Question: Using an achromat vs. spacers+zooming? (HV20&Brevis) at DVinfo.net

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Old March 31st, 2007, 11:49 PM   #1
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Question: Using an achromat vs. spacers+zooming? (HV20&Brevis)

AFAIK I understand the purpose of an achromat being used in these 35mm adapters. The issue is that the camera's lens can't focus onto the GG because it's too close, correct?

My question is, wouldn't it be possible to use spacers to move the GG a bit further away to permit focusing and then zoom the camera's lens in a bit more to grab the entire ground glass?

I'm asking particularly in reference to the new Canon HV20 and the Brevis. My understanding is that other 43mm cameras haven't required the achromat with the Brevis so I guess I was hoping, for my wallet's sake, that the HV20 might be just shy of being able to work sans achromat and that a little tweaking of the distance and zooming might solve the problem.

I've never used any 35mm adapter nor seen one being set up, so if I'm completely misunderstanding something, please be gentle :)

Thanks
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Old April 1st, 2007, 12:22 AM   #2
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David:

I have the HV20 on order and happen to have the Cinevate achromat too, thought not with the Brevis. I use it for a self built, but I should be able to tell you when I get it.
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Old April 1st, 2007, 12:55 AM   #3
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YOU FOOL! just kidding :) The deal with adapters is that you need to find a happy medium between field of view (FOV), focal length, and MOD which I believe translates as minimum object distance. If you can focus under 2" at 30mm or so you probably wont need an achromat. Good Luck.
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Old April 1st, 2007, 05:47 PM   #4
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Originally Posted by Charles Hurley View Post
YOU FOOL!
Ahhhhhgggghhhh, I knew it! :)

Thanks for the replies. The real-life report so far (HV20 + Brevis footage from Michael Schoenfeld) says that the achromat is necessary, but I don't think Dennis has done any hands-on testing with the HV20 yet so I was/am just hoping that there might be some extension/zooming/tweaking that could be done to avoid it.

Thanks again
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Old April 1st, 2007, 07:41 PM   #5
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Suggestions on how to test

David, when I was building my adapter for a 28mm Canon ZR200 I thought I needed an achromat/macro too. I understand from the thread at HV20 that he already had the achromat so he used it (the video did look nice didn't it!).

This is an amateur's suggestion, but here's something to try: put your HV20 about 1" away at full wide from an LCD screen and try to focus on it. Make sure it's stable so prop it up. Then zoom in about halfway and wait for the regular AF (instant AF will be useless with an adapter) to focus. See if regular AF can focus. You might want to try dimming your LCD to something like halfway and see if it can still match the focus (make it seem like indoor light).

If the above worked, put your camcorder to MF then put a 3.5x2.5 cm sheet of paper (use the back of those MiniDV stickers) and see if it covers the whole frame. If you can do this, there is a chance you may get along without an achromat (can't hurt to have one later).

Just for reference, my ZR200 AF can handle being stuck against my laptop's LCD screen and keep sharp focus. In any case, halfway is the max zoom I can do with the AF still able to work. The 1" I used is based on distance between my ground glass and the camcorder lens. 3.5x2.5cm is the size of my Nikon Type D ground glass.

Looking at pictures of the Brevis, it uses a rectangular ground glass within a well that hooks up to 72mm ring. The ground glass is probably a bit larger than my Type D. I can't tell what the exact distance between ground glass and adapter ring but it looks like 1" to me. If it's 2" then you may want to test from 2". Anyway, that's a lot to try if it's worth your time. 8-)
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Last edited by Mike Dulay; April 1st, 2007 at 07:48 PM. Reason: error
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Old April 2nd, 2007, 03:14 AM   #6
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Snap! The real issue is the angle of diffusion of the gg. Many camcorders can focus on the front element or within 5mm, it doesn't matter. It's an optical thing if I have to explain it to you.........
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Old April 3rd, 2007, 12:31 AM   #7
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We have the HV20 in hand, and other than figuring out how to pseudo manually control both aperture and shutter speed (important for adapter use) we haven't strapped it on to the adapter yet. We've got a busy week here getting caught up, but I will have more tips on the HV20 for Brevis use shortly.
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Old April 5th, 2007, 01:42 AM   #8
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HV 20 and Brevis achromat

Okay, I have a Brevis 72mm achromat that I purchased from Dennis's company about 2 or three months ago. I use that achromat on a different adapter, but I had been lamenting that I was unable to adequately focus my FX1 up close. I had to have it 3 to 6 inches away from the ground glass to focus properly.

I was hoping the HV20 would be different, and be able to focus a little closer. It is not, at least with the achromat I got from Cinevate. It was supposed to be the same one that goes with the Brevis. It looks like to me, assuming I was sent the same achromat that sells with the Cinevate, that some extensions as well as the achromat will need to be employed.

Dennis, is it possible that I got a "weaker" achromat than what is selling with the Brevis now, or have all of the 72 been the same ? Or have you confirmed the same thing ?
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Old April 5th, 2007, 10:05 AM   #9
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Hey Chris (or somebody) can you tell us what the close-focus distance is on the HV20? Or, maybe somebody can test the HV20 using the Brevis focus chart as described here: http://www.cinevate.com/phpBB2/viewtopic.php?t=13

Thanks
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Old April 5th, 2007, 10:37 AM   #10
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First of all, the good news, I can put the camera in its widest angle mode, and fill the screen the screen with 35mm slide sized image from one inch, and IAF (instand auto focus will bring it in to focus). There is some barrel distortion apparent there, as the edges of the 35 slide I a using for reference are bent in the image.

When you back off, say to two to three inches, you are just able to keep the edges in the frame and still maintain focus. IF you try to zoom, the focus goes away.

This is a problem, because, in my experience, most of time you need to fill the screen, for best performance with these adapters, with a less than 35mm size image.

Infact, I think if you had your Ground glass about 1/2 inch from the ground glass, you may have the ability to just fit everything right. I do not know if Brevis is designed that way, but with my adapter, I have over two inchesand , at best, between the lens and the ground glass.

I will certainly have to review my view of this, and certain the limitation I put on the adapter by the desing I used, may be the issue.
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Old April 7th, 2007, 03:05 AM   #11
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We did some testing today with the HV20 and it definitely needs an achromat...no way around it. Some pics of the rig.
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Old April 7th, 2007, 02:48 PM   #12
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Quote:
Originally Posted by Dennis Wood View Post
We did some testing today with the HV20 and it definitely needs an achromat...no way around it. Some pics of the rig.
Thanks for the confirmation Dennis. And, BTW, that rig looks AWEsome!
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Old April 11th, 2007, 09:13 PM   #13
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Chris, the issue isn't the achromat...but more likely your setup specifically. The HV20 is close to not needing an achromat at all, and would be fine without it if you were willing to do a slight crop. Most of these cams suffer significant barrel distortion with their lens fully zoomed out, making some degre of zoom a good thing. I'll have some footage posted shortly once I sort the bugs out of the 2:3 pulldown removal workflow here.
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