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Old May 11th, 2008, 11:34 AM   #1
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Another DoF question

I've run into a potential issue I need advice on. I've been experimenting with various configurations for my DoF adapter and have gotten "pretty good" results. However, I seem to have one issue I am unable to resolve.

NB - I am using a salvaged focus screen from an old Canon AE1 w/focus aid in the center.

Basically, when my rig is attached to my Sony HC1, the focus is sharp in the center and then fades in a circular pattern moving away from center. I've changed the configuration numerous times now, trying three different macros (+10 diopter, 0.6x WA, and two 50mm objectives from an old pair of binoculars) - the macros seem to flatten things out to some degree, but then I have issues of color separation and/or softness.

Since the HC1 has a 37mm thread, and Daniel's site says that camcorders with less than 43mm thread don't need macros, I changed the configuration again to remove the extra lenses. This made matters worse with regards to focus.

I then tried the rig on my Sony D8 camcorder - it also has 37mm threads. Lo and behold, the focus appears even across the FOV (though barrel distortion is worse). What is going on and can it be fixed?

Sony HC1 HDV: http://i120.photobucket.com/albums/o...ony_HD_pic.jpg

Sony TRV740 D8: http://i120.photobucket.com/albums/o...ar/Sony_D8.jpg
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Old May 12th, 2008, 12:57 AM   #2
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I thought the ground glass on a DOF adapter had to be completely plain. No focus aid or marks. Maybe you should just change your GG. Nikon D? Canon Ee-A or Ee-S? They're quite cheap and well documented.
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Old May 12th, 2008, 01:33 PM   #3
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Quote:
Originally Posted by Jose A. Garcia View Post
I thought the ground glass on a DOF adapter had to be completely plain. No focus aid or marks. Maybe you should just change your GG. Nikon D? Canon Ee-A or Ee-S? They're quite cheap and well documented.
That is correct, and I have an Ee-A on order (back order), but I've been experimenting with this Canon focus screen as a learning experience. What I am wondering about is the difference in focus - I thought that if the focus was set for, say 5', then anything at 5' would be in focus regardless of whether it was right in front of the lens or off to the side. What I am seeing is that anything directly in front of the lens can be in focus, but anything off the center axis is out of focus.

I then tested with my D8 which seems to yield much better results indicating that the "problem" is with the optics in the HD cam. I don't know if this is a result of this particular focus screen or something else (I'm not concerned with the distortion in the exact middle as this is where the focus aid is located, so I would expect funky results).
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Old May 12th, 2008, 04:41 PM   #4
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This is likely an issue with a curved focal plane as you zoom the camera in. Using a macro lens (like our 72mm non-flip achromat) would require less zoom-in from your camera, and likely resolve the issue. We don't recommend a macro-less adapter for any cam as the smaller cams show both edge softness, and for the HV20, pronounced edge falloff as you zoom in over 60% or so. That's with a "bare" camera, so these issue must be resolved apart from the adapter.

A good way to troubleshoot is just use a small 24x36 target and get your camera dead square to it. Test first on the target to baseline your camera's behaviour!

There's a pdf document you can print out to test in an old post (but pertinent to your issue) here.
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Old May 12th, 2008, 08:58 PM   #5
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Quote:
Originally Posted by Dennis Wood View Post
This is likely an issue with a curved focal plane as you zoom the camera in. Using a macro lens (like our 72mm non-flip achromat) would require less zoom-in from your camera, and likely resolve the issue. We don't recommend a macro-less adapter for any cam as the smaller cams show both edge softness, and for the HV20, pronounced edge falloff as you zoom in over 60% or so. That's with a "bare" camera, so these issue must be resolved apart from the adapter.

A good way to troubleshoot is just use a small 24x36 target and get your camera dead square to it. Test first on the target to baseline your camera's behaviour!

There's a pdf document you can print out to test in an old post (but pertinent to your issue) here.
Many thanks Dennis. I did the test and the results are, well, unfortunate I think. Take a look:

http://i120.photobucket.com/albums/o.../D8_zoomed.jpg
http://i120.photobucket.com/albums/o.../HD_zoomed.jpg

Both of these were taken in photo mode, but I think it's clear the HD cam has an issue around the edges. Both cams were zoomed in until focus was no longer possible. Any less zoom and the side lines appear.
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Old May 14th, 2008, 11:47 AM   #6
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Could someone please explain what's going on with these shots.

http://i120.photobucket.com/albums/o...tte_to_cam.jpg
http://i120.photobucket.com/albums/o...iny_to_cam.jpg

The first is how I've had my focus screen configured - with the matte surface toward the camcorder and the shiny surface toward the 35mm lens. It makes sense to have the focus screen this way around, and yet...

When I flip the focus screen, something very odd happens. In my view both the lighting and focus are more even. The only issue I see is a sort of shadow from the Fresnel patten. It's hard (if not impossible) to see it on this shot, but in the full res image it definitely shows up.

Why would the second image be better in some important respects than the first?
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