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Old January 21st, 2005, 08:46 AM   #1
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Hot spot problem again

Hello!

I m a little confused about the solution of my big problem. My picture on a GG has hot spot (overexposed circle in the center of image). Some of you kind people here told me time ago that there is something missing in my configuration.

I have

canon gl2 > GG > sigma lens 35mm

What is missing here is something like FRESNEL lens or something like that that will fix the problem. But I don't have one of these lenses. Where can I find it? Where do I have to place it in my configuration?

Thank you for the answers...

Marko
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Old January 21st, 2005, 11:46 AM   #2
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Hi Marko,

What macro lens are you using? Sometimes this can be the main problem. Also what are you using for a ground glass? You tend to get hotspots when a lot of light is passing through the adapter without diffusing. This is often related to grain size as well.

What you want to find is a material that is maximally diffuse and maximally transmissive. At first thought this seems contradictory but they two are not mutually exclusive. Diffuse just means that the light has been bent from it's normal path. You can bend the light without loosing it.

As for the fresnel lens: these tend to be generally bad for imaging systems. They provide poor resolution. Have you tried putting a condenser lens in front of the ground glass (as in between the 35mm lens and the GG)? This might solve some of your problem.
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Old January 22nd, 2005, 12:09 AM   #3
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Marko.

My guess is, with the GL2 you are having to zoom all the way in to get a frame on the gg image. I doubt you are able to get close enough in on a small enough area.

For the 35mm movie frame you are looking at a target of 24mm x 18mm or abut the size if the "Intel Inside Pentium 4" sticker on many computers with the sticker resting sideways.

For the 35mm still-camera frame your target needs to be about the size of a grocery barcode.

Anything bigger that that and it won't work.

Your first step should probably be a good close-up or macro lens. It should ideally be an achromatic diopter of 5+ to 7+ with an internal diameter of better than 52mm approx. Century Optics make them for 58mm screw in filter thread. It is something worth buying in because it enables tasks other than the AGUS to be performed with your camera.

If you want the still-camera sized image you probably need the condenser lens as well.
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Old January 22nd, 2005, 05:55 AM   #4
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But he said overexposed, so that looks like a GG with too much transparency.
I polished a CD as clear as I could and than glued a slide of tracing paper on top. It's not perfect because it absorbs allot of light, but leaves no hotspot, if a condenser lens is put behind(not in front!) it.
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Old January 22nd, 2005, 07:23 AM   #5
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Ocar.

You could be correct. I was only going by his description of his image path which did not include a macro lens.

In that event, he may only get a circular image in a darker surrounding field. Unless he locks off the gain on his camcorder, that centered image is going to be over-exposed.
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Old January 24th, 2005, 12:39 PM   #6
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Thanks for the answers...

Ok, I think that maybe it is totally normal, that i have a EXPOSED (i don't think it is OVREXPOSED) circle inside my image...

Maybe it is only the bright area the one that can be recorded. Becouse of that I need macro?

I have troubles getting macro diopter. The century optics are making one +7, but I must order it and wait...

In that case - I don't need any condenser or fresnel?

Thanx again...

Marko
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Old January 24th, 2005, 01:38 PM   #7
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I didnít use a good condenser on my first prototype and got very dark corners. Now I used a very big and thick lens from a photo developer, like I said. It makes the picture brighter and a little smaller, so you get more your shot.
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Old January 25th, 2005, 06:45 AM   #8
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Century optics must be wondering what is going on if there has been a run on their +7 achromatic diopters. Maybe they would like to query their waiting customers to see how many are Agus/Aldu builders and perhaps offer an integrated achromatic and condenser package.
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Old January 25th, 2005, 03:07 PM   #9
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Now I put the magniffy glass between the 35mm lens and GG and it looks like the hot spot is bigger, that means, it is better than it was before... Now I have bigger record area...

Is it important which side of the GG must be grinded? Or the both sides? Is it important to grind it for an hour or two? I grinded it for 10 minutes... I know it is more grain now on it, but is it possible, that also that is important for my hot spot?

Thanx again...

Marko
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Old January 26th, 2005, 11:28 AM   #10
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Grind one side only.
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Old February 3rd, 2005, 06:38 AM   #11
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I have noticed same problem when I tried to make agnus for first time. My CD was not frosted enough !!! That means if you look through the frosted CD, you should see nothing, just the CD. Otherwise you will have hotspot circle on CD, where DoF does not work !!! Because you are looking directly through the CD not on IT !
Maybe its your problem...
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