Effect of macro lens aperture on background lights

[priller]

Hmm, sounds like the title of a research paper, doesn't it? I guess it is a little research on my part. Using my old manual-focus macro lens, I was taking some photos of my Mom's many fun little Christmas ornaments, using the Christmas tree lights for a background. I noticed that when I had the lens stopped down a little (say f/4 or f/5.6), I could clearly make out the aperture shape in the background lights. For example:



Whereas if I kept the lens wide open (at f/2.8), the lights appeared nearly perfectly round:



I really like the round look, but a 105mm macro lens at f/2.8 is almost useless, since the depth-of-focus at that aperture is literally paper thin. However, if I backed away from the object I was photographing a good bit, then the depth-of-focus increased (and with 10.2 MP to play with, you can crop down pretty small and still not lose too much detail). However, when I did that, the cool blurring effect on the background lights decreased. But, if I then moved the object further away from the tree's lights, then that restored the blurring effect.

So it made a pretty funny sight with me moving furniture to get a clear line of sight to the tree lights, laying on the floor about 10 ft away, with the ornaments propped up on a box about 6 ft away. And why was I doing this, my parents asked? To get a "close up" shot! "I'm sorry I asked", said Mom.

[WannaJumpMyScooter]

1. Yes, and there have been books written on

just this subject.
You are seeing the shape of the lens diaphragm, the actual aperture of the camera.

The wider open it is, the closer to round.

Make sense?

[Blue_In_AK]

2. Cool pictures - I really like the second one.