"Focus" problem that is driving me insane

[wmokrynski]

Hello,

I have a problem that has been occurring for quite a while, which has been driving me crazy, and I believe is getting worse. Yet another shoot down the tube last week has me at at the end of my rope. I'm now questioning everything, my technique, camera, lenses, tripod, shooting environment, sanity. I'm hoping someone here might have a solution. Is the problem something I'm doing, or is it a technical issue?

The problem I'm regularly having is that I'm getting part of the image coming out sharp, always the lower central part of the frame, then falling off to a blur in the upper area of the image. The unsharp part is not a soft out of focus lens blur, rather what looks to me like a motion blur.

All the work is shot at night with long exposures, almost always at f32. The long exposures in itself shouldn't be an issue as I've had dozens of successes under nearly identical setup.

Here's the most recent image. The white line indicates the border between sharp and unsharp.

A detail of the sharp area:

A sample of the problem:

A closer view:


As you should be able to see with the closer views of the problem, the blur is more of an overlapped or offset exposure as you'd expect with some sort of motion. However, if there were motion from ground vibration, wind, or from a tripod leg slipping, there shouldn't be a sharp area anywhere in the image. So I've ruled this out. (This image was taken on a still night with almost no traffic).

Admittedly my perspective correction technique is self-taught from books, so this is where I have my largest suspicion. But I have had success with this technique. Here's my technique:
- Set up and level the camera
- ensure the lens is in the center of the vertical rise on the front standard.
- frame the scene, which usually involves tilting the camera angle up (with the tripod head)
- To correct the perspective I tilt the camera back forward till the grid lines on the ground glass line up with the structure
- I then tilt the lens standard so it is perfectly parallel with the camera back.
- Sometimes I then need to tilt the camera angle further up and repeat the standard tilts to get the scene framed correctly.
- After focusing, the frame appears to be in focus throughout.
- I then ensure everything is tightened down (and have been paying particular attention to this lately), and expose

I would have expected that if there was a problem with my tilt adjustments not being perfectly parallel, that it would be compensated for with the stopped down lens (at f32), and any issues would appear as a soft lens blur. Again, I'm no expert in this. The problem I believe has also occurred in images without perspective adjustment. (Though I'm questioning my memory now too)

I'm working with an Ebony 45RW, 135mm Rodenstock lens (used in the image), 90mm Rodenstock lens (which I'm having the same issues with), on an older Manfrotto Art 190 (silver) tripod.

I can gladly post more problem images if needed.

Thanks in advance for any help.
William

[wmokrynski]

I should add that this problem happens randomly. I'm still able to produce perfect night images, but 25-40% of the time now I'm having this problem.

[wmokrynski]

Also, when the problem occurs, most of the time all of the images taken with the setup will have the problem. I always take a second exposure as a safety measure, and check focus in between.

[domaz]

Could be a film holder not loaded correctly or one is not holding the film flat Time to think about labelling your holders and recording which exposure goes to which holder. That way you can narrow down if it's a holder or not.

[Liam:]

Pinhole in the bellows? It certainly looks like motion blur or two images overlapping each other.

Are you able to upload a full size image?

[Eric Biggerstaff]

Film can "pop" during long exposures which can cause this, it sort of bows outward due to tempreture or humidity changes. I was just talking to a very well known photographer who is now doing a night project and he told me this just happened to him. The night was cool and foggy and he did not let the film adjust to the climate prior to exposure so it "popped" a little in the holder. Part of hte neg was sharp but a section of it was soft. Perhaps this is occuring.

[Kevin Crisp]

I was going to guess a film pop too.

[Oren Grad]

Film pop #3. The tell-tale is that the exposure in the affected areas looks doubled-up, indicating that the film was sitting in one position for a while, and then in a different position for a while. I've had this happen to me very occasionally.

Further to what Eric said, try pulling the darkslide and letting the film equilibrate to the prevailing temperature/humidity before you open the lens to make the exposure. You may need to experiment a bit to find out how long you need to wait for the film to "pre-pop".

[Gem Singer]

Have you tried mounting your camera on a heavier, more solid tripod?

[rdenney]

I don't see how this problem could be:

1. Diffraction.
2. Unstable tripod.
3. Camera shake.
4. Above, caused by cable release.

All of these would affect the image with a different-looking pattern. A corkscrew motion of the camera might affect one end of the negative more than the other, but I do not see how such vibration could caused the double images. And given that the sharp end of the negative is at the bottom of the picture, which is at the top of the negative in the camera, the tripod-end of the camera would have to be wiggling back and forth with the top remaining still--the center of rotation would have to be near the top of the camera. I can think of no vibration mode that could cause that. And that sort of a shift seems to eliminate the possibility of the camera flopping from one side of the slack caused by a sloppy tripod head to the other.

Diffraction would affect the whole image, and a diffraction pattern leading to a linear double image does challenge the mind somewhat.

As Doyle said in the voice of Sherlock Holmes, when one eliminates the impossible, then what remains, however improbable, must be the truth. Film popping is the only suggestion so far that could cause the effect.

Rick "who has certainly experienced film popping in scanners--and slide projectors" Denney