How to get a 13-minute exposure?

Tjibs · Dec 4, 2007

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?

Dan Henderson · Dec 4, 2007

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.

Tjibs · Dec 4, 2007

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.

bdial · Dec 4, 2007

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.

Tjibs · Dec 4, 2007

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.

Ray Heath · Dec 4, 2007

that's easy tj

a 4x ND will reduce exposure by 2 stops

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

Tjibs · Dec 4, 2007

Ray Heath said:
that's easy tj

a 4x ND will reduce exposure by 2 stops

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.

Thanks for the calculation! I will try it out.

bdial · Dec 4, 2007

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.

Lee L · Dec 4, 2007

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.

Ray Heath · Dec 4, 2007

tj, RTF is the failure of the normal exposure "maths" at very short or very long exposure times

e.g. theorectically f2 @ 1/4 = f2.8 @ 1/2 = f4 @ 1 second = f5.6 @ 2 seconds = f8 @ 4 seconds = f11 @ 8 seconds = f16 @ 15 seconds = f22 @ 30 seconds

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

Ray

Steve Smith · Dec 4, 2007

To do what I think you are trying to achieve, you will need the moving slit to be just in front of the film plane rather than in front of the lens.

With a slit in front of the lens, the area of film exposed will vary with the aperture and may only approach what you are looking for when stopped down completely. A bit like the way a neutral density graduated filter only has an effective edge when stopped down.

Despite that, it sounds like an interesting thing to try out and I would be interested to see the results.

Steve.

Tjibs · Dec 4, 2007

bdial said:
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.

Hm, true. The tested exposure was with the slit in place, f5.6, no ND filters in place. I really didn't think of that, and it's so obvious. I'm sorry!
The slit is 0.7 or 0.8mm wide. I stopwatched it, and it moves 1cm exactly in 55,56 seconds.
The lens size is 49mm in width.
@ Steve:
It does indeed move right in front of the lens, not against the film plane. I thought of this as well, but I thought it was worth giving it a shot. What aperture would you suggest to at least get something close to this?

Thanks for helping me out so far, I didn't expect everyone to be so helpful!

Steve Smith · Dec 4, 2007

Whatever aperture you use, you are going to get some coverage of the whole frame regardless of the position of the slit.

i.e. you will not get a sharp slit image on the film which represents the slit in front of the lens. Think about the aperture blades in the lens. when you close them down, you don't get a small circular image, just a darker, full image. Selectively blocking light from the front of the lens has a similar effect (although the aperture blades are in the optimum position not to reduce image size/shape).

As an experiment, look through your viewfinder whilst pointing the camera at a well lit subject then cover half of the lens with a piece of card. You will notice that you will probably still have a full frame of image showing but at about half the previous brightness.

If you have a depth of field preview on your camera, you can use that to see what difference the aperture makes with regard to the edge of the card.

I think your method will make some interesting images but I don't think you will be able to make the distinction between areas which you are hoping for.

Good luck,

Steve.

Tjibs · Dec 4, 2007

Thanks for your reply.

I've tried holding a piece of black cardboard right in front of my lens, however, on all SLRs I've tried it on, I do not see this happen. The edges of the paper are blurry, but it does block out the right half of what I see through the viewfinder, while keeping the other half intact.

Lee L · Dec 4, 2007

Tjibs said:
It does indeed move right in front of the lens, not against the film plane.

In this case, the film is not exposed from side to side as time progresses. The entire film is being exposed for the entire time, it's just a different part of the lens capturing the image as the slit moves in front of it. Try looking at the projected image at the film plane if you can while the slit is moving (a bit of wax paper or something like a ground glass in place of the film will work to show this). You'll see that all you're introducing into the system is a moving vertical slit aperture, not a moving shutter.

So if what you're really after is a changing time frame captured on different parts of the film, your current set up won't work for that. You need to get the moving slit between the lens and film to achieve that. Closer to the film is better.

Lee

Tjibs · Dec 4, 2007

Lee L said:
In this case, the film is not exposed from side to side as time progresses. The entire film is being exposed for the entire time, it's just a different part of the lens capturing the image as the slit moves in front of it. Try looking at the projected image at the film plane if you can while the slit is moving (a bit of wax paper or something like a ground glass in place of the film will work to show this). You'll see that all you're introducing into the system is a moving vertical slit aperture, not a moving shutter.

So if what you're really after is a changing time frame captured on different parts of the film, your current set up won't work for that. You need to get the moving slit between the lens and film to achieve that. Closer to the film is better.

Lee

Thanks, this is what I thought would be a problem as well. I've used a piece of frosted plastic and held it at the film plane as I moved the piece of paper right against the lens. It certainly does block off the right side of the image! The other half darkens a bit, but is visible, while the other part where the paper is held, is absolutely black. I've tried to make a picture of it, but it's impossible to hold that many items and still make a decent picture out of the hand.

