On the subject of Depth of Field/"Bokeh"

[mrpengun]

I've seen lots of people advertising that a larger number of <EDIT> Aperture </EDIT> blades means a more pleasing blur in depth of field.

Then it struck me: the smaller depth of field means, of course, that the lens is wide open--e.g. no shutter blades impeding... I've never noticed a DoF difference between the two most open apertures (where, if the # of <EDIT> Aperture </EDIT> blades matter, then the difference should be largest). Therefore, the claim that the number of <EDIT> Aperture </EDIT> blades has any affect on the appearance of things outside the focal plane must be completely false... right?

<EDIT>
Thanks to Bjorn below for pointing out my miss-speak; should be aperture blades; not shutter blades.

[Oren Grad]

No, that's not right. The shape of the iris, along with the optical design of the lens, indeed affects the blur character. Read Harold Merklinger's article:

http://www.luminous-landscape.com/essays/bokeh.shtml

[mandoman7]

At most aperture settings the the edges of the aperture blades are defining the shape of the opening.

[Bjorn Nilsson]

I've seen lots of people advertising that a larger number of shutter blades means a more pleasing blur in depth of field.
...

In short, the number of shutter blades doesn't have any impact on the any parts of the picture. This because the shutter operates by being fully closed, then it opens very fast to fully open, to eventually fully close very quickly again. What you've probably read (At least what I've read) is "the number of aperture blades ...".
But the aperture (opening) blades on the other hand are in the same position over the whole exposure time and in a way shapes the blurry parts of the picture. (This of course given that you stop down the lens from fully open aperture.)

//Björn

[Frank Petronio]

I think having a round iris probably makes a difference around the middle apertures, when the lens is wide open or only partially open, even a hexagonal shaped opening isn't going to give you little hexagonal flares -- you need to be stopped down a bit for those hexs to show up.

I don't know if the quality of the bokeh is determined by the shape of the iris, all I know is that hexagonal flare patches are ugly and rounder ones are kind of nice.

[Ken Lee]

Those old-time lens designers were some pretty smart physical scientists.

It seems to me that used lots of blades, because they explored and understood the issues.

[GPS]

I dare to say, the new-time lens designers are not less smart physical scientists. They too understand the issues...

[Struan Gray]

<nitpick>

In short, the number of shutter blades doesn't have any impact on the any parts of the picture.

Not quite true. At high shutter speeds and wide apertures, the true aperture becomes the time average of the shutter opening, which in LF and other leaf shutter lenses is a rather beautiful flower shape. Seen here:



<nitpick>

And to the OP: if you know what to look for, the difference between a circular aperture wide open and a polygonal one when slightly stopped down is clear - if you have the sort of subject which shows this sort of thing up. The texture of busy backgrounds can change quite dramatically, especially with lenses with pentagonal apertures.

[mrpengun]

I've read through the Luminous Landscape article now; but I think the question still remains for "real-world" situations.
The article shows that the shape of the iris matters, but not how much in relationship to the properties of the lens. (I remember reading a primer on MTF charts that made a claim as to be able to get a rough estimate of the Bokeh from the charts).
Additionally, it does not show how much more "standard" iris blade options will change anything. A circle made of 13 blades vs. a circle of 8 or 10 will not be near as pronounced as the triangle used in the article.
Nor will they make any difference if your aperture is wide open (providing the iris blades clear outside edge of the lens--as the last image in the article shows).
I think what I was trying to say is that the "answer" is much more complicated than simply "more iris blades = better".
Which we seem to be in agreement over :-)

<edit> having some typing issues all around...

[GPS]

Those old-time lens designers were some pretty smart physical scientists.

It seems to me that used lots of blades, because they explored and understood the issues.

It seems to me that the bokeh "story" is quite a young one. Did anybody speak about bokeh in the beginning of the 1900s?
Beside that, if it is true what Struan says (citing Merkling) does anybody think that the old-time lens designers were after a pleasing bokeh in their lens design (not in the number of aperture leaves)?? I don't think there is any mention of it in the scientific optical literature.