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I'm about to make some long exposure photos, but I don't want them to have this star effect on light sources. Can I avoid that with any filter? For this shot I used a Canon 18-55mm lenses. Note that I didn't use the star filter, but used a ND 8, f/32, ISO 100, 60s of exposure. Thank you.

The image is at this link http://on.fb.me/OlaV3Y

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3 Answers

up vote 7 down vote accepted

Its probably due to a narrow aperture. On professional grade lenses, you can usually stop down a bit and still maintain a rounded aperture, however on cheaper lenses, or on all lenses at very narrow apertures, the opening becomes polygonal. That causes the diffraction of light as it passes through the aperture to produce a star pattern (the exact nature of which depends on the number of sides/points in the polygonal opening.) Use a stronger ND filter or and/or a lower ISO setting to reduce the effect.

Given that you are using the kit 18-55mm lens, its unlikely the diaphragm uses rounded blades. Star patterns will start to exhibit pretty early, and get stronger the more you stop down. If you have the cash, you could try getting a higher quality lens. Most of the L-series lenses have rounded aperture blades, which eliminates the effect at the wider stops, and mitigates it in the middle stops (not much you can do once you get down to f/11, f/16, f/22 though.)

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Thank you very much for your answer/explanation. Yes, I used a very narrow aperture. f/32. I'll probably buy another ND8 or an ND fader and use larger aperture though I'll lose depth of field, and I didn't wanted that. I was looking for a L-series lenses, but kind of impossible right now for it's a bit expansive. Thanks for the aperture tip for I probably wouldn't try using other than the narrowest value and I'll try to improve the equipment in the future. – Luis Carlos Aug 13 '12 at 19:25
If you really need deep DOF, you might want to look into a tilt/shift lens. Canon makes at least four of them in several focal lengths. With tilt, you can change the nature of the focal plane, and effectively make it stretch back to "infinity" at a wide aperture. They tend to be fairly expensive, however if thats the kind of photography you do, its definitely worth it. T/S lenses are very flexible for a wide variety of uses as well, including architecture, landscape, etc. – jrista♦ Aug 13 '12 at 20:00
up vote 7 down vote

I think the main cause of that effect is using a very small aperture. If you're trying to get a long exposure by stopping down to f/22 or smaller you'll most likely get that effect.

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up vote 1 down vote

You can make a physical mask with a circular hole much like people use to make the "shaped bokeh." This just becomes a larger pinhole camera.

Challenge becomes how to make a nicely round hole in a fairly sturdy material. Consider using aluminum foil pierced with a pin or other small device.

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Clever! But where, precisely, should this mask be placed? Moreover, it's unclear that aluminum foil will work at all: even small irregularities seem likely to produce similarly irregular stars in the photo. And wouldn't a perfect pinhole produce a halo of blurred light around each point of light? – whuber Aug 14 '12 at 18:19
Yes, a pinhole would produce a diffraction effect. Still, the circular hole doesn't have to be tiny - it could be larger, so long as the sides are smooth. The mask would be placed over the front element of the lens and the lens would then be wide open. Aluminum foil is cheap enough to make trial holes to perfect a technique. – smigol Aug 14 '12 at 22:53
I tried it and it sort of works. The image quality is heavily degraded with small holes and the radial falloff is intense. I can't see any practical way to make truly smooth large holes. But perhaps an alternative, such as a machined disk (think of a washer) might work. – whuber Aug 15 '12 at 13:04

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