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Old May 4th, 2007, 06:33 AM   #1
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Join Date: Mar 2005
Location: Los Angeles CA USA
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DOF boxes and internal reflections

Hello all

I'm building a DOF adaptor based on a spinning GG and have noticed that I'm getting a pretty definite internal reflection - I think it may be coming off the rear element of the stills lens and the inside rear surface of the GG.

No matter how much I polish / matte out the GG's ground surface, it's a surface after the ground one, I surmise, that's doing the reflecting, since the reflection is sharp and not "ground out".

Everything inside my box has been painted matte black, and I have anti flocking material installed as well, so I'm pretty sure that it's none of those surfaces.

Another thought: I have a 72mm achromat +10 around an inch away from the rear of the GG, maybe my video lens is too close, and that's the element that's reflecting?

I have an FX7 and can pretty much put the camcorder where I want, especially with the achromat.

Zoom to cover and prevent vignetting is approximately at 50%.

No condenser or second closeup unit in my optical path at the moment. I had a temporary GG installed (just a frosted end CD, without proper grinding) and had a tiny image circle and terrible vignetting problems.

I thought I'd need a condenser for sure, but then I re-read some of these threads, went Ettore's path and ground out my own CD. The image circle is now much wider and the illumination across the image is much more even.

Now, if I could just sort out that pesky internal reflection...
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Old May 4th, 2007, 12:04 PM   #2
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Is your groundglass surface on the front face of the groundglass closest to the back of your SLR lens or on the rear face closest to the camera.

If it is, try turning the groundglass over so that the groundglass surface faces the camcorder. No guarantee it will work but it might move something shiny far enough from another focussed reflection source to fix it.

(If your condenser optics are wide enough and they are bare glass, you might try painting their outer frosted surfaces with flat black or proper lens black and maybe about a 1/16" or 1mm approx rim in from the outer edge to mask off any stray reflections going on.

If there is a condenser causing your problems the reflection may be an evenly balanced pattern across your image. If it is not balanced, then making sure your condenser lenses are accurately centered relative to the optical center axis and also spot on 90 degrees across the center axis may make the problem go away on its own.)

Is your achromat mounted onto the camcorder or somewhere between it and the GG. It really needs to be mounted to the camera.

Last edited by Bob Hart; May 4th, 2007 at 12:14 PM. Reason: error
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Old May 4th, 2007, 01:21 PM   #3
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Join Date: Mar 2005
Location: Los Angeles CA USA
Posts: 507

Thanks for your help.

Setup right now is

a) Camcorder - 62mm to 72mm ring - 72mm +10 achromat

Into box

b) rear of GG (facing camcorder) is the clear side, front (facing subject) is the ground side

c) Nikon mount

d) Nikon lens

No condenser to speak of, no flocking of achromat or GG "gate" yet.

Actually, the reflection is there regardless of the orientation of the GG.
In fact, it's clearly visible to the naked eye even without the apochromat, just looking at the GG with the stills lens attached.

In fact...


Never mind.

I was using big glass.
Nikkor 50/1.2, full open.
So I stopped it down.
Reflections went away slightly.

SO I swapped it out for a 105 Micro.
The reflections went away totally.

I thought... "has to be the 50...right? nah..."

Of course, then a minute later... voila.

It was the Tiffen 52mm Skylight filter on the 50mm lens.

Removed the filter, bye bye big reflections.
There are still some internal reflections that go with having glass that big, but not in focus and not distracting. Good ole lens flare, in fact.


"Adding any extra glass (at all) to the optical path may adversely affect the desired purity of image".

QED, lesson learned.

Thanks again, Bob!
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