Experience with chinese helical focusing devise.

**sparq** · 5-Jul-2007, 15:28

Here are comparison shots of the device w/the wide Nikkor and of two flat lens boards. On the left, you can see SA 8/65 on #0 board; on the right the device w/Nikkor and a #1 board on top of it. This test is far from being geometrically correct, but I hope that it will give you an idea about the actual position of the hole on the wide angle device.

**David A. Goldfarb** · 5-Jul-2007, 15:32

The last image in the first three is taken from above, rather than straight on, so it's hard to judge whether it is centered with respect to the groundglass. From the first two pictures, it looks like the helical is positioned below center as it should be, but then from the next two shots it looks like it might be above center.

Let us know how the pictures look with the 65/4. Are you getting vignetting from excessive front rise?

**sparq** · 5-Jul-2007, 16:21

Originally Posted by David A. Goldfarb

The last image in the first three is taken from above, rather than straight on, so it's hard to judge whether it is centered with respect to the groundglass.

How can you tell?

It is taken from below actually; the asymmetrical bellows makes it look like from above. The center of the hole is a few mms (gross guess: 5mm) higher than centers of my flat boards.

I haven't had an opportunity to make any pictures with the device, yet.