I'll be shooting some footage of city streets and highways (some of it from inside a car) at dusk. The intention is to get it as "dusky" as possible, which I think gets me to the very limit of the lense and filmstock (f2 at 320 EI for 500T stock). The reason for that is to hopefully get at that point when the light levels of sky and sodium street lamps meet, so as to not overpower each other, but provide an even light level with changes in color only. But when it gets this dark, it gets even darked in the viewfinded, especially with a 85B filter in there. So I was thinking of maybe using 250D instead. I lose a third of a stop compared to 500T with a 85B filter, correct? I doubt this is a noticable loss with modern filmstocks; I mean if I had to rate 250D stock as 320.
So any other considerations here? In case I want to go deeper into the dusk, how much can I expect to get away (without pushing) with slight underexposure (about 1 stop).
I could be doing it wrong... so wait for other comments... but...
When shooting anything within the golden hour and after I use tungsten balanced film anyhow. Your light at sunset and sunrise is very yellow/orange and is not as bright white as daytime. Daylight films are balanced for broad daylight. Your sunset light will be very close to tungsten.
In your scenario, I would likely shoot 500T at around F4 depending on light meter readings or maybe 200T wide open.
Assuming you are doing a DI, all of these things can be corrected for in post. You can even shoot 500T wide open for thirty minutes before and 30 minutes after sunset and get very good results with some post adjustment. I also enjoy using 500T for bonfires/campfires without additional light.
Generally dusk stuff wouldn't be at key exposure anyway, you can decide that the look should be, let's say, 1-stop, 1.5-stops, or even 2-stops under, it just depends on how dominant you want the dusk light to be compared to practical sources. For example if most of the light is coming from the sky, then probably 1 to 1.5-stops under would feel like dusk instead of overcast day, but if the scene were people standing around a campfire or next to a brightly lit store or had a lot of car headlamps, then the dusk could be dimmer in comparison since the practical sources are adding highlights.
I usually use 500T without the filter or with an LLD filter because you usually want to keep shooting until you run out of light and you can't open the lens up anymore. Also, if you are going to be shooting into a lot of car headlamps and streetlamps, then it is safer to not have a filter in front, though early into the dusk, you'll probably start with ND filters.
The only reason to use 250D would be if you wanted to emphasize the orange-ness of the sky and sodium street lamps rather than get a cool skylight look, but then you'd have to live with stopping shooting 1-stop earlier than if you used 500T. So it just depends on how late into dusk you want to shoot.
Maybe I should have been more specific about what I mean by "dusk". I did some measurements a couple of weeks ago. I can't remember the time exactly, but it was after the sun had set. The reading was about f1.4 at EI of 320. This is the kind of light I'll be shooting with. And David, you are right, I forgot about that, I need to underexpose by at least a stop. With a f2 lens, that leaves me right at shooting full open on a 250 or 500 ISO stock, give or take a third of a stop.
As for using (or not using) filtration, I understand completely what both of you are saying, but I'm not sure the light is very warm at the deeper levels of dusk I'm speaking about. In fact, it gets quite blue, and I'm a bit worried that it might get too blue without an 85B on a tungsten stock. For example, I've taken exposures with daylight E6 still film at such conditions, and often I have this problem that the image on film is a little too blue as I get deeper into the dusk, even though it's a daylight stock. I certainly never have problems with the film being too warm. So we are talking about late dusk.
Later on I'll try to find some images which are representative of the light conditions (and general look) I'm talking about.
I also didn't say anything about the workflow. It's going to be shot on 35mm academy, and It will be telecined to HD. That's the final product. But I will make some contact prints just to see how this dusk footage turns out in prints.
Here are some examples of the look I'm trying to achieve, and also these photos show the exact light conditions, and also some of these locations are where some of the footage will be shot.
The first two images are the typical example of the light conditions I have been describing. Notice how the blue light of the sky mixes with the sodium lamp light in the first picture (I don't know what type of light fixtures are in the second picture), and the result is a pinkish tone. Whatever type of filmstock I use, I'd like to be able to capture this pinkishness. It will be easier when it gets graded digitally, but I'd also like to see that captured well in contact prints.
there is a lot of red/magenta here because of the clouds, which are illuminated by the sun (which had already set at that point). So I guess this has to be treated differently then the first two cases. But then again, in this case I'd also like to capture some redness, but not too much; just as it is in this digital picture.
Twilight is generally very blue, particularly on a clear evening. I think the issue is whether the noise from pulling the blue out of a 500T shot looks worse than the grain from the same moment shot one more stop underexposed on 250D. Exposure = information so I would think that the 500T shot, if one stop denser than the 250D shot, might actually contain as much red information but just have too much blue information. On the other hand, the 250D stock is a stop slower and thus less grainy so might hold up to being less exposed than the 500T shot.
Keep in mind that the sodium lights are going to read orange-yellow either way because they are a narrow spectrum source, just a bit more orange on daylight stock.
It's pretty blue but it is correctable to neutral. It's not the same thing as color-correcting a JPEG from a digital still camera or an 8-bit 4:2:0 HD recording because there are layers of color information that can be brought out in a film negative. But you certainly would have a more neutral color to start with by using the 250D stock, it all depends on whether you need that last stop of exposure at the tail end of twilight - often if you are shooting a movie with a dusk scene, you can be pretty sure you'll still be rolling long after you should have stopped. No one wants to hear "time to quit -- we've run out of light!"