Anyone else excited about the 40mm pancake?

[Agilulfo]

Because sometimes you care about the quality of the blur when not completely wide open.

So, you want a large DOF, close the diaphragm, and also you are worried about the shape of the not so blurred circles. Surely there must be situations like this but I don't know if they are common enough to justify all this concern about the number of blades.

[dsiegel5151]

So, you want a large DOF, close the diaphragm, and also you are worried about the shape of the not so blurred circles. Surely there must be situations like this but I don't know if they are common enough to justify all this concern about the number of blades.

Here's a pretty easy explanation: When taking a portrait you often want your subject to be in compete focus. Thus, you would calculate the depth of field required to keep your object in focus. When doing this, you rarely will calculate a wide-open aperture. Everything outside of your subject will be blurred, or partially blurred, depending on focal length and fstop. Any type of light or reflective surface will produce nice little bokeh balls if out of focus enough. With a crappier lens you will get stop signs or petagons instead of nice little bokeh balls.

Below is an example. Notice how the dog's face is not completely in focus? I shot this wide open with the 50 f1.4. If I took more time and had a dog that would sit still longer I would have probably shot this at f2.8 or above to try to get the dogs whole face in focus (notice that the nose and ears are blurred a little). The lights in the background would still be far enough away to be out of focus. With the 50 f1.4 these lights would have been stops signs at f2.8, and not as pleasing. Thus, I would have had to make the decision...face in focus or pleasing out of focus highlights. With more advanced aperture blades I wouldn't have to make this decision.

[Agilulfo]

Great shot dsiegel5151. And it is a perfect example of what I'm trying to say: in a picture like this, shot wide open often produce better results.

Yes, the nose and most of the hair are out of focus. But for this reason you had centered the attention in the sight of the dog. With a large deep of field, you would have captured more details of the head but -firstly- in a portrait like this I don't think those details are needed. On the contrary, you would have loosed the softness of the hair.

And secondly, no matter the number of blades, the light spots in the background would be more focused. I don't think focus is needed in this area. It's hard to say without comparison, but I think you had chosen the best aperture in this case.