Spherised Bokeh

Hi all,

I have noticed certain lenses seem to "spherise" the out of focus areas, whereas other lenses don't seem to do so. A recent example is the clip for Robbie Williams's "Radio". It looks like the distribution of bokeh discs has been squished into a disc itself, the ones in the centre relatively untouched while the farther from the centre the more squished they get and also tend to be cut off on the outside, looking a bit lopsided. Sorry, best description I can come up with!

Now I'm sure there's some nice technical term for this phenomenon, with an equally nice explanation to go with it -- so are there anyone out there that can help enlighten me here?

Cheers,

Kim Sargenius
Cinematographer
Sydney Australia

>Now I'm sure there's some nice technical term for this phenomenon, >with an equally nice explanation to go with it -- so are there anyone out >there that can help enlighten me here?

Wow. I know the effect you're talking about but I don't know for sure what causes it.

The circular bokeh that looks like a donut is just plain considered bad bokeh. I see it on a lot of video lenses. Neutral bokeh is when the field is flat and even across, which is what most film lenses do. Good bokeh is the opposite of the donut: solid in the centre but dropping off toward the edges. That's considered an aberration so you'll rarely see it.

I think I know the effect you're describing, where the bokeh turns elliptical towards the edges of the frame. If that's what you're talking about I'm not sure what causes that other than shooting with a poor lens or shooting through something that might cause that effect. I always use a net on the back of the lens when shooting video so maybe the effect that I see when that happens is partly driven by the net.

The short answer is: I don't know. Where are you seeing this video?
I'd love to take a look.

Art Adams, DP [film|hidef|video]
San Francisco Bay Area - "Silicon Valley"
http://www.artadams.net/

Art Adams wrote:

>the bokeh turns elliptical towards the edges of the frame.

Bingo! I'm not the only one then…It's as if all the bokeh's (is there an accepted plural here?????) have been photographed w/ a fish eye while the focus plane is still rectilinear.

See the thing is, 99/100 this is just plain annoying and distracting, maybe even downright ugly but there is that 1/100 where it can be somewhat funky and cool, so I would really like to know how to avoid it (most of the time) and once in a blue moon how to achieve it...

>The short answer is : I don't know. Where are you seeing this >video?

I've seen this effect lots of times but seeing it again last night in this video spurred the original post. It was on one of the Australian free-to-air music video shows, the clip is Robbie Williams "Radio".

Personally I've noticed it with an old weather (and student) beaten Zeiss 10-100 and also an equally weather (and student) beaten 12mm Zeiss Super Speed.

Cheers,

Kim Sargenius
Cinematographer
Sydney Australia

Spherized Bokeh ????

I have decided I would be the first to admit that I don't know this term.

Hey Art, do I get points for asking publicly (for the benefit of others, of course) instead of calling you directly?

Alan Hereford
Cinematographer
Northern California, USA

>Hey Art, do I get points for asking publicly (for the benefit of others, of >course) instead of calling you directly?

Yes, although your points are redeemable for little more than what the equivalent number of blue chip stamps will get you. :) 1200 more and you get a free coffeemaker.

Check this page out :

http://www.kenrockwell.com/tech/bokeh.htm - The "spherical bokeh" is poor bokeh. Scroll down the page until you see the example.

What was described in a previous message, however, is poor bokeh that is elongated into ellipses due to some further distortion of the lens. The ellipses seem to revolve around the centre of the image to some extent. (This is hard to explain; please don't ask me to draw a picture!)

I've seen this frequently with industrial video lenses when there are a lot of out-of-focus highlights in the background (like light filtering through tree branches) when the lens is zoomed to a medium focal length. The out-of-focus highlights look like stretched donuts, or ellipses, and they are stretched perpendicular to the centre of the image.

You can see a slight version of this effect in the first picture on this page:

http://www.luminous-landscape.com/columns/sm-04-04-04.shtml

If you follow the shapes of the ellipsoidal highlights your eye will find that they are revolving around the centre of the picture. It's a subtle effect but it's there.

My best guess as to why this occurs in some lenses is that, while they are corrected for barrel distortion where the lens is in focus, they do not correct for barrel distortion areas that are out of focus.

If I can make this more confusing please let me know. I don't see how but I may have missed something.

More pages on bokeh :

http://www.luminous-landscape.com/essays/bokeh.shtml
http://medfmt.8k.com/mf/bokeh.html

http://www.cartage.org.lb/en/themes/Sciences/Physics/Optics/

Optical/Lens/Lens.htm

style="font-family:arial;font-size:16px;">http://www.seittipaja.fi/data/Pontification/b_Photography/h_

style="font-family:arial;font-size:16px;">Testing_lenses/a_How_to_test_a_lens.html

Art Adams, DP [film|hidef|video]
San Francisco Bay Area - "Silicon Valley"

Thanks Art,

>You can see a slight version of this effect in the first picture on this page

http://www.luminous-landscape.com/columns/sm-04-04-04.shtml

Yes! That's what I'm talking about!

Now, still waiting for someone to give me some more technical background on this -- anyone?

Another recent example of this optical phenomenon -- some of the MCU's of Nicole Kidman in the ludicrously expense Channel 5 ad...

Cheers

Kim Sargenius
Cinematographer
Sydney Australia

Anamorphic lenses?!?

Florian Stadler, D.P., L.A.

>Anamorphic lenses?!?

Not on the video cameras I'm using.

