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Old February 10th, 2006, 06:11 PM   #1
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static 35mm adapter issue

Im having some trouble building a 35mm adapter. I have spent over $100 now and have not yet had any success. I was hoping someone could help me out

I first tried building a plastic CD spinning adapter but i had a very large hot spot. I tried different forms of ground glass, but the hot spot remained. I used a Fresnel and condensers but was unable to get better results.

Recently i bought a F3 Nikon D focus screen. It appears to be joined with a condenser lens. I threw together a PVC assembly to hold it together and block out light, but when i connected it to the camera, there was that hotspot again!

Minolta 50mm 1:1.7 -> (| -> || -> Panasonic PV-GS150

The only thing i have not tried is a macro, but i can zoom in fine.

Is there something wrong with my Minolta lens? I can point the camera through an SLR viewfinder with this same lens and get perfect results with absolutely no hot spot.

If anyone has any advice i would really appreciate it.

Thanks for your time.
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Old February 10th, 2006, 07:12 PM   #2
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It could be the lens or your GG, if the GG is too thin, your hotspot is going to be there - try a different lens. I built my adapter over 1 year ago and I am still tweaking it and it is not right. It just takes time I am afraid. Try different combinations. Even with the Letus35 I had to put a condenser in front of my DV lens to get rid of vignetting, so there is no 'perfect' solution. It is not the answer you wanted, I am afraid.
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Old February 10th, 2006, 09:14 PM   #3
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Playing around with the distance between the gg and the camera sometimes helps, but only if you're using a condenser.
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Old February 10th, 2006, 09:54 PM   #4
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Daniel.

How large is the image you are taking off the groundglass.

The 35mm 4:3 movie frame is 24m x 18mm. Use this size and you should not have a hotspot issue with f1.8 camera lenses wide-open. This choice however is only viable with moving groundglass designs.

Some builders are using a larger area about the size of a 35mm still-camera frame. It seems a condenser arrangement is needed for the larger image to appear best.

The Mini35 as I understand it uses a smaller frame off the groundglass. I understand it to be 21mm corner to corner which makes it more like the digital SLR size frame. My imagining of this choice is that it effectively deals with the hotspot issue and enables a compact device.

In a larger image off groundglasses of equal surface quality, the camcorder will see in larger image frame a finer groundglass texture. In a smaller image, the camcorder will see a coarser groundglass texture. This affects the apparent resolution of the groundglass image.

Static solutions need as much groundglass area as they can get to reduce the scale of that groundglass texture.

As I understand it, this area includes edge areas of the groundglass image where the brightness begins to fall away.

Condensers deal with this problem but they are too high a science for my feeble brain to comprehend so I can't explain why.

I suspect you may be taking a lager image off the groundglass than even the static solutions are using.

You may have to use an add-on close-up lens (macro) to get your camera close enough to be able to zoom in on a smaller frame on the groundglass.


There are better brains than mine at work on these projects so take that advice over mine.
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Old February 10th, 2006, 10:33 PM   #5
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Thank you everyone for your contributions.

If i measured correctly, the Nikon focus screen im using is 35mm x 20mm. I don't want to have to zoom in much into it because it would make the grain a lot more visible. Here is an example of what the hot spot looks like:

http://www.buysmartpc.com/gg.jpg

I would probably have to use a microscope to zoom in enough on the gg to avoid the hot spot.

The ground glass has a condenser lens attached to it. I read on this forum about other people that use this same setup and get great results without having to use a macro. I have tried to contact them and waiting on a reply.

I have tried different distances and other side of the gg with no luck.

Tomorrow im going to visit a camera store and try out some macro lenses, and anything else they have. If something works i guess ill have to spend more money.
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Old February 11th, 2006, 01:28 AM   #6
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That vignette is huge.

I have a Nikon screen and it looks evenly lit all across. So I am guessing your minolta lens is the issue here. Are you closing the apperture much? What's the diameter of the back lens on your Minolta prime?

My screen measures 36x24 mm btw.

Also, the PCX curvature needs to face your camcorder, not the minolta lens. (judging from your diagram you got it backwards)
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Old February 11th, 2006, 10:20 AM   #7
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That is a very small image area for a 50mm f1.7 lens.

Is that lens from a full 35mm still camera or a "half-frame" still camera?

I think Olympus made one and called it the "Pen". The image frame for these was smaller and was widest across the film, not along it, closer to the 35mm motion picture image frame in area.

The only other thing I can think of is that the Minolta lens aperture is set at f11 or smaller.

