moment it drops below the horizon. It's actually a huge sign, which also has lights inside that must ALSO be properly exposed so that it reads correctly...just keeps getting better, doesn't it?

So, if I pull it off, I'm the hero!

If I blow it...well, that's not an option. I have kids to feed.

Thanks for the input guys.

mark_hubbard · May 8, 2002

I'm glad I misunderstood the moving "rake"! Thanks for the clarification.

I was checking Art's math, and I think he meant to write 5 miles per minute, which would be 300 miles per hour.

If you do a single exposure, the problem of light fall-off still remains: if you try to light an entire block of buildings from two point sources, and the first building is 50 feet away, at 100 feet there will be 1/4 the illumination there was at 50 feet, at 200 feet 1/16 the illumination, right? Or am I getting the arithmetic all mucked up in my tiny pea-brain? I keep "seeing" two relatively bright areas closest to the HMIs that fade quite noticeably with distance. What part will you meter?

Please share more of your project when you can!

Best wishes,

moses_sparks · May 8, 2002

Mark,

Each building gets it's own HMI, so falloff is not an issue.

I actually ran a quick test with my Dynalite 2000 pack last night, and I was able to get f8 at 50 feet, ISO 320, so I may end up flashing this after all...I only need one more stop, and a quad head running from 2 packs would give me that (4000 w/s).

Still working out the details...

art_haykin · May 9, 2002

I stand corrected! It would be 5 miles per minute, or 300 miles per hour. I KNEW the idea was preposterous, but I goofed on stating it properly. He'd need the Batmobile.

cecil_thornhill · May 10, 2002

Moses,

It sounds like you have some good alternatives, but I did have a few ideas about metering the HMI's in daylight that might work.

My first thought was to use a foil or paper cylinder to block the sunlight path to an incident foot-candle meter and do a sort of 'spot' style reading on direct light from the HMI. I do think this would work, but I think consistency might be an issue in field conditions.

The second thought I had was to take either an analog or digital foot-candles meter, mark the zero position, then zero it out for the daylight, and add the HMI. If the readout is accurate enough (I think I'd trust my spectra), you could also just mark the sun only position and add the HMI. This should tell you how many foot-candles the HMI is adding to the party.

Both ideas call for a fairly normal TV style approach to the scene (treat it like a set and light to a pre-determined value for the sensor). If you are going for flash none of it may be an issue anyway.

A third possibility that occurred to me while posting was to use a spot meter on the subject (building) and get cumulative readings with and without the HMI. My spot to FC conversion is a pain on my old meter, so I use more direct F-stop read on my spot meter, but it should work as well as the incident meter if you get the math right.

Good luck. PS - I'm discounting clouds - the sky is always blue in sunny SoCal...:-)

Cecil Thornhill

moses_sparks · May 10, 2002

Cecil,

I've been contemplating the same idea you mentioned, i.e. metering the wall with and without the HMI, and then determining the variance. I don't have a cine meter and I'm not familiar with them, so I would want to use my Sekonic L408's to get a value in f-stops.

So am I figuring this correctly? If I get the HMI in position to cover the area desired, and then take a reading of the wall where the beam's center will hit, WITHOUT the HMI burning, and then take a second reading WITH the HMI, will the variance give me an accurate idea of the HMI's output in f-stops?

Let's say the ambient light alone reads f4 at 1 second, and adding the HMI reads f16 at 1 second. When the daylight is gone, should my exposure be f8 at 1 second? (Splitting the 4 stop difference).

I've shot everything imagineable with strobes, but hot lights are foreign to me!

Thanks.

cecil_thornhill · May 10, 2002

Moses,

I posted this as well. As I read your scenario the ambient light if at F4 at 1 sec, and the HMI brings that to F16 at one sec. Lets see...up all night fighting bugs in a release (yeah...I sold out to computers, but once upon a time I made a meager but honest living in film and TV)...sure I can do the math...what the heck sleep is over-rated...:-). Let's call the F4 reading 100%, and just simplify it to 100 'units' for now since we keep the exposure constant at 1 sec.

Given F4 as 100 units, F5.6 would be 200 units (twice as much light), F8 should be 400 (twice as much again), F11 is 800(same as before), and F 16 is 1600 'units' (16 times as much light - 4 stops). I may be a bit off by hand calculating, but I think that if you loose all the ambient light you may only be talking part of a stop here. I don't think the ambient (daylight) is contributing as much to the exposure as it might appear. Of course this is making the world 18% gray and the center of the HIM may represent a highlight not a mid-tone, so opening it up a bit more might be OK. Also, once the ambient light goes, the contrast on the un-lighted areas will pull them down into the toe for sure. Still, If I haven't done my sums wrong, I think you may be closer to F11 or F11.5 at 1 sec. without the daylight than you would be to F8 at one sec.

The real story on film may also be told by the shadow detail. With the ambient light, even though it brings only a small part of the total 'units' to the picture, the contrast range is not much beyond 4 stops. If you have constant lighting on the highlights (the walls in this case - center of the HMI), they won't show a lot of difference when all the ambient bounce goes away. A lit object in a dark studio should act the same way. The rest of the scene, is another mater. It was only registering on the film (or sensor for TV/digital) because of those 100 units that keep it out of the toe. When that ambient light goes away, there is nothing to keep it in the scene at all, and the effect is totally different for the final image. I know it is un-natural, but to use the studio analogy, the HMI's are your key strobe, and the sun (in this case) is just a white reflector providing some fill. I know how odd that sounds, since we don't see that way with our eyes (having a million to one sensor is so cool - thanks Nature). Still, the meter doesn't lie, so I think you're F-stop for objects in the main HMI path won't change that much when the sunlight fades. Your picture will look a lot different, but the highlights should stay pretty constant.

That's all I can think of for now.

Cecil Thornhill

ellis_vener_photography · May 10, 2002

If ambient is 1 sec @ f/4, and and ambient +HMI is 1 sec @ f/16,

I'd shoot at 1 sec at f/16, or maybe two seconds at f/11.5. Wait

maybe I'd shoot at 2 seconds at f/16! in other words I'd brcket

and use polaroid as I you go so maybe rent a third body to do

that with. In case the calculations are wrong I'd also go ahead

and have double the lighting handy -- just in case. I'd probably

rent a grip truck. your bill is goingto be a fraction of the total cost

of the project and, depending on the usage, having the backup

gear is going to be cheap insurance..<PNow who do I make the

consulting bill out to? (just joking).

ellis_vener_photography · May 10, 2002