I'm telling you, it's got to be a side effect of barrel distortion. I'll bet they correct for barrel distortion where the lens focuses but not where it doesn't focus... which means that the normally spherical bokeh can, on occasion, end up being distorted out of its spherical shape.

If someone else has an explanation, though, I'd love to hear it!

Art Adams, DP [film|hidef|video]
San Francisco Bay Area - "Silicon Valley"

Florian,

> Anamorphic lenses?!?

I was thinking the same when I first saw it a few years ago , until this spherical 10-100 Zeiss zoom kept doing it to me

Cheers,

Kim Sargenius
Cinematographer
Sydney Australia

>If someone else has an explanation, though, I'd love to hear it!

This is starting to get a bit frustrating! I went to the library today and hit a stack of optics text books but couldn't make heads or tails from all the diagrams/curves/equations of different aberrations -- pretty much made aberrations of my brain!

Now there used to be a couple of lens techs around here...and anyone know if Steven Morton is still around on the list?

Cheers,

Kim Sargenius
Cinematographer
Sydney Australia

Hi,

Well, so I finally found what I was looking for -- this website has a very clear/concise explanation for what I've been seeing, namely the "cat's eye effect" (which the deformation of the bokeh disks along towards the edges of frame do indeed resemble a cat's iris).

Link :

http://www.vanwalree.com/optics/bokeh.html

A very interesting assertion about the effect of the size of the mirror chamber, which can, at wide apertures, clip the light from the lens before it reaches the film plane/sensor -- never heard of this before, any experiences out there?

BTW, this is a really good layman's resource on photographic optics and much recommended! Thanks to Mr. Van Walree for putting this to rest for me!

Cheers,

Kim Sargenius
Cinematographer
Sydney Australia

On the subject of bokeh, can someone explain why anamorphic bokeh are vertical elliptoids? My brain tells me they should be horizontal (like with lens flares), but clearly my brain doesn’t know enough about anamorphic optics...

Thanks

Ross McWhannell
DP. Leeds, UK

>On the subject of bokeh, can someone explain why anamorphic bokeh >are vertical eliptoids?

The original 2X anamorphic CinemaScope lenses had a problem that as you focused closer and closer, usually necessary for close-ups, the movement of the spherical elements forward of the film plane, relative to the anamorphic element, produced a larger image along one axis, resulting in the fact that the image had less than a 2X squeeze. When projected with a standard 2X unsqueezing lens, the subject now looked "fat", not particularly attractive for close-ups.

Panavision solved the problem mechanically using counter-rotating cylindrical elements that come into play as the lens focuses closer and closer, correcting the squeeze so that it remains 2X on the subject. However, the net result is that the background, out of focus, has MORE than a 2X squeeze, so even after unsqueezing by the 2X lens on the projector, the background looks skinny or stretched vertically.

This is better explained by John Hora in his article on anamorphic photography in the ASC Manual.

David Mullen, ASC
Los Angeles

> http://www.vanwalree.com/optics/bokeh.html

What a great page! It sounds like the solution to "cat's eye" bokeh is not to shoot wide open... but then there's not much bokeh left! Hmmm.

>On the subject of bokeh, can someone explain why anamorphic bokeh >are vertical eliptoids?

The way I've always thought about it, in my non-scientific yet intuitive way, is that an anamorphic lens is really two lenses in one. A 180mm lens, for example, is 180mm vertically but only 90mm horizontally.

Those two virtual lenses are designed to render a subject normally at the point of focus. Outside the point of focus all bets are off, so one virtual lens dominates over the other. In the case of background objects are stretched vertically; in the opposite case, foreground objects are stretched horizontally.

David's answer is much better technically, but I think mine is just generally more fun and artsy.

Pick the one you like.

Art Adams, DP [film|hidef|video]
Mountain View, California
San Francisco Bay Area - "Silicon Valley"

Art,

>It sounds like the solution to "cat's eye" bokeh is not to shoot wide >open...

From my reading on it, it seems that the cat's eye effect is related to optical vignetting in the lens, most notably small front element and exit pupil -- probably why you don't see it on a Cooke S4 or Ultra Prime. By this token it would seem like something like a 10mm Switar would be a good choice if you wanted to achieve it

Cheers,

Kim Sargenius
Cinematographer
Sydney Australia

Kim Sargenius wrote:

>By this token it would seem like something like a 10mm Switar would >be a good choice if you wanted to achieve it

Don't make fun of the 10mm Switar! It was the only lens I used for nearly 25 years -- even have one adapted to fit my Aaton! A fine fine lens.

Jeff "and can always make a special Kinetta with C-mount" Kreines

>By this token it would seem like something like a 10mm Switar would >be a good choice if you wanted to achieve it

Um, yes, it should be relatively easy to throw something out of focus on one of those. Not.

Art Adams, DP [film|hidef|video]
Mountain View, California
San Francisco Bay Area - "Silicon Valley"

Jeff,

> Don't make fun of the 10mm Switar!

I'm not knocking it -- it was the first lens I fell in love with ( more like five years ago, so nothing like your knowledge of it ) just trying to illustrate the point of front element/exit pupil; I don't think there's any doubt it is at the opposite end of that scale from a 14mm Cooke S4!

Art,

>Um, yes, it should be relatively easy to throw something out of focus on >one of those. Not.

With focus down to a couple of inches and (from memory) a stop of 1.6 sure! Still got a shot like that on my reel!

Cheers,

Kim Sargenius
Cinematographer
Sydney