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Double exposure in film


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#1 Francisco de Burgos

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Posted 23 January 2010 - 03:05 PM

Hi there, I'm going to shoot a short film in about 3 months, it was asked that I can't use any lights in exterior day nor any type of light "rebound" (excuse my english, it's not so good) it was also asked there were no contrast or minimum, so I thought I could expose my film to a light blue cardboard and then shoot the film in the exterior, double exposing the film, but I'm quite confused about iris and the proper way to this, which I should do first, exposing the movie or exposing the cardboard, should I expose the cardboard like 3 stops down for example so the movie shows no contrasts but it's also no so overexposed, etc.
So I'm planning on doing some tests of this process but surely I could use some heads up on what's going to happen, or if someone could explain to me the proper way of doing this. The way I'm planning to do this is shooting the cardboard first, my lightmeter should show to expose it at 5.6 so I would do it at 2.0, then I would shoot the film trying for my exposition to be at 8 using ND filters and exposing at 5.6 for later in the lab push the film 1 stop down.
Thanks.
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#2 David Mullen ASC

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Posted 23 January 2010 - 06:45 PM

You're talking about flashing the negative to lower contrast by exposing the film first to a blue card (why blue?).

First of all, you don't get a lot more shadow detail with flashing; above a small percentage and all you are doing is lifting the blacks, not increasing shadow detail.

My feeling is that if you want to experiment with flashing, that's fine (I've done it a couple of times on features, but using the Panaflasher, not double-exposure). You really have to test though to get it right. You may also want to combine a very light flash with overexposing and pull-processing the negative by one stop.

Also realize that lowering the contrast will also be lowering the sharpness. You didn't say whether this was for 16mm or 35mm film.

A blue card is going to tint the shadows and the blacks blue. Most people would use a grey card, shot at the color temp of the film stock (i.e. if using a tungsten-balanced stock, you'd either shoot the grey card inside under a tungsten light, or outside in daylight with an 85 filter on the camera.) But you can use blue if you want to cool off the shadows more.

The card has to be underexposed. If you are using a spot meter so that whatever card you measure will be exposed at 18% grey in brightness, if you shoot at the stop the meter tells you, you probably want to underexpose from that reading by three stops or so (that's a total guess -- this is where testing using different stops of underexposure is important.) Also, keep in mind that the same flash will look heavier in scenes with a lot of black areas (like at night) compared to a day exterior scene. Basically you see the flash in the blacks.

If your meter says f/5.6 and you shoot at f/2.0, you are overexposing the card by three stops, not underexposing it. You would shoot at f/16 to underexpose it by three stops.

It takes very little flashing, hence why you need to test your method to make sure you aren't doing too much of it. But shoot a non-flashed test roll as a frame of reference so you can project the results of both the flashed and unflashed rolls to see the effect.

If you are going to flash the film and then rewind the roll and expose it again, remember that the framelines will be unexposed in the first pass, so the second pass has to line-up with the first. This is only an issue with 35mm since it uses 3 or 4 perfs per frame, you can't really misthread 16mm and not have it line up again since the perfs are at the top & bottom corners of the frame. But you would have to rewind the roll slowly in a completely dark room.

The flash will cause the film to start aging, so use the flashed stock right away, don't flash it weeks in advance of using it.

Exposing your scene correctly and consistently is important because the flash has its own exposure and then there is your scene exposure on top of that. So if you accidentally overexpose the scene a little, then the flash will look lighter in comparison, and if you underexpose the scene a little, then the flash will look heavier in comparison.

The actual f-stop used for the scene and for the flash doesn't really have anything to do with each other, what matters is the exposure needed.

I don't know what you mean by "push the film 1 stop down". Either you push or pull the film, but you wouldn't say "push down", you'd just say "push" (if anything you'd be pushing "up" and pulling "down" the film since pushing increases density and pulling decreases density.)

And since pushing increases contrast, I don't know why you'd want to push if the goal is to decrease contrast.
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#3 Francisco de Burgos

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Posted 23 January 2010 - 08:02 PM

First of all thanks, again sorry about my bad english, the part about pushing the film is like my translation of what I say in spanish, but now I know, and yeah I was a little distracted about underexposing the film, I know you're right, thanks for "getting me right". The film is a kodak vision 2 and I was thinking about using the 7205 that is a 250 D, this is 16 mm in an arriflex 416. The idea of the short film is to use mainly long shots in daylight.
I should ask again that if I overexpose a film 1 stop and pull it in the lab would give me more contrast, I was trying to get a little more information in the blacks, not creating more contrast, although this process is diferent of what I'm trying to do about flashing, again thanks for getting me right. I said about the blue cardboard because of the colors I was trying to reach in the film, to know if that would change any of the colors, and I never thought of using a grey card which now seems better, even so I'm doing both tests with the grey card and the blue one, telling later how it went.
Last of all I didn't quite understand why do you say the f stop is not the important part as the exposure needed,I would appreciate if you could please be a little more specific. I know the scene and the card should be exposed at what it's needed, different one of other.
Thank you again for answering me.
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#4 Francisco de Burgos

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Posted 23 January 2010 - 08:16 PM

First of all thanks, again sorry about my bad english, the part about pushing the film is like my translation of what I say in spanish, but now I know, and yeah I was a little distracted about underexposing the film, I know you're right, thanks for "getting me right". The film is a kodak vision 2 and I was thinking about using the 7205 that is a 250 D, this is 16 mm in an arriflex 416. The idea of the short film is to use mainly long shots in daylight.
I should ask again that if I overexpose a film 1 stop and pull it in the lab would give me more contrast, I was trying to get a little more information in the blacks, not creating more contrast, although this process is diferent of what I'm trying to do about flashing, again thanks for getting me right. I said about the blue cardboard because of the colors I was trying to reach in the film, to know if that would change any of the colors, and I never thought of using a grey card which now seems better, even so I'm doing both tests with the grey card and the blue one, telling later how it went.
Last of all I didn't quite understand why do you say the f stop is not the important part as the exposure needed,I would appreciate if you could please be a little more specific. I know the scene and the card should be exposed at what it's needed, different one of other.
Thank you again for answering me.


I should add this is a school project, I can't get my hands in a Panaflasher, low budget.
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#5 David Mullen ASC

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Posted 23 January 2010 - 08:44 PM

Pulling reduces contrast, pushing increases contrast. If you overexpose and pull-process by one-stop, you get LESS contrast.

What I mean about the f-stop not mattering is that I don't know why you are telling us what ND filter and f-stop you are going to shoot at because why does that matter to us? What matters is the exposure for the scene and we don't know that since we aren't there. What f-stop you use doesn't affect the flash unless it's the right f-stop for the exposure you want. For all we know, f/2.8 is the correct exposure or f/11 is the correct exposure; either way, as long as it's the correct exposure, it won't affect the flashing.

If you are using daylight stock then the grey card for the flash has to be lit with daylight balanced lighting (or if lit with a tungsten light, you need a blue 80A filter on the camera.)

Talk to your lab because some labs don't offer pull-processing for 16mm, only push-processing.

Obviously you'll need ND filters outdoors with 250D stock. Even with 50D stock, with no ND filters, you are normally at f/16 in direct sunlight on a clear day, at 24 fps with a 180 degree shutter.
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Metropolis Post

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Wooden Camera

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