Snod35 V1 - Focusing Element
I have tested various types of focusing elements including the Nikon D screen, AOL case (translucent plastic), and the Canon Ee-a and Ee-s screens. Hands down, I have had the best results with grinding my own glass and making my own wax screens. Grinding your own glass is a fairly simple process and only takes 20-30 minutes. I obtained most of the info for grinding my own glass from an article by Dick Dokas in Photo Techniques Magazine (May/June 2003).
There are several types of lapping powder that you can use to grind your own glass. These powders can be purchased from Willmann-Bell (willbell.com or (804) 320-7016). The thing to keep in mind with the focusing element of a vibrating adapter is that it needs to be rough enough that it will diffuse the light properly and yet not too rough as to create too much light loss or grain in your images. Of course with vibrating adapters you will not actually see the grain, but instead the pattern that the grain is making as the vibration is occurring. When the focusing element does not diffuse enough of the light (spread the light out evenly over the surface), your images will suffer from hot spots and/or vignetting at higher f/stops and with short focal length lenses.
The image below to the left is from glass that was ground with 5 micron aluminum oxide. The f/stop of the 50mm lens was at f/22. Though the images with the 5 micron ground glass were beautiful up to f/5.6, they slowly began to break down as the aperture size was decreased. The 5 micron aluminum oxide did not create a rough enough surface to hold the projected image and diffuse the light evenly. In this case, some of the light from the projected image "burned" through the ground glass. The center image below shows some of the various types of focusing elements that I have tried. The image below at right shows some of the aluminum oxide and silicon carbide that I have tested.
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For the focusing element in the Snod35, I use the plano-convex lens taken from a Nikon F3 series focusing screen. A plano-convex lens from any of the F3 series screens will work. I removed the plano-convex from the frame and separated the plano-convex lens from the plastic focusing screen (image below at left). I ground the plano side (flat side) of the plano-convex so that it works as a condenser and focusing element all in one.
To grind the glass, I used a 4" X 4" X 1/4" piece of glass as the grinding platform (purchased from a local glass store for $2). I used a small amount of 12 micron aluminum oxide powder and water (about 60% water and 40% powder). It is important to make a figure eight pattern while grinding and periodically rotate both the 4" X 4" piece of glass and the plano-convex lens. It is best to grind in this manner for around 15 minutes, stopping every 5 minutes to rinse both pieces of glass with water and to refresh the lapping powder and water mixture. Rinsing the glass off every 5 minutes ensures that there are not any large glass chips floating around in your mixture (large in comparison to the size of a single grit of aluminum oxide that is). Though I have yet to scratch or smudge the convex side with my fingers while grinding, I still try to grasp the plano-convex lens on the edges as much as possible.
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The image above at center shows the items needed to grind the glass. You can see the circular pattern created on the 4" X 4" piece of glass from being used for many grindings. When rinsing the plano-convex lens, it will look as though nothing has been accomplished. In reality though the surface is becoming rougher. The surface tension of the water causes the water to form a nice smooth layer across the surface making it appear as though it is still smooth. Once the glass begins to dry however, it will become apparent that the surface is in fact rough.
The final rinse of the plano-convex needs to be a thorough one. It is a good idea to hold the glass under warm, fast running water for several minutes to ensure that the aluminum oxide is fully rinsed from the glass. A heat gun or a blow dryer can be used to help the surface dry evenly. In some cases, after the glass is dry, it needs to be cleaned with lens cleaner. Though it may make some cringe, I use Windex and a paper towel to gently clean the ground surface. If you try it this way, spay the Windex on a portion of the paper towel so that it is damp and then gently and evenly wipe the ground surface. Any of the tiny fibers from the paper towel can be wiped away with a lens brush once the ground surface is dry. The glass can be reground several times, so if the surface is ever damaged, it can easily be ground again. The image above at right shows the plano-convex after having been ground and cleaned.
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