Re: Focus problem on Century Graphic

Posted by Dennis Lee on November 08, 2000 at 23:01:27:

Hi Bob,

Thanks for asking, here's where I'm at.

I did a version of the 'fence shot' focus test using the railing posts on our front deck. I clamped a small caliper to one of the posts and focused on it with about four posts showing to the left (ie, foreground)and several more showing to the right in the background. The caliper located dead smack in the middle of the full frame 6x9 format.

I removed the ground glass from the GG back so that I could move it between the gg back and the film back. I focused on the numbers of the caliper where they intersected with caliper arm closest to me. This was done while holding the gg tightly in place in the gg holder and using my Leica 50/2.0 lens as a lupe. Crisp focus achieved I carefully placed the film back in place on the camera. Removing the film casssette I placed the gg on the backs film plane and rechecked the focus.

Focus matched perfectly, I checked it a couple of times and then loaded the back and shot a roll of RDP II bracketing 4.5 through 32.

Next I took the camera out to the schools athletic field and did some shots at long distance. I shot two rolls out there of scenes at two different distances using the same focusing technique as mentioned with the caliper shot. I should also mention that I did not use the preset infinity stops here because I didn't want to be limited by the focus racks movement.
One shot was focused at a row of trees about 500 yards away with a soccer game going on in the foreground. The other scene was focused on a football upright about 30 yards away with a line of trees another 30 yards behind it. Same film, same bracket.

I processed the film and all the caliper shots looked great, well, sharp. The caliper numbers were sharp through the whole sequence with a slight degredation occurring around f32. One frame was a little soft (f8) and I attribute it too camera shake. In all this testing I didn't have a cable release. I wish I had, but....

Depth of field didn't carry to the edges of the scene in the caliper shot. But it did prove a certain solidity of focus in the center of the frame.

The long distance field shots where another story though. A few of the long distance shots weren't tack sharp at all. Some where sharp in the middle with a general degradation out to the edges. There's at least one where the edge is sharp and the center is soft.

The shorter distance shot was also very inconsistent. Generally the standard I focused on was pretty close to sharp but once again not tack. The background trees which I also focused on for a couple frames where also inconsistent.

Oddly the camera seems to focus OK closer up but not further away. It is possible that my lens standard was a little twisted for the 'distance' shots and hence the inaccurate results but I don't really think so. I think the problem is with the film back. This is the '23' back with the knob wind, but it does have the small pin rollers that supposedly help hold the film flatter.

I'm going to rent a horseman 6x9 back and do another test. I'm hoping this will be the answer.
I've never had so much trouble with getting a sharp picture before and it's becoming a bit frustrating.

Sorry this was so long winded, but it should give you a pretty good idea of what I did. Let me know what you think.

Thanks again, DL

Follow Ups: