I really need some advanced help, I can't seem to figure it out myself.
Never really was a star in mathematics anyway.
I've done some testing in a situation, and I can most likely shoot the shot I want on f5.6 (1:1.7 50mm lens) in 19 seconds, correctly exposed.
However, I want to have a 13 minute exposure (Any f/stop will do, but around 5.6 would be the best if possible.) to get the desired effect. I have 3 ND 4x filters I can work with. How can I achieve the 13 minute exposure?
why do you want exactly 13 minutes? Can it be done at night? A year or so ago I did some full moon photography, and was happiest with 8 minute exposures at f/8, with FP4 film. I suspect that with reciprocity, 13 minutes would provide a usable negative as well.
What I'm basically trying to achieve is what a scanner does. I've made black plastic slides with a narrow slit in the middle, which will move from one side of the lens to the other, in 13 minutes. So if correctly exposed, I should just get a complete image. I hope that made any sense at all. I'll be using Ilford HP5 film.
I'm willing to try different f/stops / ND filters, so any pointers would help as well.
More or less your "shutter speed" is the time it takes a side of the slit to cover the width of the slit. So you need enough ND to get a correct exposure at that time at f5.6.
At least if the sides of the slit are sharp in the viewfinder, the whole frame isn't getting 13 minutes with of light.
Yes, that is indeed what I'm looking for. I've timed the slit-movement, and it takes 13 minutes.
The sides of the slit aren't going to be sharp, since I will be focusing on something behind the slit.
I have 3 ND 4x filter to use. If it's impossible with that alone, I'll have to use a higer f/stop.
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so, if you use 2 of them, your f5.6 @ 19 seconds becomes f5.6 @ 304 seconds or about 5 mins (19x2x2x2x2 or 19 plus 4 stops)
if you then allow for RTF (Reciprocity Time Failure) your 5 minutes becomes about 12 or 13 minutes
is your idea somewhat like a falling shutter?
what is your subject?
what do you hope to achieve?
Ray
My idea is more like a scanner. As subject I will have be using a model
that will be asked to stand still and hold that nice, tough, or cute pose for 13 minutes. As with a scanner, you will see that everything on the left side of the picture will be taken relatively early in those 13 minutes, and the pose still holds, while at the other end, you'll have more of the picture at the end of the 13 minute range, and the pose will be a little off. I'm hoping for an uncomfortable look at the right side, and your average portrait look at the left side. I'm not sure if it will work, but it's worth a shot.
What I mean is that your effective exposure is not 13 minutes unless the slit uncovers the entire frame for 13 minutes.
For example, if the slit is moving from right to left, your effective exposure is the time it takes for the right slide of the slit to cover the width of the slit. On a large scale, if the slit is one inch wide, and the slit moves at one inch/second, you have a one second exposure. Since this is in front of the lens, the light is spread, so the actual exposure will be a little more.
How much more will require some experimentation.
Where is this moving slit relative to the lens and film?
If it's too far away from the film, or in front of the lens, it seems to me that it would work like a moving aperture exposing the whole frame through only part of the lens rather than a shutter slit blocking all but a thin strip of film. So your exposure length calculations would be off. Is the slit width variable like a focal plane shutter? In the case of a moving slit just in front of the film, your exposure length is not the time it takes for the shutter to make the full trip, it's the time it takes between the leading and trailing edges of the shutter to pass the same point on the film. That again throws off your 13 minute calculated time exposure.
More info on the mechanical set up would be helpful for those giving advice.
Lee
editing while bdial posted, but we're both getting at the same potential problem concerning the real exposure time.
but at exposures of longer or shorter than a certain time, which varies from film to film, the theoretical time will not be long enough, typically consider RTF at shorter than 1/1000 and longer than 1 second
RTF data should be available from the manufacturer
How to get a 13-minute exposure? 
Linear Mode
