CA for monkeys
 

OK so I understand that "CA" stands for Chromatic Aberration. I understand Chromatic Aberration is purple, or green (etc), is annoying as all hell, and makes a lot of people want to aim their credit cards at yet another nine thousand dollar target. I understand that it occurs mostly near the end of the zoom on the stock 16X lens, but that even the nine thousand dollar lenses don't clear you of it completely. I even understand that it especially occurs on edges, between high contrast areas. But I don't understand why, really. Something about bending light like on the Pink Floyd Dark Side of the Moon album cover? (Don't worry, I already know I'm shot).

Please hang with me for one minute more..

I can't quite digest two things lately: One, that a 900 square foot condemned house in Palo Alto sells for 1.5mil in one day - and Two, that I need to spend another ten thousand dollars if I want to zoom in on my girl slopping mud all over herself WITHOUT her having purple teeth (almost) once I get back to the Mac. I know, I know, "Don't zoom"

I'd like to ask just why it is that my fifteen year old son can wind up my 70-200mm Canon EOS lens (which sells for around $1,600 new) to 190mm and there's no CA, while I can't do even close to the same with this Fujinon 16x under the same conditions at the same angle, same moment, same subject? We stood elbow to elbow.

Just what is CA? What is happening inside this Fujinon lens/HD200 that is not happening inside the Canon.

Thanks in advance for any help at all.

Chromatic Aberration is where the colors of the spectrum don't all come into focus at the same focal plane. It can be caused by a few different things, with the lens being one piece of the puzzle. Another would be the prism block inside your JVC camera. Three chip cameras have to use a prism to split the colors out onto their respective sensors, red, green, and blue. The reason your son isn't having this problem is because still cameras use only one imaging chip, so there's no need to split the incoming light rays. Video lenses are made to focus while recording an image without excessive breathing. This requires more lens elements and that can contribute to chromatic aberration. Particularly at the near and far ends of the zoom range. Still lenses will breathe when focusing, but it's not a problem because you focus, then shoot a single frame. The lens design is simpler and helps reduce chromatic aberration.

-gb-

and the ratio between lense diameter/sensor size makes that video lens are more difficult to build.

See also the Canon white paper HDTV Lens Design: Management of Chromatic Aberrations.

Thank you guys. I believe I'm getting warmer.

I knew Pink Floyd had something to do with it...

Each day I learn something new on this site. Only problem is these lessons come in Costco sized packages. You can never just buy one answer at a time.

So some of the mechanical differences between the stock lens and the 18X (for example) might be - less breathing while focusing, better glass, better build, faster zoom servo (?), a more accurate "aim" from the prism color split to each respective chip, and a more reliable/consistent backfocus - for starters?

The short answer is that the Canon 70-200 only has to look good over a range from 70-200mm, that's a 2.85X zoom, while the Fuji 16X has to look good over the much larger range from 5.5mm to 88mm. It is much more difficult to design a lens that has to perform at many different focal lengths.

The basic reason CA happens is because different wavelengths of light are bent at different angles when they meet a refractive material. The shape of a lens element and the refractive index of the material it is made from affect the amount that rays of different wavelengths will diverge. CA can therefore be somewhat corrected using several elements of different shapes and materials. However, CA is not the only abberation that lens designers must take into account, and complicating the design of the lens with the need for a high zoom ratio and low cost (as in the stock fuji 16X) can make things very difficult. Any lens is a compromise between hundreds of different factors, the best lenses are so expensive because they compromise less on cost-related factors such as pricy materials for the lens elements. The 70X and above lenses used for sports broadcasting are possible because being tripod mounted they don't have to compromise on size and weight. They can also cost as much as a small house.

As to the house in Palo Alto ... I can't help you with that one. I live in San Mateo up the road and its no better. Pretty crazy huh?

I suppose the excuse might be 'location... location... location'.

Hopefully Stephen didn't mean the lenses can cost as much as a small house in Palo Alto. I'm destined to rent everything, forever..

I think if George Carlin was a member of DVinfo he might chime in here with some "different" perspective:

1 - If one of the vital selling points to the HDxxx line-up is that you can change lenses, why did I buy the camera with one lens that "doesn't need to be changed"?

2 - If it's impossible to make cheap a lens that can cleanly zoom from 5.5 to 88mm, then why build it? This IS high definition after all.

3 - Perhaps we should re-name the 16x. Maybe the "8x" is more appropriate since that's about the range where you can actually use it?

I for one would rather spend $3,000, three different times, as the needs presented themselves. Especially if this meant better images. Perhaps you'd get to the same $9,000 mark, but maybe you'd get there cleaner. Maybe a 4.5 to 25 that did what it did - well. Then an 30 to 55. Then finally a 60 to 88?

