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Old January 8th, 2007, 09:04 AM   #1
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How to focus GL2 with 7+ diopter on focusing screen?

OK, I have a 7+diopter on a GL2 camcorder and I am trying to get it to focus on a 35mm canon focusing screen inside some extention tubes.

The ONLY way I can get it in focus is by doing the following:
1. Zoom in loosely so there is still a black area showing all around the focusing screen.
2. set focus to manaul.
3. zoom in some more till the image snaps into focus
4. Usually I zoom in too much so I have to back out the zoom a bit to get it refocuses.

This is time consuming and frustrating. If I accidently move the focuing ring I MUST repeat all the steps above. I CANNOT simply remanually focus the image onto the screen. I believe this is just the nature of the zoom lens on this camera.

Has anyone made their own 35mm DOF adapter with a GL2 and a 7+ diopter and how did you solve this issue of focusing on the ground screen?

Peace, Rolland
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Old January 8th, 2007, 11:37 AM   #2
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If you bump the manual focus ring from an existing setting, try a short burst of autofocus then go back to manual to prevent hunting or drift when you adjust the focus of the SLR lens on front of your adaptor.

If the image from the SLR lens on front of your adaptor onto the groundglass is fairly sharp, autofocus will trim to sharpness as well as manual focus onto the groundglass with a PD150.

I understand the optical system on the GL2 to be similar or even directly related to the PD150, so this method may work for you as well.
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Old January 8th, 2007, 12:20 PM   #3
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Quote:
Originally Posted by Rolland Elliott
OK, I have a 7+diopter on a GL2 camcorder and I am trying to get it to focus on a 35mm canon focusing screen inside some extention tubes.

The ONLY way I can get it in focus is by doing the following:
1. Zoom in loosely so there is still a black area showing all around the focusing screen.
2. set focus to manaul.
3. zoom in some more till the image snaps into focus
4. Usually I zoom in too much so I have to back out the zoom a bit to get it refocuses.

This is time consuming and frustrating. If I accidently move the focuing ring I MUST repeat all the steps above. I CANNOT simply remanually focus the image onto the screen. I believe this is just the nature of the zoom lens on this camera.

Has anyone made their own 35mm DOF adapter with a GL2 and a 7+ diopter and how did you solve this issue of focusing on the ground screen?

Peace, Rolland
Rolland,
I used the CO 7+ diopter on my GL2 with a Letus35 and didn't have the issue you described at all. I do remember a short period of time where I had this issue you describe, and it was because the focusing screen was either too far away or too close to the camera. I do know the CO diopter works very well on a GL2 if the focusing screen is the right size and the right distance from the camera.
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Old January 9th, 2007, 01:01 PM   #4
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thanks for confirming gl2 and 7+ diopter will work

Thanks Ben and Bob,

Any idea approximately how far away the ground glass was from the front surface of the CO +7 diopter?

Right now I think my ground glass is about 1 to 1.5" away from the front of the diopter.

Peace, Rolland
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Old January 9th, 2007, 08:51 PM   #5
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Roland.

To my thinking that 1" to 1.5" seems a bit too close for a 1/3" CCD camcorder. I'm surprised you can get a sharp focus that close at all so the close focus ablity of the GL2 is not the same as the PD150 but better.

I think that if you make up some sort of test where you mount the camera so the front of the dioptre is about 120mm from the groundglass, you may find the whole thing a lot more controllable and pick up a bit more resolution as a side-benefit.

I suspect you are imaging off a very small groundglass area in the centre by the time you get it focussed.

Before you start doing this, just for interest sake, you could cut a disk of paper to slip inside that tube to rest over the groundglass as a mask on the camcorder sise. In centre of that mask, draw a 24mm by 18mm rectangle.

This is the standard 35mm 4:3 motion picture frame. Inside that rectangle, draw a few smaller ones so they all fit one inside the other, the smallest about 10mm wide. Circles are just as good if they are easier to draw.

Use no SLR lens on front of your adaptor. Shine a light in front bright enough to light up the paper mask sufficient for your camcorder to see it and the rectangles. If your setup is good enough, you should be able to frame the 24mm rectangle.

If you can only frame the smaller rectangles, your resolution and field-of-view will suffer because you will be over-enlarging a smaller groundglass area and not realising the field-of-view advantage these adaptors offer.

You don't need to do this paper mask inside the tube test. Instead, you can conduct this same test by simply drawing the rectangles on a paper sheet and fixing the paper to a vertical surface like a book or breakfast cereal box.

Set your camera with its Century fitted on a tabletop and slide the camera closer or furthur away from the paper until you can get the ideal frame and focus, then measure the distance paper (groundglass) to front of camera.

Then make up the right spacer to achieve that rearward offset with your adaptor.

I think you may find zoom 54mm or thereabouts will be what you need as a starting point to get the best frame.

You could frame wider, to get more of the groundglass but you then may also experience edge or corner brightness fall-off (otherwise known as hotspot) in the image.

Sorry to be making a simple idea complicated by this description.
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