MTF testing & vibration

This is a continuation in a discussion of spatial frequency response (SFR) and modulation transfer function (MTF) testing reproduciblity. The series starts here:

Towards a reproducible MTF testing protocol

A reader brought up the subject of vibration and its effect on SFR testing.

I suppose we should deal with that sooner rather than later. However, I warn you that I have very little to say about vibration and sharpness testing that constitutes good news.

Let new tell you about my testing setup from the perspective of vibration.

I’ll start with external (out of the camera) vibration. I do my work in my basement, which has a six-inch concrete slab covered with unpadded vinyl tiles. My house is a mile from the nearest public road. I use heavy RRS carbon fiber legs on the tripod to which the camera is mounted, and I use the rubber indoor hemispherical pads on the feet. In theory, spikes would be better, but to preserve domestic tranquility, i wish to avoid scratching the floor.

I also mount the razor blade carrier to a set of (smaller) RRS legs, which rest on the same vinyl tiles with similar pads.

Because of where I live, in the absence of earthquakes, there are no material vibrations from things not on my property. I have found this setup to be resistant to local vibrations such as footfalls. I have not tested what would happen if someone were using a flail mower right outside, but I expect that that would cause problems.

When I use charts mounted to a wall in the basement, footfalls can be a problem with sharp lenses measured on-axis. I expect that they vibrate the door jamb and shake the door to which the targets are mounted. Thus, when I work with charts, I need to be concerned with who else is in the house and what they’re doing.

In an environment where there are low-frequency external vibrations, mounting both the target and the camera to tripods supported  by the same floor structure can help get consistent results, since low-frequencies will tend to lift and lower both the camera and the target synchronously. That only works for translation, though. If the floor twists as it rises an falls, all bets are off.

What about camera-induced vibrations?

Before electronic first-curtain shutters (EFCSs), I never could achieve the results that I currently get with focal plane shutter cameras. If your camera has neither a leaf shutter nor EFCS, I think it would be a huge uphill battle to get reproducible SFR results, and I’d advise you against trying.

With EFCS, camera-induced vibration is rarely a problem. The trailing curtain launch acceleration does cause some measurable SFR degradation at shutter speeds around 1/125 to 1/30 second, so in a perfect world I’d adjust the lighting levels or the camera’s sensitivity to avoid those speeds when testing the best apertures of the best lenses.

Unless specifically testing for shutter-induced vibration, I’d avoid portrait orientation, since tripods are less stiff horizontally than vertically.

If you live in an urban environment, or one near train tracks or highways, you need to be concerned with vibrations external to the building in which you do the testing. If you plan to test in a large building with heavy machinery (including HVAC equipment) and people whose movements you cannot at least briefly control, you should think about the effects of that vibration on your testing.

I wish I could provide some quantitative guidance. I had a couple of sites that had some good material on the subject of environmental vibration, but my old links are broken now. I’ll do some more looking.