Just thinking out loud.

Ha, no, I meant a small house in Minnesota. But thanks for making me glad I don't live in California. =D

They build it because they want to be able to aim this camera at the prosumer market that is used to getting a lens with the camera as a package deal, and the overall package has to be competitively priced with the other prosumer HD cameras out there. Notice that I didn't say "image quality" anywhere in there.

You're sounding a bit like a still photographer there. =D The crucial difference between still photogs and video photogs is that a still guy never needs more than one focal length in a single shot. Because we have to deal with time in our shots, we might need the entire 16X range of focal lengths in the course of a single shot. Also imagine having to cover an event and having to swap lenses every time you need to change focal lengths, missing shots every time you did. Still guys get around that by carrying several camera bodies with their different lenses, we don't have that luxury since our cameras are a lot bigger and harder to carry several of.

The appeal of being able to swap lenses on a video camera is just the freedom to buy the best glass you can afford, not necessarily to swap lenses regularly in the course of shooting.

Hey at least I'm sounding like a somethin-grapher. That's progress in my mind ; )

Points well made, and taken. Thanks for the help

This is a very good thread. Thank you, everyone, for clearing things up for me as well.

We all should really look into creating an HD100 website run by the users of the camera to compile all this great information.

I'd like to point out that the HD100 stock lens is just like any other video lens in that THEY ALL have CA somewhere.

Even the most expensive DigiPrimes money can buy somewhere have CA though it may not be noticeable at first glance and very minimal where you don't really notice it.

Every lens has its problems somewhere and at the same time every lens has a "sweet spot."

Is CA a condition strictly of video lenses, or does it occur on film lenses as well?

I know it occurs with still photography lenses, so I bet it occurs on film lenses as well (depending on the quality of the lens). In fact I've attached a screen shot of a window in photoshop which is designed specifically to help you get rid of CA. However I don't think it happens so much on fixed lenses in either world. From what I gather from here on DVinfo.net, CA seems to be a trait that comes along like a monkey on the back of most zoom lenses. Like Mark said, all lenses have a "Sweet spot." Fixed lenses must be built at that spot (hopefully). But zoom lenses seem to pass from before it, through it, and then finally to beyond it as the shooter zooms through the mechanical (instead of the "usable") range of the lens.

I'd like to know if there are techniques, settings, filters, or more ideal lighting situations that can help minimize CA for those times where you simply have to zoom way in - like shooting into a mud pit from behind a crowd for instance ; )

There is no such thing as a "perfect" lens, period. The laws of physics make it pretty much impossible. Even a very well-corrected lens has *something,* even if you can only measure it with a computer. If you're willing to pony up the cash, however, it's pretty possible to get good-looking lenses even at high zoom ratios. You probably never notice CA in a sports broadcast, for example, even though they're using zooms with ratios of 70X and up.

I'm enjoying the debate over the finer details of life, but it's fair to ask - is any of your CA distracting from what's going on in the shot? I mean, really - it's fun to compare and understand, don't get me wrong - but when it comes video, aren't we just interested in being absorbed by what's going on in the scene?

Ok, I don't want to shoot home video class video...and I did get the 17x "upgrade" - and I do notice CA - but don't let it get in the way.

I like the still shot sooo much better. Perhaps we should be trying to figure out how to get that lens on the camera instead of how to get the CA out of the lens! What do you need to get the image on the right at 24p, instead of the image on the left...

The whole discussion is intriguing to me as I'm interested in all aspects, even the unpleasant ones, of my business and the career I've chosen and have committed myself to learning as much as possible. It's more of a personal philosophy, really, as I view every element of an art form as a feat of engineering. A conversation I've carried on with my non-film friends about the decision between viewing a movie in widescreen or fullscreen led me to this metaphor I used to explain my preference to my engineering friend. Yes, you only see "a little bit more" on the sides, but that little bit more was consciously incorporated by the filmmakers, just as a certain sized bolt is selected and used in the construction of a bridge. Sure, if you choose a smaller sized bolt, you could probably still build a bridge and have it look more or less like you originally planned, by the stability of the entire structure is compromised when individual elements are out of place and not correct. CA, similarly, is an element that threatens the integrity of a shot. That's why it deserves to be discussed and fretted over. If I was a client, I'd trust the guy with the big bolts.

Personally, I think you might be more suited to stand up than videography. Also, that screen shot, i've never seen that much CA if you're referring to the purple stuff. In fact all the CA i've seen is green.

