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Thread: Slit camera

  1. #1

    Slit camera

    No pictures right now as I'm just off to bed. Try post some tomorrow. But a quick question. I have just constructed a new camera. Cardboard box + stanley knives. This is my very first slit camera. The measurements weren't quite perfect. I was aiming to create a gap between the blades measuring 0.47mm but in the end settled for 0.44mm. The focal length was originally 150mm but having lost some on the lens I decided to mount it inside on mountboard and ended up with a focal length of 143mm. My calculations led me to an f stop of 325. However that is assuming a pinhole and what I have is a slit measuring 5cm. Is the slit going to bring the f.stop back into a more familiar figure [325 is waaay out there] or does it make no effective difference? But if there is some difference, how is it calculated?

    I'm planning to shoot on flashed paper with an anticipated ISO 6 which is looking like sunny day exposures in the 1 + minutes region.

    I'm dead excited but got to go to bed and then to work and then ... it'll be dark when I get home. Grrrrrr.

  2. #2

    Slit camera

    The single slit 0.44 mm x 50.0 mm will expose the ISO 6 paper as you have planned.

    I'd be anxious to see the results. Your endeavor is definitely outside normal pinhole photography, and for this I applaud you.

    The usual approach is to use two stacked perpendicular slits to form a double slit camera. See:

    http://pinhole.stanford.edu/ds.html

    and,

    http://f295.f295.org/uploads/Blah.pl?m-1144888858/s-15/

    Your calculations would be correct except for the following:

    1. A square pinhole has 27% greater area than a round pinhole that would fit inside it. This introduces about a 1/3 stop decrease in exposure. That alone would not affect your design significantly.

    2. Black and White paper records actinic light in the green-blue-UV spectrum. B&W paper does not respond to red/yellow light and that's why they make darkroom safety lights in those colors. So instead of 550 nm (green) in your Raleigh Equation, I might try 430 nm (violet). I'm sure there are others at this forum that might better guide you on this aspect.

    3. The single slit aperture as you have described is quite a bit longer than a simple 0.44 mm square pinhole. The slit area is 114 times larger [114 = 50 mm / 0.44 mm]. If the luminous flux doubles with each doubling of aperture area, then there is a 6.84 stop difference [114 = 2^6.84]. So your 1 minute sunny day exposure should actually be in the neighborhood of 1/2 second.
    Throw in the 1/3 stop exposure decrease from above and you are probably closer to 1/3 second.

    But the design you propose is intriguing in another regard. The pinhole dimensions discussed at this forum are chosen so the wave fronts diffracted by opposite edges of the pinhole either reinforce or cancel each other when arriving at the focal plane. What results is something new - greater definition. Because most builders use round pinholes, light from any direction results in the same increased definition. Nice round pinholes make for high definition photographs. On your camera, the greater definition will only occur perpendicular to the slit. However, the light in the direction of the slit simply forms a blur.

    But even blurs of any photographic interest will have dark patches. So it is possible that in amongst all those nice, long blurs you will have patches of something interesting. I can't visualize what you're doing in Black and White and keep seeing it in color, like a kaleidoscopic rainbow.

    The attached photo was taken from promotional material for a single slit aperture once offered by 8 Banners. No dimensions were given but the slit appears to be shorter than what you propose using. Orientation of the slit is uncertain.

    I couldn't find a clear photo for a side-by-side comparison. But from other less blurry examples in the same promotional material I was able to determine the following:

    The red and gold thing in the center is a pack of cigarettes, the black and silver thing to the right is a hand-held calculator, and the black thing on the far left appears to be a detached SLR lens. The dark coins in front of the cigarettes are completely blurred.

    Of course the dust was added, on purpose, to achieve needed dimension.


    [img]aje_slit_pinhole_1494.jpg[/img]

  3. #3

    Slit camera

    It's been a weak of bum exposures and wasted paper. As far as this camera goes I'm now shooting at f.32 and god knows what I've created, it's prooving a very difficult camera to tame. In fact I'm beginning to wonder if it is at all possible to pull anything out of it that is even slightly interesting.

    Below it the best of the worst. amazingly, for the first time, I have an image in which I can discern something. If anyone cares - and I don't mind if you don't - you are looking at some 120mm wooden decking panels (this is clearest in the upper left area) and in the middle of the leftside frame my hands are held out splayed (these are barely discernable yet they are there.)

    Now that I have something, I might be able to start figuring it out a bit better. Being that it shoots around f32 maybe even f22, I at least have speed on my side. My early exposures ran at over 20 minutes and developed to pure blacks.

    Anyway thanks SlipRing, you gave me much to ponder and mull over as I tried to make sense of this strange object. One thing I'm drawing towards now is that this is to be used as a telephoto with a minimum 5m distance from subjects. Another try tomorrow ...

    By the way the paper is not preflashed. Attached files

  4. #4

    Slit camera

    Here it is. I had problems resizing which is why I never got it up sooner.

    Attached files

  5. #5

    Slit camera

    I'm glad that things turned out as well as they did. f/32, f/22 looks about right.

    What's surprising is the amount of detail. Especially along the left edge.

    I kind of see where the black and white thing is going. Its growing on me, I kind of like it.

    Just a thought: How about a shot directly into the sun?

    Pinhole flare can be interesting. Light gleaming off those Stanley blades might even be more interesting.

    Good luck,

    SlipRing

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