If it is an automatic lens with a manual aperture ring, you may have a lens which remains closed unless the actuation device in back of the lens, usually a small lever or a pushrod which sticks out a little, is moved to open the aperture up to the manual selection chosen on the aperture ring.

I am only wildly guessing here as I don't know Minolta. Whatever, It definitely does not seem quite right.
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Old February 11th, 2006, 11:02 AM   #8
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the screen is probably 36x24, i did not want to risk damaging it so i measured it from about 1 inch in the enclosure.

Well the lens works perfectly when attached to a SLR film camera when i point the gs150 through the viewfinder.

The lens is open wide, i can see the iris if its closed.

I have tried flipping the focus screen so the lens curves to the camera without any change. Ill try again.


I just had a thought... The SLR camera is a Minolta, with a Minolta focus screen, and a Minolta SLR lens. This focus screen i bought is a Nikon F3. Is it possible that the Minolta SLR is not compatible with the Nikon focus screen?

When i go to the camera store today ill try a Nikon lens on it.
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Old February 11th, 2006, 11:13 AM   #9
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What kind of Minolta lens are you using? Off the SLR and with aperture wide open, if you look through it, do you see a wide open aperture?

Min focus distance on the GS150 is likely 1.6 inches. If you are not using a macro you must have the cam this close, and only in 4:3 mode.

Is that picture you sent actually from the adapter? Try taping a 24x36 rectangle over your GG (not touching it) and take a shot through that. If you have the barrel dimensions I can post up a jpg from Visio for you...you can just cut it out.

Last edited by Dennis Wood; February 11th, 2006 at 11:45 AM.
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Old February 11th, 2006, 12:03 PM   #10
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The lens reads

MD 50mm 1:1.7 JAPAN o49mm
MINOLTA

Here are a few pictures i took:

http://www.buysmartpc.com/slr/front.JPG
http://www.buysmartpc.com/slr/back.JPG
http://www.buysmartpc.com/slr/side-back.JPG
http://www.buysmartpc.com/slr/top.JPG

The original hot spot image i made in photoshop because i dont think i have a memory stick that will work in this camera. I just had to show an example of what it looks like through the gs150.

The gg is about 1.4 or so inches away from the gs150 lens. I think i can zoom in fine because i get an image in focus (overlooking the blinding hot spot).

The shots were taken as

Minolta lens -> || -> |) -> (not mounted)
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Old February 11th, 2006, 12:26 PM   #11
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I found a memory stick compatible with gs150. Here is a shot with the adapter on:

http://www.buysmartpc.com/slr/gs150.JPG

The hot spot remains the same at any distance between the gg and camera.

*edit*

the camera is zoomed in on the gg looking at a plain evenly lit textured wall, although a bit out of focus. I can focus on objects and get a clear view in the middle.
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Old February 11th, 2006, 01:00 PM   #12
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Daniel. Something seems wrong.

For comparison here is a shot of my Slide-viewer adapter.

http://mentemagica.com/35mmAdapter/Frame2a.jpg

The whole frame is 40mm wide on either side (yes, the actual apperture of the diffusion screen frame) and as you can see it fills the entire frame. I think I can even go wider but I would have to remove the slide mount on the slide-viewer. You can only see vignetting on the bottom corners, and that's because my Nikon lens is mounted a bit off center. I can record a 36x24mm image perfectly with no vignetting.

The setup is like this:

Nikon 1.4 lens || |) |) GL2 camcorder

Even without the second PCX lens the vignetting is pretty much gone after the condenser.
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Old February 11th, 2006, 01:47 PM   #13
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Daniel, what does the zoom level on the LCD say? I can't tell, but are you zoomed in so the nikon D screen fills the frame? Also, make sure you have at least 500 watts illuminating that wall.
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Old February 11th, 2006, 02:02 PM   #14
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What is the distance between the back of your SLR lens and the GG?
I take it is gg facing the SLR lens.

regards

Dave
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Old February 11th, 2006, 02:08 PM   #15
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the zoom level should be about 2x or less. If i go 3x the camera cant focus on the gg.

The frame was completely filled with the gg. If i zoomed out just a hair i could see the edges. I should of taken a picture of that as well.

that wall was lit by the sun, behind me were 4 large windows. Even looking outside on a bright day that hot spot is still there, just a little brighter.

Even without filling the frame with the focus screen, i think it still should be bright and evenly lit.

I flipped the focus screen so the optics are in between the camera and the gg with no change.
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