It's all in the slider.
Lensbaby adds a twist to its latest special-effects lens, the Burnside 35. In addition to the native swirly bokeh like that of the Twist 60, there's a second aperture with which you dial in the degree of vignetting and saturation.
The second aperture closes above the first, which adds vignetting and allows in less light, making the image look darker and more saturated.
Because it has six blades instead of eight, the out-of-focus highlights also have a slightly different quality.
The lens has a traditional round eight-blade aperture; the effect slider has a polygonal six-blade aperture that overlays the first to cover the edges of the frame.
The lens comes in Canon EF, Nikon F, Sony A, Pentax K, Micro Four Thirds, Sony E, Fuji X and Samsung NX mounts.
The design of the Burnside is most like Lensbaby's Velvet line, all-metal with no drop-in lenses like the company's lower-end lenses.
Like the Velvet lenses, the focus ring has good tension, and you do have to rotate it a lot to get from one end of the focus range to the other.
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