Influence of diaphragm shape on Bokeh

[Thierry Schreiner]

Hi to all,

A very quick question please.

What is the influence of the diaphragm shape on the bokeh of a lens?

By changing from an old shutter to a newer type, from a nice "round" opening with lots of blades to a "squarer" one, with less blades, am I changing the initial characteristics of the lens, especially its bokeh?




Thank's a lot for your imput.

Best regards

Thierry

[Peter K]

Also recent softfocus-lenses like the DC Nikkors are equipped with a "round" opening with many blades to get this special bokeh. So it's a good idea to repair the old shutter to avoid the change of the lense's character.

[sparq]

While bokeh and its appeal is highly subjective and is a product of many design factors, there is a simple relationship between the aperture shape and the way it renders out-of-focus highlights. The squarer the aperture is, the squarer OOF highlights it will produce.

There are plenty of resources covering that topic on internet, see i.e. http://www.luminous-landscape.com/essays/bokeh.shtml

[Ken Lee]

"By changing from an old shutter to a newer type, from a nice "round" opening with lots of blades to a "squarer" one, with less blades, am I changing the initial characteristics of the lens, especially its bokeh?"

Sans doute !

[CCHarrison]

From all of my reading on bokeh over the last 10 years, there is only a minor correlation between bokeh and aperture opening. The simplest explanation around this is this - when different lenses are ALL shot wide open, they do and can display different bokeh.

The most obvious impact of different aperture openings is when lenses are shot into a light source and that source takes on the shape of the aperture. Same thing with flare...

In the end, lens design impacts bokeh far more than aperture. I believe aperture shapes only play a very minor role in bokeh.

My on page on bokeh http://members.aol.com/dcolucci/bokeh.htm

Dan

[Jim Galli]

Yes it matters. If you have paid good money for an old lens with a pleasing bokeh character, you should not compromise it with a 5 6 or 7 blade aperture shutter.

[Ole Tjugen]

http://www.trenholm.org/hmmerk/ATVB.pdf is H. M. Merklingers explanation.

[sparq]

http://www.trenholm.org/hmmerk/ATVB.pdf is H. M. Merklingers explanation.

I've posted a link to that article already, only hosted on LL.

[CCHarrison]

Jim

With all due respect, do you have sample images where bokeh was changed in any noticeable way when a low bladed aperture was used ?

thanks
Dan

[Emmanuel BIGLER]

I believe aperture shapes only play a very minor role in bokeh.

Depends on what we call Bokeh here.
For strongly defocused images, any bright point in the initial scene appears as the projection of the iris.
So if we include the shape of out-of focus bright points projected on film in the definition of bekeh, obviously the shape of the iris does matter.
See a provocative example here with a home-made star-shaped iris !

initial image (not super-sharp !)
http://farm1.static.flickr.com/127/3...4e496be7_b.jpg

strongly defocused and circular iris
http://farm1.static.flickr.com/133/3...e44c708d_b.jpg

strongly defocused and star-shaped iris
http://farm1.static.flickr.com/155/3...e84bf2ef_b.jpg

Be for weakly defocused images and reasonably shaped iris blades, e.g. the good old 5-blade Synchro Compur pentagon, yes, I would support the idea that residual aberrations produce most of the specific character of each lens.