CAUTION WHEN USING TEMPORARY CARDBOARD SHIMS.

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robert Hart

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CAUTION WHEN USING TEMPORARY CARDBOARD SHIMS.

PostWed May 12, 2021 4:45 pm

Whilst testing a behind-the-lens IRND filter option, I reset the URSA Mini 4.6K EF-Mount slightly forward using temporary thin cardboard lens mount shims to research thicknesses for metal shims to be made later.

Cardboard and paper shims with complex shapes and patterns of holes are easier to cut, punch and stack, then measure once the correct flange to focal plane distance including the indwelling filter has been restored. One need only measure the thickness of the cardboard/paper stack to choose the correct thickness of metal or plastic shim material.

Be aware that the shiny layer on some cereal packet cardboard turned out to be chemically antagonistic to the magnesium alloy and had set off a fine white powder corrosion on the surface it touched after about five weeks.
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rick.lang

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Re: CAUTION WHEN USING TEMPORARY CARDBOARD SHIMS.

PostWed May 12, 2021 6:16 pm

Who would have thought that coating is ubiquitous on many food products. What does eating food from those packages do in your stomach if you’re also taking a calcium-magnesium supplement? Would be interesting to give that issue to a post-graduate student.
Rick Lang
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robert Hart

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Re: CAUTION WHEN USING TEMPORARY CARDBOARD SHIMS.

PostThu May 13, 2021 6:18 am

Rick. I suspect it might be something as simple as the polished side of the cardboard attracting moisture and setting off the corrosion bloom. You only have to look at some magnesium alloys and it goes to white powder, CP16 cameras being the notorious examples. In this instance, I was removing the indwelling IRND filter and the shims for an unlit indoors event shoot. I thought first that the shiny layer of the shim had peeled off and stuck to the magnesium, but it was actually a coating of fine white bloom and some loose white dust fell into the camera throat. I have been putting off cutting the metal shim as it is complex and will need to be done by an engraving bit on a Dremel tool.

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