terichie Loc: Ma.

I just got my first macro lens. Some of my pictures have this octagon shape. What is causing it?
Shooting with canon rebel t2i
canon 100mm macro lens
f/8
shutter 1/100
ISO 100
shooting manual mode

St3v3M Loc: 35,000 feet

The 'octagons' in background are known a Bokeh and can be very pleasing to the eye.
http://en.wikipedia.org/wiki/Bokeh
"In photography, bokeh is the blur, or the aesthetic quality of the blur, in out-of-focus areas of an image, or "the way the lens..."

terichie Loc: Ma.

I am aware of bokeh. I was worried, not to sound like an idiot, I was thinking something was wrong with my camera. The octagon just reminds me of the aperture opening. When taking pictures of fireworks I sometimes get a huge octagon right in the center of the picture?????
Thank you for helping. I still consider myself a beginner. Just teaching myself as I go. :-)

St3v3M Loc: 35,000 feet

It almost looks like raindrops in the background.

martinfisherphoto Loc: Lake Placid Florida

Exactly. My Tokina does the same thing. Not the most pleasant affect but result of the way your lens is build. Read the section on Bokeh, for your lens. It mentions the octagon shapes. http://www.kenrockwell.com/canon/lenses/100mm-macro.htm

Blurryeyed Loc: NC Mountains.

It is the aperture ring, bokeh does not refer to the out of focus blurry area but rather to the quality of the roundness of the small out of focus light sources. Most people understand that the quality of the "Bokeh" is supposed to improve with the number of blades in the lens iris, or even better with curved blades... this is because the small out of focus spots of light appear as circles rather than sided objects... Many people confuse out of focus areas with bokeh and that is not correct, it is the spots of light, in this case the octagon looking spot as you so well described it. Don't worry, that lens with its 8 blades is producing pretty good bokeh.. ;-)