Hi, I'm curious about something I noticed today in one of my photos. Here's a bit of background info: The attached photos (one is cropped to show a close up of bokeh) were taken with a Canon 100mmL lens (Rebel T3i camera). The settings for the water droplet are f/8, 1/200, ISO 200 and for the sunflowers: f/8, 1/320, ISO 400. If you take a look at the top left in the close up photo of the water droplet, the bokeh looks octagonal shaped. Yet, the bokeh looks circular in the sunflower photo. This is the first time I've seen the octagon shape. If bokeh is, among other things, the shape of the lens aperture, why would it appear round in one photo and octagonal in another using the same lens and similar settings?
"...bokeh is, among other things, the shape of the lens aperture, ..."
You are forgetting the "other" things. Bokeh is effected by the size of the aperture, by the number of aperture blades along with the shape of the blades. Besides that, the distance from the light source and the DOF in the shot will effect bokeh. Even the type light, diffused or pinpoint will change it.
B&H has a pretty decent basic explanation of bokeh with a link to more information. Google B&H bokeh.
Under some conditions you can also get light reflection from inside the lens assembly with high brightenss off angle points of light in images which projects back onto the subject and further complicates matters.
Any lens will give round bokeh shooting wide open. When you stop down you start to see the shape of your aperture blades. The blades will form a more round shape if they are rounded rather than just straight. It also helps with roundness if there are more blades rather than fewer.
You can even make funny shapes by putting a black card over the lens with a shape cut into it. Stars, triangles, airplanes, kitties, whatever you like. Look at Bokeh Masters.
All lenses are not created equal. Even no aperture blades dose not guarantee perfect bokeh
Moreover, there's no unanimous agreement as to what constitutes perfect bokeh. Among those with strong opinions on the subject, arguments erupt from time to time.
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