I FINALLY Understand... Bokeh & Aperture Roundness

[Oren Grad]

Aperture design? What's that? To a first approximation, a hole is a hole.

+1. Short of contrived test targets with simple geometrical shapes, even really pathological hole shapes will have a modest effect at best on anything other than the shape of specular highlights. Pathological blur disks, OTOH...

There are all kinds of wonderful aperture shapes out there in the lens kingdom. If you're* inclined to worship classic portrait lenses, take a gander at the opening in a Studio shutter.

(* != Dan)

[Dan Fromm]

Unicum, too, I think, Oren. I just recently passed on a 120/6.8 Dagor in one. I'd swear, but could of course be mistaken, that the aperture was shaped like a cat's eye.

[Frank Petronio]

Quality bokeh may not depend as much on aperture shape as you think but if you shoot backlit enough to get flaring spots, rounded aperture spots as circles and ovals will look better than hexagonal polygonal spots.

[Moopheus]

I don't see any out-of-focus areas.

[jp]

There's two parts to it... One is distribution of light across the out of focus spot. A mirror lens might produce annoying rings with the light distributed primarily to the edges of the spot. An old "buttery" portrait lens might have a more gaussian distribution. A well corrected lens might have a even distribution that looks like a square wave pulse. How the light is distributed also leads to things like wires or branches turning into annoying double lines. The distribution/correction is different for foreground versus background out of focus, generally foreground is less pleasant.

The other part most people notice is the shape of the spot. This is where the many rounded blades thing comes in. I prefer a round spot over an octagonal spot or a hex spot. Sometimes those other non-rounded things can be OK to create some impressionist background where there are not bright spots that create shapes, but generally rounded spots are preferred by normal people and snobs alike. Far off center the round spots can be unround from distortion and make some nice swirliness.

[Old-N-Feeble]

Quality bokeh may not depend as much on aperture shape as you think but if you shoot backlit enough to get flaring spots, rounded aperture spots as circles and ovals will look better than hexagonal polygonal spots.

There's two parts to it... One is distribution of light across the out of focus spot. A mirror lens might produce annoying rings with the light distributed primarily to the edges of the spot. An old "buttery" portrait lens might have a more gaussian distribution. A well corrected lens might have a even distribution that looks like a square wave pulse. How the light is distributed also leads to things like wires or branches turning into annoying double lines. The distribution/correction is different for foreground versus background out of focus, generally foreground is less pleasant.

The other part most people notice is the shape of the spot. This is where the many rounded blades thing comes in. I prefer a round spot over an octagonal spot or a hex spot. Sometimes those other non-rounded things can be OK to create some impressionist background where there are not bright spots that create shapes, but generally rounded spots are preferred by normal people and snobs alike. Far off center the round spots can be unround from distortion and make some nice swirliness.

That is precisely my concern. Pinpoint lights, light, and dark out-of-focus areas all seem to look different with out-of-round apertures.

BTW, I do realize that lens design plays a big roll too. However, my wide lenses I want really W-I-D-E so I'm stuck with modern plasmats but my medium/long lenses are mostly dialytes.

[Dan Fromm]

Not to be a complete idiot, but what does the rendition of specular highlights, in- and out-of-focus have to do with the transition from in- to out-of-focus? Or, for that matter, with the dread double line bokeh?

[Old-N-Feeble]

Dan... I know nothing about "double-line bokeh". But I've looked long and hard at images and, as far as I can tell, a well-rounded aperture makes a big difference in out-of-focus areas. It's a purely pragmatic nonscientific approach, I know. I'm just trusting my tired old eyes.

[Frank Petronio]

I've had Veritos, old Brassies, and Aero Ektars, Fast 135 Xenotars, etc. what I use now is an older 1960s Linhof-Select Symmar and I like it a lot. The lens doesn't need to be super fast and shooting at f/5.6 provides enough depth of field that you don't have to worry as much as shooting more open. The rounded shutter opening makes nice spots, and the overall transition from sharp to blur is quite without being overbearing or becoming the most important part of an otherwise ordinary picture. I'm not sure if that is bokeh or if most people don't like it? I do.

IMHO most of the "portrait" and fast lenses look like mushy crap, no offense to anyone who spent a lot of money on them. And I would make an exception for one of those nice new Cookes.

In fact, I'd do your portrait in trade for a new Cooke ;-p

[jp]

Not to be a complete idiot, but what does the rendition of specular highlights, in- and out-of-focus have to do with the transition from in- to out-of-focus? Or, for that matter, with the dread double line bokeh?

I can't easily explain it for certain, but this website explains it using the very nice Nikon defocus control lenses, which require more instruction than Nikon can provide.

http://www.stacken.kth.se/~maxz/defocuscontrol/

Russ Young's soft focus lens thesis has a chapter on Bokeh too, for LF lenses, not specifically soft focus. Google it for a pdf.