I have to recreate late afternoon light to shine through a window. ... My question is do I have a strong enough light source?...
My main light source is an Arri Tungsten 5k.
How I would approach this question is to start with how many foot candles (FC) the T5 will give you at 21ft. According to the Arri Photometric calculator, at med. flood it will provide 477 FC. I would then multiply that by the transmission coefficient of 1/2 CTB which according to the Rosco websiteis 52% to calculate that 248 FC will come out the other side of the gel. According to the Lee website, the light lost to Hampshire Frost is less than a ¼ stop which means that you will lose, conservatively, another 31 FC to the diffusion (1 stop = 124 FC x .25 = 31 FC.) Subtracting the 31 FC from the 248 FC that came out of the ½ CTB, you can expect to have 217 FC on set.
To calculate what stop 217 FC will provide, all you have to remember is that 100FC at an ASA of 100 at 24fps (180 degree shutter) will give you a stop of 2.8. So 217 FC at ASA 800 should give you a stop of about 11, which means that a 5k should be plenty strong enough, especially since you are probably going to want to balance your sun source at a half key because late afternoon light should be fairly weak.
Where it will draw 42 Amps at 120V, you can’t plug a 5k into the wall. You will need some other means of powering it. A 60A Transformer/Distro on a modified 7500W Honda EU6500 will handle it without a problem.
I think the 5K should work -- the problem with going lower in wattage isn't the output so much as the balance to existing daylight coming through the windows. Even if it is soft daylight, if it is too bright relative to the lamp, then your effect will get diluted and washed out, which is one reason I don't think you need the CTB gel on the tungsten, the mix with daylight on the subject will cool it off a little.
Thanks for the replies, every time I post a question I get great responses.
Thanks Guy Holt for your in depth reply.I will be using a HondaEU7000IS and a Transformer for power for the 5k. The DP prefers a tungsten light source over HMI's for this scenario.
I see your point about the lower wattage David, do you think the natural light will dilute the power as well from the 5k not just its temperature? Cancelling out its directional quality as a key light?
We plan to flag some of the natural light during the day from the window. Also the 5k is going up roughly the same angle the sun will be as it rises in the morning. Hopefully mimicking the suns angle to some extent for the morning. After noon any direct natural sunlight won't be hitting the window, only bounce.
If the director wants to throw up some curtains or wants the light stronger I may be in some trouble with the 5k? Maybe I should just get the 10k to be safe. However the 10k will require more hands. Or we could just open up in the camera if we need to get in more light.
Maybe I should just get the 10k to be safe. However the 10k will require more hands.
A 10k doesn't necessarily require more hands. You can parallel two unmodified Honda EU6500 or EU7000 generators for 100A output thereby eliminating the need for a large diesel tow plant. But, since that 100A is at 240V, you will need a 100A 2 40-to120V step-down transformer/distro. Use this link for more details about this approach.
Guy Holt, Gaffer, Lighting and Grip Equipment Rental & Sales in Boston
I don't think your over thinking anything. but would be suprised if the DP wants to shoot at an f11/16 ....probably wants to put some ND in there. so I wouldnt worry about the stop so much unless he needs that super deep stop for a reason... your world is quality of light. Bigger lights farther away are generally going to appear more natural with better quality.
The thing about size of the head is more about fall off then anything. If you have to move in your light to get a stronger exposure the light will fall off across the room much more drastically and start to look false. A bigger light farther away will fall of less as you move into the room.
and also as david said combat natural light coming in, which you might want some of as it helps raise the ambiance and can look nice.
The dp should be aware of the curtains and such and you guys should talk about it though. you can alway open up some of the curtain to let the light in more direct if need be. assuming the light it self will not be in frame
Edited by Albion Hockney, 07 April 2015 - 10:07 AM.
Yes the balance is more important, if the net stop ends up too high, just put an ND filter in the camera. For all you know, the natural daylight in there at 800 ISO is already an f/8 so if your 5K is giving you an f/11, it may not look bright enough in comparison to feel like sunlight.
The other thing to keep in mind that atmospheric conditions affect the color, sharpness, and intensity of a setting sun but as it gets lower, it can often be much more red-orange compared to the skylight, which is getting bluer, and the difference in intensity between the sun and sky is less extreme.