I'm probably the farthest thing from a nitpicker most of the time. My desk is a mess, my truck is a disaster, and if you walked through my garage with your eyes closed you might impale yourself on a C-stand, then trip on a circular saw and bash your head into the tail section of a superbike. It's just the way I'm wired I think. But, then, when it comes time to focus in on something - that's when the the other me shows up. And when this HD200 is in my hands, it's time to focus.

Ironically, this still I have linked to here is NOT in focus. What can I say, I'm learning. But it's still a good (better) example of just why I posted this question originally. And I think it's a good example of when a "Little" is just too much. I would have used a more horrible shot like this before, but I wanted to use the same shot as my boy got with the 20D.

By the way I've been fumbling around with filters in FCP and I have had some (very limited so far) success zeroing in on what I'm affectionately calling "Purple Haze" Something about Saturation in the highlights?

I've been kind of hoping someone would chime and say, "You idiot! You've got the DPA way off in that shot. Adjust your hootinany to B-17 between 3 and 5 and you'll be all set!"

Told you I'm shot... Been hit in the head with too many bolts

ps - Brian, how bout purple AND green in one shot? I think it must have been the subject matter exaggerating everything. Dark, brown, WET mud, on a very bright and sunny day.

I'm not one of gurus here. In fact, it seems I'm the only one here who is NOT a guru. But I've been loitering in the forum for a long time, and you've got some of the crappiest looking frame grabs I've ever seen. I'm assuming you just bought your 200? Maybe you just found a perfect storm of adverse lighting to bring out the worst. Hopefully someone who knows what they're talking about will weigh in here. I've never had a shot like that with my HD100.

Also, one of the dirty secrets of the JVCPRO HD series and HD in general is it's a bitch to focus.

I'm going to take a stab and say that most of that is sensor overload. You'll often see the same purple fringing around lights at night time because the iris is open to expose the darker areas and the lights will overload it and give off a purple fringe.

I've had the same thing happen with the sun high in the sky, shooting stills of a motorcycle with lots of shiny chrome.

-gb-

Yeah but were that the case wouldn't the overall shot be way overexposed?

Not necessarily, have a look at this. I have zoomed and cropped so it looks soft, but look at how all the shiny highlights have the purple haze.

Attachment 3476

No worries. Should only take me a week or so to turn that one around... ; )

"Whoever said it's lonely at the top, must never have checked the bottom."

Guru or not, if you can tell me how to NOT get this crap, or at least how to minimize it, I would much appreciate it

See, I'd say that looks overexposed though, at least the soil and grass. Eric's grab didn't look overexposed. In his frame grab, those purple thingees were so excessive that it actually looked like some kind of NLE filter FX.

I agree, all I see is a girl in mud. Others see CA. Yes, the shot is covered in CA, but no matter how hard I look I just see a girl in mud.

The still shot is far superior, it should be. For that shallow depth-of-field you'll need a lens adapter and then at least you might get rid of some of that CA.

Okay, what about Redrock micro M2 lens adapters where you can mount Nikkor, Canon etc lenses on your GY-HD?

For Vince: still photo lenses can suffer from CA as well. Generally speaking, you get what you pay for. The more expensive the lens, the less likely it will suffer from aberrations.

For everyone: also keep in mind that fringing, be it purple or green or whatever, can be caused by several different things, only one of which is chromatic aberration. Some folks tend to make the mistake of referring to *all* fringing simply as "CA," and that's wrong. It might be CA, it might be a chroma sub-sampling issue as has been previously discussed on this site in our XL H1 forum, or possibly some other cause.

The usual reaction is "fringing or CA, whatever it is, just make it go away," but in order to make it go away, you have to understand what it is and what's causing it. If you just arbitrarily call it "CA" when it's really a chroma sub-sampling issue, then you're barking up the wrong tree and it ain't about to go away.

Identify the cause of the fringing. It might not be chromatic aberration.

Alright then I've got a pretty simple question. And guess what, it involves spending more money I think! Thank the heavens it's Friday..

Since I did NOT see this fringing in the viewfinder before shooting (and I WAS looking), I'm imagining that it might be nearly impossible to identify whether it's a sub-sampling issue (here I go back to the encyclopedia), or Chromatic Aberration (I'm guessing that tweaking settings until you affect it will help you identify it?). So when a shooter really needs to know, is THIS one of the cases where you absolutely have to have an external monitor?

Hi there!
Long-time reader, first time stander-upper :)

Depending on the nature of the shot and how much color you are willing to sacrifice there is an option to "fix it in post". It's kind of a cheat depending on how you look at it.

Using color supression you can get rid of most of the CA. The downside is that the affected areas turn into a grey color. This is noticable in some shots but to save a shot it is sometimes worth it.

