I have seen photos like this https://flic.kr/p/PwNpFq where the foreground is in focus but the background has double elements. Is this just what the lens does or is there a technique which achieves this? Any ideas?
A lot of people think that this shot is achieved through Photoshop, but that is not true.
You can replicate this effect by shooting through grass at the object. You will need to have your aperture wide open, say f/2.0, and have grass physically between the lens and the subject. The grass will effect the bokeh in such a way that it gets the vertical lines. Illustration:
Lens Grass Flower Grass |] ||||| * ||||| ||||| | ||||| Foreground Background
To support my answer, if you look at the flower, you can see that there is distortion created by a heavily out of focus object between it and the lens. That is part of the grass in the foreground
I got similar effects by placing an opaque object just in front of the lens. You can use a piece of tape. Place the strip of tape right across the middle of the lens as close to the lens as possible. Since I discovered this by putting my extended finger across the lens, I use a strip of tape approximately the same width, more or less. Different widths will produce different effects. The angle I used was vertical which put two out of focus (bokeh?) images right-and-left of each other. Horizontal would split the OOF images into upper-and-lower, etc.
The objects in focus are almost unaffected while foreground and background images will show the effect to a greater degree.
Experiment with image-blocking opacity, width, and angle for more departures.
The photo in question was shot on an APS-C camera (Canon 7D) with long (300mm / f4) lens.
Even though it is likely heavily edited the key contribution to the Bokeh (tm) was from the focal length used.
As the poppy seems to be rather close and the lens has minimal focusing distance of 1.5 meter it might have required an extension ring and/or some cropping.