Lee L · Dec 4, 2007

Tjibs said:
Thanks, this is what I thought would be a problem as well. I've used a piece of frosted plastic and held it at the film plane as I moved the piece of paper right against the lens. It certainly does block off the right side of the image! The other half darkens a bit, but is visible, while the other part where the paper is held, is absolutely black. I've tried to make a picture of it, but it's impossible to hold that many items and still make a decent picture out of the hand.

What you're seeing here is the interaction between the internal lens aperture and the slit aperture you've introduced in front of the lens. If the internal lens aperture is wide open, the entire frame will see the light from the slit aperture.

In any case, this gives me an idea to help get what you want. Put a light tight box in front of the camera with the slit travelling across the side of the box opposite the lens. So you're shooting through a box with a travelling slit opposite the lens. Make sure the opening that the slit travels across is big enough to cover the angle of view of the lens and paint the inside of the box flat black. Here in the US, Krylon Ultra Flat Black spray paint is a good choice. Spacing the slit aperture away from the lens like this will restrict the lens' view to a narrow slit. As the distance from the slit to lens increases, the apparent sharpness of the slit edge will increase. You'll have to pick a convenient size for the box. This arrangement will give you an effect analogous to the slit travelling right in front of the film, and give you a different time frame as the slit travels from one side to the other.

The closer the slit is to the lens' internal aperture, the more of the frame will be covered by the light from the slit aperture. As the internal aperture on the lens is stopped down, the slit edges appear sharper and the light from the slit aperture will cover less of the film.

Lee

Tjibs · Dec 4, 2007

So this is basically what you are suggesting?
How big a box is 'convenient', what would the minimum size be for this to work?
I'm assuming the model will have to sit on the side of the box?

Thanks for the suggestion, looks like I have another chance before I have to switch to digital..

jeroldharter · Dec 4, 2007

The difference between 19 seconds and 13 minutes (780 seconds) is about 5 1/2 stops.

If you stopped down to f 8 1/2 that leaves you with about 4 stops to go. Add an 8x ND filter (3 stops) and a light yellow filter (~1 stop) and you are there.

Lee L · Dec 4, 2007

Tjibs said:
How big a box is 'convenient', what would the minimum size be for this to work?
I'm assuming the model will have to sit on the side of the box?

Thanks for the suggestion, looks like I have another chance before I have to switch to digital..

Looks to me as if you have the slot and camera at 90 degrees to each other. I was suggesting the box as a "spacer" to get the slit further from the lens, with the camera and travelling slit on opposing box faces. You'd need the internally blackened box to limit light to what passes through the slit and cut down on internal reflections and flare from inside the box with a long exposure.

By "convenient" I mean you'd just need to make it work with a manageable slit size and box size. You also need to make sure that the box and slit window are large enough for the angle of view of the lens at the distance it's set. Nothing tricky.

Lee

Tjibs · Dec 4, 2007

Lee L said:
Looks to me as if you have the slot and camera at 90 degrees to each other. I was suggesting the box as a "spacer" to get the slit further from the lens, with the camera and travelling slit on opposing box faces. You'd need the internally blackened box to limit light to what passes through the slit and cut down on internal reflections and flare from inside the box with a long exposure.

By "convenient" I mean you'd just need to make it work with a manageable slit size and box size. You also need to make sure that the box and slit window are large enough for the angle of view of the lens at the distance it's set. Nothing tricky.

Lee

Ah, okay! (With slit at the side, I thought you ment side of the box.)
That's actually quite easy! Thanks. Problem solved, I suppose. I'll try to post results later on.

Lee L · Dec 4, 2007

Good luck. Let us see your results. I suspect you and your model will learn a new definition for the word patience.

BTW, for adjusting your exposure time you could take a lesson from the standard focal plane shutter and use slit width as a control for exposure.

Lee

Steve Smith · Dec 4, 2007

I'm sure that the box idea Lee suggested is used in cinema photography to 'slide' one scene into another. I can't remember what it is called though.

Steve.

Lee L · Dec 4, 2007

Steve Smith said:
I'm sure that the box idea Lee suggested is used in cinema photography to 'slide' one scene into another. I can't remember what it is called though.

Steve.

I think the term I've seen used for the transition effect is a "wipe". I hadn't thought about how it was done, but assumed it was in editing with two different pieces of film as input. Haven't heard of the name for a box to create the effect. For the cinematic transition you'd use only one curtain at a time for "wiping" each scene in or out rather than a travelling slit. You'd still need one leading curtain and one trailing.

Lee

Steve Smith · Dec 4, 2007

Thats exactly what I was thinking of. I can remember Stan Laurel 'pulling' the next scene into the frame and pushing the old one out.

As you say, in this case it is a single curtain but I imagine a slit could be used as well.

Steve.

How to get a 13-minute exposure?

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