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Old June 12th, 2007, 01:12 AM   #1
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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.
Attached Thumbnails
CA for monkeys-tarpit_jvchd200_16x_75_web.jpg   CA for monkeys-tarpit_eos20d_70-200_190mm_web.jpg  

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Old June 12th, 2007, 01:36 AM   #2
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Quote:
Originally Posted by Eric Gulbransen View Post
Just what is CA? What is happening inside this Fujinon lens/HD200 that is not happening inside the Canon.
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-
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Old June 12th, 2007, 01:41 AM   #3
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and the ratio between lense diameter/sensor size makes that video lens are more difficult to build.
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Old June 12th, 2007, 01:58 AM   #4
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See also the Canon white paper HDTV Lens Design: Management of Chromatic Aberrations.
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Old June 12th, 2007, 02:46 AM   #5
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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?
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Old June 12th, 2007, 03:48 AM   #6
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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.
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Old June 12th, 2007, 07:19 AM   #7
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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'.
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Old June 12th, 2007, 12:14 PM   #8
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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.

Last edited by Eric Gulbransen; June 12th, 2007 at 01:12 PM.
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Old June 12th, 2007, 02:05 PM   #9
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Ha, no, I meant a small house in Minnesota. But thanks for making me glad I don't live in California. =D

Quote:
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.
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.

Quote:
Maybe a 4.5 to 25 that did what it did - well. Then an 30 to 55. Then finally a 60 to 88?
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.
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Old June 12th, 2007, 02:53 PM   #10
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You're sounding a bit like a still photographer there. =D
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
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Old June 13th, 2007, 11:11 AM   #11
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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.
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Old June 14th, 2007, 11:49 AM   #12
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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."
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Old June 14th, 2007, 12:44 PM   #13
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Originally Posted by Mark Silva View Post
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?
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Old June 14th, 2007, 04:01 PM   #14
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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 ; )
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CA for monkeys-chromatic_aberation_fix.jpg  
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Old June 14th, 2007, 06:49 PM   #15
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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.
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