Archive for November 17th, 2011
The macro lens adapter took shape around a nice 25 mm doublet lens from the Box o’ Lenses. The Canon SX230HS has a lens opening that’s just about 25 mm in diameter and a larger lens would be better, but at maximum zoom the image pretty much fills the camera’s entrance pupil. I ordered a pair of 50 mm LED ring lights from halfway around the planet and built the snout to hold the ring around the lens:
That’s the first pass to get the sizes right and work out some details. In particular, that small white ring inside the aperture below the lens didn’t work at all, so I made the main tube opening a bit smaller.
The solid model shows the details a bit better:
The inner cone shields the lens edges from (most of the) scattered LED light. I considered angling the side walls to concentrate the ring light, but wasn’t convinced it’d be worth the effort because the LEDs have such a broad beamwidth anyway. The little hole is for the LED power cable, which goes to a 12 V switching wall wart. The 5 strings of 3 LEDs draw about 70 mA, which suggests I should hack the ballast resistors down a bit to boost the current up to 20 mA per string. FWIW, the resistors give 25 mA per string at 13.8 V, so I could probably goose the current a lot higher.
The bottom has four shallow holes for the alignment pegs cut from ABS filament:
The hole in the front end of the main tube is marginally smaller than the lens diameter, as I used the OpenSCAD cylinder primitive instead of the PolyCyl module that slightly enlarges holes to make the answer come out right. The difference is just enough to form a solid stop that aligns the lens and prevents it from sliding into the body before the glue cures.
The whole affair looks pretty scary from the victim’s point of view:
But the camera’s view seems OK, albeit with some vignetting:
Limited by vignetting & entrance pupil filling, zooming controls the horizontal subject size from 25 mm to about 10 mm. Depth of field is a few mm, at best; the printing on the far end of that battery is fuzzier than it seems.
Best results so far come from:
- Manual aperture at f/8
- Manual focus at infinity (move the subject to focus)
- Shutter delay = 2 seconds to let the camera stop shaking