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
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