Bokeh question for Steve.............

[MARINERMAGNUM]

My wife says this image gives her the creeps,[why, i don't know]but i was wondering why the bokeh looks so swirly and weird? or is this normal or something you have seen before? thanks in advance!

[Steve Foss]

MM, the amount of blur depends on several factors, including the aperture the lens is set at, the number of aperture blades (more blades, more blur), the pattern of the background and how close it is to the subject. In this case, the background elements were too close to the subject to blur out completely, and I've seen branches and grass and tree trunks like this do a kind of double-take blur before.

Probably there's some technical explanation for that double-vision effect (isn't there a technical explanation for EVERY techie thing in photography?), but I don't know what it is.

Which lens was it, what was the focal length, aperture and number of aperture blades for that lens? It could be your lens doesn't have a lot of blades, and the branches were rendered more sharply than if it had more.

[MARINERMAGNUM]

it was my 70-300 @300mm 5.6. i don't know how many blades it has. i think you are right about the distance of the background.

also,i am starting to shoot in RAW mode almost exclusively. it does seem to capture a little more detail. it's nice when a hobby gets more intersting along the way-instead of boring.

[The_Duckslayer]

Mariner, I agree with the Lil Mrs there. Not that it gives me the creeps but is has a very "different" look to it. Even the bird seems to be very intesne. I like it!

As long as we started with a questions what is a blade?

Take care and N Joy the Hunt././Jimbo

[MARINERMAGNUM]

i think i'll let Steve explain aperture blades. i'm sure he explained it to alot of people in the past,so he must have a good way to describe them. thanks.

[Steve Foss]

Geez, thanks MM!

It's hard to explain the aperture's mechanism using words. A picture immediately shows what language has a hard time with.

Anyway, the aperture is made up of flat metal blades that overlap each other. As the aperture opens wider, these blades withdraw from the center of the circle, continuing to overlap each other but making the hole (aperture) wider. And, of course, when stopping down the lens to gain a higher f-stop number, the blades tighten down to make the hole smaller.

So, if you have a pretty cheap lens with only five blades, for example, the aperture opening formed by those blades isn't really a circle, it's a five-sided pentagon. Each side of the pentagon is flat. The wider the aperture is open, the longer the sides of the pentagon, and the longer those flat sides, the more angularity and sharpness your background will have, and the less buttery the bokeh will be. The physics of the cause is right on the edge of my memory, but I don't remember it enough to write it here with any authority.

A lens with eight blades, for example, has an aperture that much more closely looks like a circle, because the eight flat sides of an octagon will each be much shorter than the five sides of a pentagon, and that allows the background to be broken up more finely.

I've seen comparison images taken with lenses of the same focal length but different aperture blade constructions. Same distance from subject, same aperture reading, etc. An out-of-focus streetlight in the background was rendered into a pentagon by the five-blade lens, and those five-straight sides were easy to see. The eight-blade lens turned the same streetlight into a circle.

Clear as mud? Good, my work here is done.

Actually, I believe the luminous landscape Web site has a couple tutorials on aperture. A google search will turn up the site right away.

[Dbl]

MM I've seen this a few times myself, in fact here is an image with the 300/2.8 which has one of the nicest bokeh in the land. This was shot the other day in the Corkscrew Swamp just NE of Naples, FL. This is a throw away shot for a number of reasons but it illustrates what you experienced.

And here is a shot from just 10 minutes prior to this with the same lens.

Like I said I've seen this a few times but can't put my finger on what causes it.

[MARINERMAGNUM]

thanks for the info DBL. i was hoping it was something that didn't just happen to me alone.

Duckslayer here is a diagram of what a camera aperture looks like.

the smaller hole would be a larger number,such as f22. the largest hole would be called something like f2.8.

a larger number [smaller opening] tends to keep objects in focus from near to far. [like squinting your eyes]

a smaller number [larger opening] tends to isolate a particular subject and blur the rest.

i hope this helps.

[Steve Foss]

MM, thanks for posting that illustration. Says more than all my words can.