Playing around with Jim Galli's 14 inch

[c.d.ewen]

A really, really embarrassingly long time ago, Jim Galli did me a favor, and I sought to repay that favor by offering to put one of his lenses - a Goerz 14 inch Blue Dot Trigor - into a shutter. After that project was finally finished, he asked me to play around with the lens, to see what I thought of it. I'm still playing.

I was quite confident that I had gotten the cell spacing correct to within a few 10,000ths of an inch, but it made me wonder: How close is close enough?

Pondering the problem for some time, I eventually mounted the cells into the ends of two brass cylinders, one screwing into the other. This changes the cell spacing in a reasonably accurate way - 0.5mm per revolution. The inner cylinder, holding the front cell, was internally threaded to allow a center-drilled disk, functioning as an iris, to be mounted inside at various distances behind the cell.

I've coupled this device to a digital camera and have taken many hundreds of pictures. I thought I'd present some results of this activity to the collective, and ask smarter folks than I what I should do next.

Below are a couple of pictures: the first is an uncropped shot of a scene, and the second is a composite made from crops of seven separate pictures of that scene. Each crop is marked with the cell spacing used for that photo. What I find fascinating is that there's no appreciable difference in sharpness between the various cell spacings.

Here're some tech details: I used a Nikon D90 with MC-DC2 wired remote and shutter delay (no mirror lock-up available) to minimize vibration. The photos were taken through the approximate center of the lens. According to SKGrimes' website, the correct cell spacing is 32mm. A disk with a 20mm center hole was used; this would approximately equal f/16 on the BD Trigor. The disk was set 17.7mm behind the front cell. The only post-processing done was an Auto Levels in Photoshop. The sensor width is 23.8mm, and the cropped area is about 1.6mm wide.

One last note: I've avoided using the t-word (test). To me, lens testing requires a far more rigorous set-up and protocol than I've used. I'm just playing around.

Oh, and Jim....your lens is ready.

Charley

[John Schneider]

If it were me, I'd have silenced any ambiguity by entitling this thread "Experimenting with a 14" Trigor belonging to Jim Galli."

And by the way, would you mind posting pics of the finished lens? I have a 14" Goerz WA Process lens that I intend to someday mount into a shutter.

[Jack Dahlgren]

The samples are too small to tell, but it looks like the middle one has a bit better look to it. A few percent difference in the cell spacing doesn't look like it makes much of a difference, but I'd not expect that it need be accurate to the 1/10000.

[Mark Stahlke]

To my eye the foreground rocks and bushes look sharpest at 32mm and the background door and windows look best at 34mm.

Is it possible that some other defect in the experimental setup is masking the effect of different lens cell spacings?

Cheers,
Mark

[PBrooks]

yeah I think 32 also, but I wonder if I would have picked it over the others had it not been in the center or having read skgrimes said 32mm. Don't know?

[pandachromatic]

I came in here expecting porn.

[c.d.ewen]

Here's a larger version of the composite photo:

http://www.flickr.com/photos/cdewen/4037273456/sizes/l/

Some of the differences in sharpness people are seeing in the photos is the result of differences in focus. I used a DG-2 eyepice magnifier - a 2x loupe - for focusing. I found I had to make multiple exposures, refocusing each time, in order to get the sharpest photo. What's interesting to me is the lack of significant differences in sharpness between the various cell spacings.

Charley

[Bill_1856]

"What is a "Blue Dot Trigor?"

[cowanw]

Here's a larger version of the composite photo:

http://www.flickr.com/photos/cdewen/4037273456/sizes/l/

Some of the differences in sharpness people are seeing in the photos is the result of differences in focus. I used a DG-2 eyepice magnifier - a 2x loupe - for focusing. I found I had to make multiple exposures, refocusing each time, in order to get the sharpest photo. What's interesting to me is the lack of significant differences in sharpness between the various cell spacings.

Charley

Yes, I could not really see a difference over the 1/2 centimetre you measured either.
This reminds me of the difference in old instructions and some modern practices of the variable front lens diffusion spacings such as the diffusion velostigmats. The original instructions suggest not refocussing after focussing at zero diffusion and resetting the diffusion setting.
I have found that if you refocus after setting the diffussion ring the effects are much less.
So are you just changing the focal length of the lens by spacing it a half centimetre or so, in or out?
Regards
Bill

[c.d.ewen]

Yes, I could not really see a difference over the 1/2 centimetre you measured either.
This reminds me of the difference in old instructions and some modern practices of the variable front lens diffusion spacings such as the diffusion velostigmats. The original instructions suggest not refocussing after focussing at zero diffusion and resetting the diffusion setting.
I have found that if you refocus after setting the diffussion ring the effects are much less.
So are you just changing the focal length of the lens by spacing it a half centimetre or so, in or out?
Regards
Bill

Bill:

This is the kind of thought-provoking comment I was hoping for. I had this rig on a monorail Horseman, moving only the rear standard for focusing. I never looked at the scale to see how different the focus points were. I'll look next time.

RE: diffusion - below is what you get when you don't change the focus.

Charley