How large should an oval aperture mask be to create stretched bokeh on a 50mm f/1.8 lens?
Asked 3/7/2011
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I want to make a DIY front-of-lens aperture mask to produce vertically stretched, anamorphic-style bokeh with a Canon 50mm f/1.8 on a crop-sensor DSLR. The idea is to cut an oval opening in black card or vinyl and place it over the front of the lens.
How do I estimate the size of the oval opening? Is there a way to calculate the maximum useful size for the mask based on focal length and shooting aperture, so it actually becomes the effective aperture shape instead of being ignored by the lens? I'm mainly looking for guidance on sizing; I understand the exact oval proportions are somewhat subjective and may require experimentation.
Originally by user4192. Source · Licensed CC BY-SA 4.0
user4192
15y ago
2 Answers
6
- Well "best" is an extremely subjective term and will largely be determined by individual taste.
- In general, what you're describing sounds alot like a Bokeh Master
- Your cutout must be at least smaller than the real aperture you'll be shooting at (shooting at, not the maximum for your lens). This is because you need it to become effectively the new aperture shape. If you're shooting at 50mm f/2 - then it must be at least smaller than 25mm across. If you have a 50mm f/1.8 and are shooting wide open, all the time - then its 50/1.8. If you have a 50mm f/1.8 lens and are shooting around f/2.8, then its 50/2.8. In order to keep as much light, you want it as large as possible without going over (focal length / f-stop).
Originally by user1917. Source · Licensed CC BY-SA 4.0
user1917
15y ago
0
Generated from our catalog & community — verify before relying on it.
Yes: the mask opening must be smaller than the lens’s effective aperture at the f-stop you’re using, otherwise the lens aperture will dominate and your cutout won’t define the bokeh shape.
A useful estimate is:
aperture diameter ≈ focal length ÷ f-number
So with a 50mm lens:
- at f/1.8: about 27.8mm
- at f/2: 25mm
- at f/2.8: about 17.9mm
Make the oval’s largest dimension no larger than that value for the aperture you’ll shoot at, and usually as large as possible without exceeding it, to keep more light. The exact oval proportions are a creative choice and are best refined by testing.
In practice, cut the mask from opaque black material, mount it cleanly over the front of the lens, and shoot with out-of-focus highlights in the background. This creates anamorphic-style stretched bokeh, but it does not produce true anamorphic squeeze—only the bokeh shape (and, with some products, flare effects).
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