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.
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!
Last edited by Tjibs; 12-04-2007 at 08:22 AM.
Reason: Added extra information. Hit reply too soon.
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.
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.
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
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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.
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
Last edited by Lee L; 12-04-2007 at 11:15 AM.
Reason: reworded for clarity
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..
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.
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.
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