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Old April 17th, 2004, 12:05 PM   #1
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Making mini35 - I see overexposed circle ???

Hi!

I am making a static mini35, on the one side of device is GL2 and on the other my Sigma lens. But when I attach my device on my GL2 camcorder, i see a circle inside a picture. The circle is overexposed, outside the circle the picture is less exposed, if you know what i mean :(

Is it possible, that I was not grinded my GG enough (I was grinding an UV filter for about 10 minutes)?

Or is it possible that my focus lenghth (between lens and GG) isn't allright?

Thank you for understanding and the answers, this forum is revolutionary...

marko
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Old April 18th, 2004, 02:13 AM   #2
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Hi again! That is really big trouble for me, so somebody please help me... Where I made a mistake?
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Old April 18th, 2004, 04:12 AM   #3
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To me it seems like it could be a hotspot caused by the pincusion effect of the condenser. You need two condensors or a fresnel downstream from the GG. Some recommend two condensors, but a good quality fresnel should fine. The fresnel should be as close as possible to the GG. Also you need to watch out for vignetting caused by just barely catching the lens hood or outer edge in the image.
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Old April 18th, 2004, 07:06 AM   #4
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Thanx Jim.

I don't really understand that thing - is there any site on web, where is this more detailed described? Or would you mind describe it to me once again more detailed?

Thanx alot...

Marko
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Old April 18th, 2004, 07:33 AM   #5
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Some more informations:

I have GL2, GG ang Sigma lens. The body for my device is made of three PVC coupling pieces, but now (when I am testing the whole thing) they are not finally glued together, there are small places between them. So there is some light which come inside device now. Is it possible that this thing disturb picture?

I want to see if thing works of course, before I glue the parts together. Another thing: is it important to be the focus lenfgth (between lens and GG) perfect precisely measured?

Thanx,

Marko
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Old April 18th, 2004, 03:21 PM   #6
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OK, I understand little more about it now, there is good article about it on http://topcontechnotes.home.att.net/viewingsystem/page4.html.

So, if I understans right, I need a Fresnel lens? Why didn't anyone mentioned them to be a part of Mini35-static-system?

Thanx,

Marko
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Old April 18th, 2004, 03:38 PM   #7
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For an explanation of how a slr viewfinder is set up:
http://www.camerascreens.com/PDF/Installation_Guide_For_View_Cameras.pdf

There is a better link in one of the threads, but I can't seem to locate it right now.


You might want to review these messages starting here:
http://www.dvinfo.net/conf/showthread.php?s=&threadid=17195&perpage=15&pagenumber=21

If you only spent 10 minuted grinding the glass, it might not be enough if you used AOX. Remember to use lots of water and rinse often when it starts to bind.

The fresnel helps reduce light scattering caused by the ground glass with the effect of brightening the image. It must be nearly contacting the ground glass surface. Good fresnels should not degrade the image unless you zoom in to 30x and you are unlikely to zoom in more than 5-10x.
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Old April 18th, 2004, 09:52 PM   #8
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Here's that link:
http://topcontechnotes.home.att.net/viewingsystem/page3.html

The dabate between two condensers vs. one condenser and a fesnel goes on. I think two condensers would be better if not for the light loss caused by the ground glass.
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Old April 18th, 2004, 10:00 PM   #9
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The groundglass may need more than 10 minutes work.

On the groundglass there is the image your camcorder sees? I call this the target image. How wide is the image? How high is the image.

The image bigger than normal still camera (SLR) frame will have a hot spot in centre. You may have to use a stronger close-up (macro) lens to make the target image frame smaller.

On Aldu35 and Agus35 threads, look for posts by Brett Erskine on this subject.
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Old April 20th, 2004, 09:35 AM   #10
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Marco,
I can completely understand how disappointed you must have felt when you first saw those images with the super bright hotspot in the middle of your image. I felt the same way. I hope this is some new information for you.
First, your adapter requires a minimum of three elements (most require 4)

1. SLR Lens
2. A Ground Glass Surface
3. A condenser (this is what fixes your hotspot)
4 A macro adapter for your camcorder. (required if your camcorder cannot focus ontoa 36mmx26mm area on the GG)

In my own adapter, I have combined my condenser and my ground glass surface by grinding the flat surface of a 60mm dia. 60mm Focal Length PLano Convex Lens. According to Brett Erskine, this is an F1 rated lens. That is what you need to correct for the hotspot. Brett Erskine has made the most contributions regarding the optics of these adapters. It would be a good idea to read his posts throughout the Aldu 35 thread. I am working on my second generation adapter where I'll be using two 50mm dia. 100mm Focal Length adapters. With my current adapter, I have a very even image in regards to brightness but I am experiencing some slight barrel distortion in the edges. I am hoping that the two combined lenses will fix this. Here are some simple diagrams of what I'm talking about:

[SLR Lens]------[)PCX w/ GG------[Sony Video] This is what I have now.

[SLR Lens]-----(][) two PCX w/ one PCX side GG---[Sony] this is the improvement.

My advice would be to use a PCX for a condenser over the fresnel. I also suggest you be sure you can focus your camera onto a 36mmx26mm area on the GG surface to avoid vignetting. Good Luck. I hope this helps.

Joe
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