I tried it on Eric's attached image and it sort of works since it's mostly mud :)
You don't have to suppress all the colors this aggressively, I just wanted to show the possibilities with this simple approach.
Taking things to the next level would include swinging the affected colors back into something more "normal" instead of just making them gray.

There are probably a whole slew of ways to achieve this effect. A nice plugin for After Effects is Suppressor @ http://www.fandev.com/supressor.html
(Not trying to plug or anything... Not a 100% sure about forum rules on this so go ahead and edit or complain if inappropriate)

For my money, this is CA. This lens is shocking at both ends of the zoom.

In my experience this type of CA in the highlights always looks worse on Fujinon lenses than Canon, I think it's something down to the lens coating.

Nice work Karl, you really should post more.

Eric, you didn't see it in the viewfinder because the viewfinder is low rez. I guess we must remember just how much this camera costs. It delivers a lot of bang for the buck, but it ain't never going to be an F900. BTW, love the writing style, I like to see a bit of passion.

Thanks for the help, guys. I really learned a lot by posting this here. I guess I kind of sacrificed my pride in doing so, but that's a small price to pay for an education.

And holy mother, Karl, did you do a great job affecting the "CA(?)" in that image. After Effects is still just an icon staring me down from the bottom corner of my desktop so far. I'll have to wait till my saturation level gets down below my eyebrows again before I open up that challenge.

To Jeffery's credit, in the end I don't really mind the CA either -

please excuse the stumbling. I'm not even new at this yet..
http://tracyfit.com/QuicktimeVids/Mu...H2OmarkWEB.mov

I do think the stills look hideous - but re: what Karl did - bad stuff happens in the production part and while we want to avoid as much post as possible, there are fixes out there to make things look better. But if you have no story, you have either gorgeous scenery or nothing worth watching...

Now, to your quicktime. I think I see a lot of what you're talking about. I do wonder, though, about the high shutter and ND2 (that I'm assuming you had on?) combined with the high reflection of the mud and water in that one section. Could that have caused some of that, and would a slower shutter with a deeper ND (screw on) have yielded better results? But only in that case - the wide stuff looks much better, and the sky and colors show good exposure. I thought there for a bit that you didn't have any ND on...

But seriously - while there was plenty I was just looking at regardless of the technical merits, I think you've got something going on. Your settings weren't right...for that situation....

Jeffrey, it took me two years to figure out how to use my 20D well enough to even print a picture. I can't even imagine how long it'll take me to figure this camera out. But, then, I went at the 20D all on my own. I'm trying to learn this camera/world in a more effective way (thankfully, because it seems this world is about nineteen times more complicated). That's why the post, the frame grabs, the video, and now finally the settings that produced them.

I've only really been able to shoot with this camera about four times so far. And even then, just about two hours each time. No, the rest of the time I was NOT out shooting with another camera. I was out swinging a framing hammer while dreaming about swinging this JVC.

I'm finding it real hard to keep all the necessary priority points considered at all times while shooting. I always seem to forget things - sometimes I even forget things in small groups. That's why I set the aperture on auto - so I could intentionally forget one thing. Also, I didn't want my adjustments to look clunky as I panned around the mud pit. I made a big mistake with the polarizing filter too I think. Maybe it was too cheap. Maybe it introduced some color where there shouldn't have been any. And you are right on the ND filter as well. As this thread has developed I have learned, and as I've learned I have also begun to cringe with the thought that I didn't have any ND filters on. I think the new polarizer and auto aperture kept the overexposure ND flags from waving at me - which would have reminded to activate them. Maybe the ND filters would have cut down on the glare, which might have helped reduce the "CA"? Finally (at least I THINK, finally), I have no idea how in the hell all my settings went on walkabout this day. When I checked the settings to list them on the frame grab, not one of them looked familiar. Can't wait to find out which button I inadvertently hit THIS time. Man do I have a lot to learn.

Oddly enough, there does exist a world where I am not such a chump - and I do help every single person who asks (and even some who don't). Maybe you guys helping me here is about good Karma. Either way, I thank you all.

Thanks guys! Always nice to share knowledge and to gain some from you.

While I totally agree with Jeffrey about content and story, this was an easy 2 min fix. It doesn't help the story per se, but it improves the shot by not distracting the viewers attention from what's important. E.g. no "What's that green and purple stuff...?"
Granted, this would not be the first thing I looked for in a video like this :)

It is also nice to know that when unexpected problems are discovered after the fact, there might be a way to solve them. Sometimes shots are thrown away just because of "problems" with the image. We can only hope that spreading a little more knowledge leads to a few more saved shots or, if used anyway, not so horrible looking images :)

And when looking at the quicktime it looks more like sensor overload than just chromatic aberration.