Our Letus MOD

[Dale Backus]

Hey guys,

I've been hangin out in another thread, but decided to start my own because we've had quite the adventure and i'd like to share it with you guys.

Basically, we have a JVC HD100 which we bough the Letus HD100 for about 6 months ago. We thought it was the best thing ever until we started researching other stuff and the flaws the Letus carries started becoming more and more unbearable. We've ripped the Letus apart and decided to use parts from that for our own. We basically kept the Backfocus and Achromat and the GG holder that Quyen uses. We bought the exact same pipe that he uses and extended it so we could zoom the image back out of the GG to have a wide image. Everything was working BEAUTIFULLY. We made some awesome GG using Microcrystalline wax and Microscope slides (that's a whole other adventure i spent weeks learning and doing, anyone wants to know how we did it just ask). We put our GG in the existing holder from the Letus and backed out the image and we the grain is amazingly not visible, even without it vibrating.

So we decided to conquer the light loss issue. The backfocus lens he uses is a Minolta 50mm 1.7 lens. However, if you open it up to 1.7, there is MAJOR chromatic abboration, and it stays there until about f3. Which is awful as you can imagine. We can't figure out why this is, but think it has something to do with the size image the lens is projecting in comparison to the actual size of the CCD. Anywho, if anyone has any thoughts on this, that'd be great, because we're stumped!

Any other questions about our rig, please ask - i didn't want to write everything because i know it's daunting to click a new thread and have a book to read. THanks

Dale

[Ben De Rydt]

If you read this long thread ( http://www.dvinfo.net/conf/showthread.php?t=43262 ) on TV-style zoom lenses for the JVC you will notice that you need a lens capable of resolving +/- 130 line pairs / mm in order to fully utilize the 1280x720 pixels on the tiny 1/3" chip of the JVC. Still photography lenses are tested up to 40 lp/mm since the imaging area is much larger (lower resolution necessary), thus chromatic aberation not noticeable on a 35mm sized sensor might show up on the 1/3" CCD block.

Originally Posted by Barry Green

Doing some calculations with Graeme, it looks like the lens will need to resolve at least 133 line pairs per millimeter in order to deliver the full resolution possible from the JVC's 1280x720 chip.

The big expensive high-def glass usually only resolves somewhere around 80 to 100 line pairs per millimeter (which is okay, because the 2/3" CCD chip is twice as big/twice as wide, so the lens doesn't need to be so ultra-sharp in order to deliver the full resolution the 2/3" chip is capable of). With the smaller 1/3" chip, and its concordant tinier pixels, the lens has to be substantially sharper in order to deliver adequate resolution. If the lens can only resolve 80 line pairs per millimeter, it won't matter how many pixels you have on your CCD, you'll have soft blurry low-res footage. 80 lp/mm on a 1/3" chip would give you about standard-def resolution, regardless of the CCD pixel count.

So the goal is to find a lens capable of around 133 lp/mm, or 266 lines per millimeter.

Also, further in the same thread:

Originally Posted by Dan Vance

There is a significant problem with using C-mount 16mm and 35mm lenses on 3-CCD cameras. Lenses for 3-CCD cameras are designed with long exit pupil distances. If the exit pupil distance of the lens is too short, it will result in chromatic aberration because of the camera's prism block. This shows up as loss of color registration toward the edges of the frame.

The way I understand this is that the color separation of the prism requires the incoming rays to fall fairly perpendicular onto the prism. My take on your problem is that at f1.7 the light comes from a lot of different angles while at f3 it is more focused.

If you ever find out a solution (aka better relay lens) for this I'd like to hear it. My DOP is getting a horrible backache from the JVC-HD200 with Redrockmicro M2 attached. It seems that right now only the P+S relay lens is an option.

[David Delaney]

I am interested in the Wax GG tutorial and some images of your rig, if you don't mind posting.

[Dale Backus]

Ben, Thanks for your response. What you indicated worries me a bit, because i've never thought about it. Not really even sure what that means. However, we've done quite a bit more testing and here's what we found.

Right now, we have our standard fujinon attached to the camera body. We took the GG holding mechanism out of the Letus (with our new Micro GG) and slapped it in a slightly longer tube of the same width and diameter. If you take JUST That (tube with 35mm lens, and GG) and put it up against the Fujinon, with the macro enabled. You can't zoom in to fill the image of the GG and focus close enough to sharpen it up. The macro isn't powerful enough. SO - we slapped a "Universal (Brand)" 58mm macro, not sure what the magnification rating is (it came in the Letus) between the Fujion and the GG. This magnifies the GG enough so the Fujinon can focus on it, however now we have chromatic and spherical abberation issues, they're not horrible, but bad enough to not use it.

NOW - am i correct in saying that all we need is a High quality (like the redrock micros HD achromat) achromat with a powerful enough magnification (not sure what number is, any ideas?) that will be good enough to eliminate the abberations as well as magnify the image enough to focus on?

It seems like this is the last piece. We found the fujinon is actually quite acceptable to use, because it is actually a quite fast lens (1.4) and doesn't bloom when you open it all the way up. The light sensitivity upgrade is ENORMOUS, i can't even express it to you. When we realized this, we were set on being able to get it to work.

Any suggestions on achromats or other solutions that may work here? Thanks so much.



David, i will definitely do that. I actually took some pictures when we were making it, and i'll try to get that up tonight. I don't have them with me ATM. We're not experts, but we did find some things that definitely made the difference, (basically blind luck) and made some VERY good GG. I'll get back to that tonight. Thanks

Dale

[Ben De Rydt]

We have no chromatic aberration with the stock Fujinon and Redrockmicros HD achromat, but edge-to-edge sharpness isn't as good as can be. IMHO this is a problem with the HD achromat from Redrockmicro, it acts the same on my Sony FX1. Our problem is mainly with the length and weight of the whole system, plus the zoom lens keeps getting bumped while shooting.

Perhaps try Wayne Kinneys achromat? There's a thread on it here.

[Daniel Rudd]

Has anyone considered taking some of these acromat issues to an eye doctor, who has an prescription optics lab right there?

It seems like a correcting astigmatism has a lot in common with the abberations we're all talking about.

Daniel

[Igor Babic]

72mm Achromat from Brevis is exellent! No barell distorzion and no CA. Get away from cheap Asian 10+ macros. I have them both.