Smarter material for ground glass shim?

[LabRat]

It's not just the thickness of old fresnel, but due to new optical distance added by fresnel...

Check graflex.org for the proper distance there, I seem to remember it around .047" (?) about thickness of a popsicle stick that can be sanded to the thickness, or brass strip (available in USA retail) was close... Four little pieces on 4 sides... Then do a close-up focusing test wide open of three items like playing cards in a row focusing on the middle card, and checking focus on developed film...

I did this, and worked well...

Steve K

[maltfalc]

It's not just the thickness of old fresnel, but due to new optical distance added by fresnel...

Check graflex.org for the proper distance there, I seem to remember it around .047" (?) about thickness of a popsicle stick that can be sanded to the thickness, or brass strip (available in USA retail) was close... Four little pieces on 4 sides... Then do a close-up focusing test wide open of three items like playing cards in a row focusing on the middle card, and checking focus on developed film...

I did this, and worked well...

Steve K

the lens part of the fresnel is in direct contact with the gg and shouldn't cause any significant shift in the focal plane, which just leaves the potential shift caused by 1/16" of acrylic, which shouldn't be much different from 1/16" of glass if the gg is mounted backwards in the position of the fresnel. the frames with and without fresnels differ in depth by 1/16", so apparently nobody at graflex thought there was a noticeable difference in physical and optical distances.

[Neal Chaves]

I don't like Fresnels either, especially with wide lenses. I found a type of gray plastic material in a large hobby store used for model railroads that is the exact thickness required to shim a Graphic ground glass when the Fresnel is removed. The thickness of the shim required is less than the thickness of the Fresnel lens. I had already determined what was required by measuring the film plane of a holder in place and finding that plastic in the hobby store with a micrometer was a lucky coincidence. It is used for something to do with model railroad tracks. I don't have a piece handy or I would measure it for you. It's about as thick as a bank or credit card card and about an inch wide. You need to cut strips about 3/32" wide to fit under the ground glass.

[BIATHLONEIL]

Beer cans and a micrometer. There is a great variation in wall thickness. Do your research carefully but enthusiastically.