do i really need a 10 stop nd filter if I want to do long exposure film photography like for 1 to 4 hour in the pitch dark how can one decide if he needs a nd filter or not in various situations like when we have really dark sky and some portion of road illuminated with road lights
Filter Factors to the rescue: By tradition (and some science) the basic unit of exposure change is the f-stop.
This is a 2X incremental chance in image brilliance. In other words, each f-stop change is a doubling or halving of the light energy that comprise the exposure.
This being true, we can state that if a filter that attenuates 1 f-stop of light it can be described as having a Filter Factor of 2. If a filter attenuates 2 f-stops of light, its Filter Factor is 4.
Another way to compute a Filter Factor, if the number of stops of attenuation is known, we elevate the number 2 using the number of stops of attenuation as an exponent. Using this method a table of Filter Factors can be conducted.
1 f-stop = 2^1 = 2
2 f-stops = 2^2 =4
3 f-stops = 2^3 = 8
4 f-stops = 2^4 = 16
5 f-stops = 2^5 = 32
6 f-stops = 2^6 = 64
7 f-stops = 2^7 = 128
8 f-stops = 2^8 = 256
9 f-stops = 2^9 = 512
10 f-stops = 2^10 = 1024
Now the FF (Filter Factor) can be used as a multiply factor that computes a revised shutter speed.
Suppose your unfiltered exposure is f/8 @ 1 second. You desire an exposure time of 64 seconds. You mount a 6 stop ND and the revised exposure time is the unfiltered time multiplied by the FF. Thus 1 X 64 = 64 seconds.
Another example: Unfiltered exposure time is 1/100 of a second (written in decimal form as 0.01 seconds. You mount a 10 stop ND. The revised exposure time is 0.01 X 1024 = 10.24 seconds.
Reciprocity failure is a photo film phenomenon. We must add time if the exposure is prolonged. Digital sensors have their problems but not this one.
Well, you can do some exposure calculation to see what you need.
Let's say you set up your shot and the proper exposure is ISO 400, f/5.6, 4 seconds.
You can move two stops with the ISO, and if the depth of field you want permits, you can get another two there by going to ISO 100 and f/11, respectively.
That'll move your shutter speed to 00:01:04. (64 seconds)
But if you want hours, then keep doubling and count the stops and THATS the ND you'll need. For example, adding another 6 stops gets you just up over an hour.
But, let's not forget about film reciprocity failure. Each film behaves differently but the gist is that the film will take exponentially longer to actually expose at longer speeds. So, you may not want a 6 stop to get over an hour. In fact, given your particular film and and an exposure of a few minutes (calculated) - the real exposure might already stretch into long minutes or hours.
Edit to add: Reciprocity Failure gets really tricky. Especially when you're talking about moving objects, like stars, simply because the recorded light has to be enough before the star moves to actually be recorded, given your exposure settings.
In the case of car lights - you would want to calculate the exposure for the background. This time would get your background properly exposed. ANY car light is going to be many, many stops brighter than the background and will most likely start getting recorded.
That's assuming you're not using any ND filtration. If you are, because of reciprocity failure - it may take many, many cars to get a light trail to expose. It's quite possible that, given enough ND filtration, you don't get any light trails.
Unfortunately, the only real way to get a feel for this will be to go to your intended spot and shoot it. Start with no ND filter and see the results of different shutter speeds. Add on ND as you wish and experiment. Though, I don't think you will ever need a 10 stop filter at night.