View Full Version : Double Image Off Axis - Fresnel?

Mark Stahlke
15-Oct-2006, 13:38
I recently acquired a Nikon SW 120/8 to use as an ultra wide on my Canham 8x10 (with fresnel lens). Naturally, when the lens arrived I simply had to look through it. The first thing I noticed was a terrible double image off axis. It gradually gets worse the farther from the center of the ground glass I look. The double image is so bad in the corners that I can't tell if the image is in focus.

Being curious by nature, I put a 150mm lens on the camera and carefully checked the corners of the GG. The double image effect shows up with this lens too although it's much less noticable and it's easy to check the focus in the corners. I can even see a very slight double image in the corners with a 210mm.

I haven't shot the 120 yet but images taken with the 150 are sharp to the corners with no sign of the double image on the film. This leads me to believe that the double image is being caused by wide angle lenses and the fresnel. Is that possible? I've never seen this effect on my Tachihara 4x5 (also with fresnel) using lenses as wide as 55mm.

On the Canham the fresnel lens is located in front of the ground glass, nearer to the lens, that is. If I remove the fresnel would I need to shim out the ground glass?

Are there any other solutions? I suspect I'll really like the extreme wide angle view of the 120 but currently it's almost unusable.

Thanks for your help.

15-Oct-2006, 16:22
yes, you are developing astigmatism. Get your eyes tested.

I'm not joking, I see everything double nowadays. A simple check is to look at the stars at night. If they appear as binary pairs then you have astigmatism. And tilting your head through 90 degrees will cause the binary pair to tilt through 90 degress with your head so you will know it is not a real binary star system. Check each eye individually.

Thats not to say it isn't due to something else, but you can elliminate this one possibility very easily by looking to the heavens at night.

When this first started for me I could (and still can) see red and green in the pair of stars. Because of this I was convinced I was looking at a stationary aircraft.

Brian Vuillemenot
15-Oct-2006, 17:06
When using a Fresnel, you will always see that double image when you look out to the edge of the ground glass. The wider the lens, the more pronounced it is. It's kind of annoying, as are the concentric rings of a Fresnel, but not too hard to get used to. I would rather deal with those issues and have even illumination across the entire groundglass than to go without the Fresnel. One solution would be to get a removable clip-in type Fresnel so you can have it both ways.

Canham cameras are set up with the Fresnel on the inside of the ground glass, so if you remove it it will introduce a slight focus shift. It would not be difficult to have Keith Canham or someone else adjust it for you, though. Good luck!