I'd keep the 5K un-gelled but set the color temp of the camera to pull a little orange out if you want, which would push the ambience a little bluer -- let's say 4700K or 5000K on the camera.
Thanks Guy for the info about combining two Generators, I will definitely look into that.
My apologies, I went to take precise measurements at the location, the actual distance from the window to where the DP wants the light is 34 feet NOT 20 feet. Not including the room itself, lets add 8 feet for the light to actually light the room/actors.
Total 42feet. On spot the beam diameter is about 9 feet giving me 222 foot candles. After gel and diffusion that's 90 FC.(2000 LUX)
Things are starting to look pretty grim for the 5k.
However the DP says we can move the light in. Plus he doesn't want a nice light fall off, which you would get from a 10k Fresnel Strand or a 10k Big Eyes Fresnel Mole moved further back.
How could you even find where that fall off is, other than moving the light physically or using Photometrics to find where it drops off numbers wise Albion.
The DP clarified saying he wants a harsher horizontal light source, rather than from a 45 degree angle. He agreed with Dave's about the diluting natural light to blue the tungsten. But it would be nice to have some CTB on the side.
Maybe add some Light Opal Frost to a 4x4 frame. Which is similar to Hampshire Frost but has a y transmission of 85%. Not sure why the Hampshire Frost doesn't have a rating. Sorry Guy I don't know what the Rosco equivalent is.
Thanks for the replies. Yeah it could be an overcast day or cloudless. How many stops would you say you need to bring out the 5k and the ambient lighting, 2 or 3? I guess we will see on the day of when we light meter. We could always flag off some of the natural light from above.
Yeah a 10k is the safer bet, luckily for us we are lighting a bathroom, not a larger living room for instance. Plus I don't want to have to make a scaffolding and put up a 10k and distro with our lonely, skinny grip.
What are your opinions on using a tungsten light source rather than an HMI for a sun source? Would you say its a budgetary or personal thing. Because I have seen both and not sure I have a good enough eye to spot the difference once the film has been graded. But i do think HMIs look very strange when they are undiffused.
But yeah why not just change the kelvin in camera, but I guess you would need a freshly calibrated monitor to see the difference.
I personally think a tungsten for warm sunlight is prettier than a CTO-gelled HMI, but I've done it both ways, whatever is more efficient at the moment. For regular sunlight in a day interior, I usually use an HMI with 1/4 CTO, sometimes 1/2 CTO, but after that, I prefer to use a big tungsten instead of an HMI with 3/4 CTO or Full CTO.
Plus I think with today's digital cameras like the Alexa, the color difference between daylight and tungsten doesn't look as strong as it did on film, unless you are doing a saturated grade to the image.
Light fall off can be calculated using photometrics at different distances....See what the difference in stop would be from the distant to the window and then to the back wall of the room your shooting in.
Keep in mind the sun is 100million miles away and has no fall off ....unless its hitting curtains or some sorta diffusion it will be as bright on one side of a room as another if direct, so if you are trying mimic it realistically you just always want the biggest source as far away as you can.
Not sure what you mean about the DP doesnt want "nice fall off" I donno if there is nice or not nice fall off but as said if you are going after the look of the sun you don't want it falling off much.
In regards to HMI, David seems to prefer Tungsten for the color of the light and I beleive in other posts has stated he likes the tungsten heads because since sunset is near 3200k he can use them bare without any gel frames infront of them and therefore sometimes sneak them into frame if shooting back toward a window without seeing a gel frame.
The downside of HMI's for some is color temp....sometimes they can be slightly off ....especially if they are older heads. As far as quality of light and diffusion its not really about the source being HMI or Tungsten. If you want the same look as a Tungsten Fresenel then you just need an HMI Fresenel.
Par Lights (both HMI or Tungsten) yes can look a little weird without diffusion....they are really designed to be used through some sort of diffusion. That said it depends on the lens you use in the fixure and the look your are going for. Narrow lenses can look pretty good ....Often times Par 64 Can lights are used like this to mimic direct beans of sunlight. It is the various medium, medium wide, and flood lenses that have paterns to them
Also the Arri M series is designed to have a nicer quality of light. although I'd still put it through some diffusion.
your dead on with the idea to use a 4x4 of Opal. Opal works great to diffuse a sharp sources just a little bit and looks pretty realistic.