Can a very long pinhole tube give enough resolution to photograph the Sun on an APS-C DSLR?
Asked 5/23/2017
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I’m wondering whether a pinhole setup could work like a very long focal length “telescope” for solar imaging on an APS-C DSLR. For example, using a black sealed tube about 1500mm long with a pinhole at the far end. One pinhole calculator suggested about a 1.6mm hole for that length, which would make the Sun’s image close to filling the frame.
My concern is image sharpness. I’ve read that the blur/circle of confusion from a pinhole is on the order of the pinhole size, and 1.6mm seems far too large for a DSLR image that would be enlarged for viewing. Would making the pinhole much smaller, such as 0.1mm, improve the resolution at this long distance, or would diffraction make it worse?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
9y ago
2 Answers
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The max size of the circle of confusion should not exceed 1/2 mm when viewed by the unaided eye at 250mm viewing distance (reading distance). Miniature cameras need the circle to be much smaller because typically we enlarge this image. Most tables and charts are based on making an 8X10 inch print. The 35mm camera image must be enlarged about 8X to achieve. Thus the circle size needs to be 0.0625mm to achieve. The APS-C format is 66% of the size of the full frame. Thus the image needs to be enlarged 12X to get the 8X10. Now you are talking a circle of confusion size of 0.04mm.
Because of diffraction and interference we abandoned the pin-hole for a lens. A lens suffers from the same maladies but now we can enlarge the aperture and gain resolution. We can get about 25X to 35X per inch of aperture with a good lens.
Originally by user44949. Source · Licensed CC BY-SA 4.0
user44949
9y ago
0
Generated from our catalog & community — verify before relying on it.
A longer pinhole “focal length” will make the Sun’s image larger, but it won’t make it sharp enough to behave like a real long-focus lens or telescope.
With pinholes, blur comes from two competing limits:
- a larger hole increases geometric blur
- a smaller hole increases diffraction blur
So there is an optimum pinhole size for a given tube length. Making the hole much smaller than that does not keep improving resolution; diffraction takes over and the image gets softer again.
For APS-C, the acceptable blur on the sensor is quite small because the image is usually enlarged a lot for viewing or printing. A blur on the order of tenths of a millimeter—or worse, 1.6mm—is far too large. That means a pinhole solar image may be visible, but not detailed.
In practice, pinhole cameras were replaced by lenses because lenses let you use a much larger aperture while controlling the light, giving far better resolution. If your goal is to record useful solar detail, a proper lens or telescope with appropriate safe solar filtration is the practical solution, not a very long pinhole tube.
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