Sunday, November 16, 2008

YSM35B Adjusting Infinity Focus

This is the fifth in the series of instructions on how to build the YSM35B Vibrating 35mm DOF Adapter. In the last part we assembled the mount without the electronics. At its present state it is a static adapter. We will adjust the ground glass distance to have correct infinity focus.

Adjust Ground Glass Distance

Watch the video. Pardon the bad audio, it was windy that day.

You will need the following parts to test your current assembly:

35mm lens
+10 Diopeter Macro (Achromatic is best)
Step-up rings
Circular Filter sans glass
Rubber Band

First attach your choice of 35mm prime lens unto the F-mount. Set to infinity and set to the widest setting (lower number). Aim the assembly at some far away trees. Remove the #3 and #1 rings Push the top base closer or further away until the focus screen (ground glass) gives you the sharpest image you can perceive. It should be somewhere between 45mm-46.5mm. In my case it was a tad more than 45mm. Check that the top mount is as horizontal as possible.

Screw back the #3 and #1 rings. Now attach your Macro, Circular Filter ring and Step up rings. Attach to the camcorder. Turn on the camcorder and look through the viewfinder. You should see the image on the ground glass. Rotate the assembly until the screen looks horizontal. You can "lock" the CPL with a rubber band. The friction keeps it from moving when you make manual adjustments to the lens later.

Use the camcorder zoom to cover the entire recorded image. Note the LCD doesn't show the overscan area. Turning off Image Stabilization is probably a good idea as well. Point at some far away objects and let autofocus zero in on the focus screen. Lock manual focus.

You now have the adapter adjusted. It can be used as a static adapter and you could stop right here. In lower light cases or stopped down you will notice static grain on your images. The rest of the series will go toward eliminating that.

Demonstration of Static Grain and Dust

Download the video directly from the Vimeo page. Watch it full screen.

Why did we do this?
Setting infinity focus will be important later when we make the focus screen vibrate. You will notice some lens dust and grain in the static test above (download and view full screen). A vibrating adapter will remove that.

What's next?
We start on the electronics that will vibrate the focus screen.

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