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Old January 17th, 2005, 06:56 PM   #1
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Hot Spot...Problem

OK I finally got around to making my agus 35 pro ... anyway now I have a real bad hot spot.. Looks like it comes from the aperture from the lens.. I had the Cd looking great with a few minutes with a dry SOS pad and it was very sharp.. so Now I deal with the hot spot... Help Please... I don't have time to read 2 years worth of threads for the help... thanks
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Old January 17th, 2005, 07:10 PM   #2
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I took a thick lens out of an old projector (negative projector to develop photos). It's a big lens that is placed under the light to spread the bundle.
It also makes the picture on the GG a little smaller and gains light.
Most people would use fresnels(with the circles) I think, because they are flat, but I'm sure you'd see something of the lines.
Maybe a magnifying glass will do.

You have to put the lens between the groundglass and the 35mm lens.
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Old January 17th, 2005, 09:29 PM   #3
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Depending on your setup and how much money you are willing to spend you might consider using one of these:

http://www.optosigma.com/miva/merchant.mv?Screen=PROD&Store_Code=OS&Product_Code=pg85

They are much better than normal positive lenses for this purpose (or fresnels) as they have the aberration correction power of more complex multi-element systems. Aspherics are much better suited to this purpose and these are really quite cheap.
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Old January 18th, 2005, 05:21 AM   #4
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WOW...

I was playing around with one of these.. "May be one of these" last night.. I dug up some old photo equipment and had a magnifier shapes just like this.. It was surprising how bright this was... Also I gues the image just inside the "hot spot/ bright circle" is to be recorded with the video camera..Thanks
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Old January 20th, 2005, 08:05 AM   #5
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Craig,

Also keep in mind that if your intermediate image surface is too 'thin' (ie. not ground enough / not diffuse enough), then this could add to the hotspot problem. The light from the lens would basically be coming straight through the surface.
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Old January 20th, 2005, 10:20 AM   #6
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It also helps to use f1.8 aperture SLR camera lenses. Avoid the newer digital still-camera lenses. They may state as wide as f1.4 but the actual path diameter may be smaller
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Old January 20th, 2005, 10:29 AM   #7
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I am using a canon lens

I am using a canon lens 1.8 but the problem i just found out is the cd adapter i'm using wobles..so now I'm currently fixing it... Though one question I have.. does the condenser lens help that much.. seems to me that a straight shot at the disk from the lens would be a cleaner image... thanks
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Old January 20th, 2005, 10:45 AM   #8
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Craig,

The wobbling CD issue can be a real problem. However, if you are in a situation like me - having no professional tools for precision work - then here's something you can try:

Attach the CD to the motor via some sort of short, flexible part. It can't be too flexible. (I used a suction cup, with the cup end glued to the motor and the stub end glued to the CD.) What happens is that when the CD spins, it automatically straightens out, and if it's spinning fast enough, you shouldn't have to worry about it wobbling during quick camera movements.

Be sure the CD is centered with the shaft propertly.
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Old January 20th, 2005, 10:46 AM   #9
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I guess that depends on the condenser you use. If you use an aspheric element of decent quality you shouldn't have too many problems. A simple PCX though will give all sorts of chromatic aberration. Also which side of the ground glass are we talking about?

In reference to Bob's comment: it's too bad these adapters require such an open iris. It's hard to be as creative as with 35mm film when all you can get is extremely shallow DOF.
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Old January 20th, 2005, 10:51 AM   #10
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Another thing I forget to ask was, how much of the image on the disk are you trying to frame. If it is the 35mm movie frame 24mm x 18mm then you may get away without using a condenser.

If it is the larger still-camera frame, then you may need to use a condenser. Condensers are a high science I don't have the knowledge of so I can't help you except to suggestthat they seem to work judging by some of the images posted which have apparently been created using one.

On the earlier non-erecting Agus I built, I used a telescope eyepiece lens set, the SW5042. This has one element in it which looks and seems to work a little like a condenser as the brightness across the image shot with a f3.5 Micro-Nikkor 55mm was even on a 24mm x 18mm frame off the gg. I did not perservere with it as the 44mm internal diameter was a bit tight, the path difficult to set up and at 12mm off the gg was too close to fit a prism path in.
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Old January 20th, 2005, 12:31 PM   #11
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project..

Well it seems to me that as I start to try and record from the video camera one has to come to a simple compromise about... Ground CD/ and thickness, Lens Hot spot, Where to record “inside the hot spot” or get rid of the hot spot and just trying to be happy.. Anyway here is my set up …

Lens--CD--magnifying glass--camera with no closs-up lenses at this time…

I’m been working on this for a week off and on I know to be patient and not hurry… thanks for the input
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