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Old September 17th, 2006, 09:04 AM   #1
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Just Another Medium Format Attempt

Luckily I had and ability to buy an emty body of Kiev-88 and a loop with the ground glass relatively cheap - $40 + $20 + $10 for the GG with the mount. Then I borrowed two lenses and checked it using a wider 55mm f/4 with HC1. First attempt was really unsuccesfull - not even a vignetting, but just a tiny spot of light on a GG. Then I found a fresnel with the focus point simalar to the distance frm the lens to the GG, and plased it right after the GG. It definitly helped, but still there's a lot of vignetting, please take a look at http://kinoprostir.org.ua/anry/vario...er/15.09.2006/

From my point of view MF adapter is really usefull when using wide lenses, as it gives more DOF and good looking picture, but vignetting is really auwfull even on not-so-wide 55mm.
There's a guys who can make virtually any lens for me, i just need to giv them a specs of it. Making one side matte is not a big deal also, but how to tell what lens do I need to get rid of the vignetting? Also, where shell I place it?

Dear adapter gurus, please give an advice!
Andriy Toloshnyy is offline   Reply With Quote
Old September 17th, 2006, 10:30 AM   #2
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1. Make a square mark on paper - same size as image on the medium format ground glass.

2. Draw more squares inside this square mark with cross lines.

3. Look at the square mark on paper through HC1. If the HC1 can not zoom close enough to make the square mark grow outside of HC1 frame and keep the small squares sharp, you need a close-up lens. Achromatic dioptre is best. (Achromatic dioptre is two or more lenses in one assemblage).

4. Look closely at the HC1 image. Are the small squares good? Are the lines straight? If they curve in - is pincushion distortion. If they curve out - is barrel distortion. Try moving the paper closer or distant from the HC1. Adjust the zoom to make the square fit again. Maybe the lines are straight, Keep trying until the lines are straight. Maybe it won't happen. Best if you can get the lines straight.

5. When you can make the HC1 best see the area just inside the square mark, measure distance from paper to HC1. Build your adaptor to place the GG this same distance from front of the HC1.

6. If you have a computer to edit your video, connect to it and look at the HC1 capture image. This image is complete. The HC1 viewfinder is smaller inside the true image or cropped. Sometimes this is called overscan. If you make DVDs you need to know what the complete image looks like.

7. Look close at this computer image. Search for edges of the square frame on the paper. If you can see the edge, you need to zoom in more or move the paper closer to the HC1.

This work will give you the correct distance for making your adaptor without risk damage to the medium format groundglass.

Hope this is a help for you.
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Old September 17th, 2006, 10:53 AM   #3
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Thank You, it is a definitely good idea to check the distortion and then ajust the good distance between the camera and the GG, but the main problem now is vignetting, in other words it is more about the image on a GG. I've looked through other posts, and found out at least thee points that affects it:

1. The quality of the GG. Mine is quite transparent, and maybe with more diffused GG I will get better result. The other side is the light loss - with more diffusion you're loosing more light.

2. Collector/Fresnel lens. It seems, that the angle of the light rays coming to the GG is important. The less is angle, the darker is the projection. Putting this lenses in front(?) of a GG bends the rays and helps with vingetting.

3. Distance from the GG to the camera. It was said somwhere that it shell be the focal poing of the Condenser/Fresnel lens for the best result.

HC1 is focusing well at the size of MF GG, and there's also qiute good distance range when it still can focus on the GG. What I'm thinking of now is getting some condenser lens (or two condensers, as in the MovieTube patetn) and make the flat side of the condenser matte, or try to play with wax-parrafin layer in between. But the question is how to compute the parameters of this condenser lenses to get rid of the wignetting in this case, in other words what kind of condenser do I need?
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Old September 17th, 2006, 12:37 PM   #4
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Hi Andriy,

Your second point there is an interesting one as the ground glass generally doesn't scatter light evenly in all directions but in some distribution around it's original direction of travel.

I would have assumed that some kind of condenser either side of the gg would maximise the light incident on the camcorder lens. This would mean that near to the axis of the adapter, the image would be bright and away from it it wouldn't. This would be particularly useful if the gg-camcorder distance is large as the (solid) angle the camcorder lens makes with respect to a point on the gg becomes small, so without this focussing system, a lot of light is going elsewhere other than into the video camera, especially at the screen edges - hence the vignette.

I don't know if anyone out there has gone with this particular setup, but from some very preliminary tests using 2 nikon d's back to back (with built in condensers), it looks like it works.

cheers

Dave
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Old September 18th, 2006, 12:56 AM   #5
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Hi Andriy,
i am very glad that somebody takes it seriously - medium format. And where else if not in Ukraine - famous Kiev. But ...

about vignetting:
Quote:
Originally Posted by Andriy Toloshnyy
... least thee points that affects it:

1. The quality of the GG. Mine is quite transparent, and maybe with more diffused GG I will get better result. The other side is the light loss - with more diffusion you're loosing more light.

2. Collector/Fresnel lens. It seems, that the angle of the light rays coming to the GG is important. The less is angle, the darker is the projection. Putting this lenses in front(?) of a GG bends the rays and helps with vingetting.

3. Distance from the GG to the camera. It was said somwhere that it shell be the focal poing of the Condenser/Fresnel lens for the best result.
there is fourth point which i would place in first - lens vignetting. As much i have experimented with different lens and different GG-s. It's always possible get rid of vignetting with certain lenses(expensive ones :( ). And other ones give just feeling that there is something if You change Your GG. It's important to not get confused what is hotspot and what is vignetting.

Quote:
1. The quality of the GG. Mine is quite transparent, ...
Here i guess You have actually hotspot in the game.

Quote:
2. Collector/Fresnel lens. It seems, that the angle of the light rays coming to the GG is important. The less is angle, the darker is the projection. Putting this lenses in front(?) of a GG bends the rays and helps with vingetting.
This point goes very well under same optical rule what states about lens: sinus gamma in 4th power where gamma is angle of lightray. Actually with in Your system(lens+GG) should say that rule sinus gamma in 5th power.
This is basic theoretical rule about lightloss in edge areas of optical system. I can't give You a reference to it because it written into photo book from 70's. I can't even find it right now:) But I'm sure old book stores in Kiev got such books in huge amounts.
Frank Hool is offline   Reply With Quote
Old September 19th, 2006, 06:40 AM   #6
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i was bit wron(again :) ). its cosine in fourth power(not sine). Basic reading: http://en.wikipedia.org/wiki/Vignetting . one before last section.
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