This is a continuation in a discussion of spatial frequency response (SFR) and modulation transfer function (MTF) testing reproduciblity. The series starts here:
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.
Lynn Allan says
Do you prefer to use the DPR Sony FF forum or TLW for discussion? Either is fine with me, but using both seems problematic.
If it reaches the 150 threshold … would Tech and Science be better … but I see advantages of assuming an a7Rii wrt “baby steps vs walking vs running”.
I have confidence that you will be able to refine the protocol to take into account the vibration issue. Keep up the good work.
I think we’ll get more readership on DPR.
I agree that PS&T is a good place to put the follow-on discussion. However, if there’s much action, we may want to have two discussions: one for people who are trying to make it work, and one for people who are trying to say that the project is misguided. I don’t know quite how to handle that. If you and I are the only people trying to amke it work, then we won’t have to deal with that issue, but I may lose interest.
Lynn Allan says
I concur that there will be a “critical mass” at which it is worth proceeding, and more trouble than it is worth under that.
I’m not clear if it is a case of “the more the merrier”, at least at first. Maybe later if an inexpensive, solid protocol can be put together.
With a substantial number of people, I don’t mind being the “dummy checker” and “some village has lost its idiot”, but with only two or three participants, that get’s old and has diminishing value, if any.
I am a big believer in “two heads are better than one Kaizen”.
At this point with interest seeming to fizzle (hope I’m wrong), I’m debating whether it is worth tackling the MtfMapper learning curve.
I have gotten less concerned about the vibration issue, at least at my quiet location. I think other factors might swamp that problem. My impression is that error factors are cumulative, however.
Jack Hogan says
Re: modeling vibration
Boreman has a model for it in his MTF book that shows vibration MTF being the magnitude of a bessel function of the first kind j0(pifD), D being the peak to peak amplitude of the vibration in the spatial domain. You could start with that and have it decay exponentially. That would mean convolving the bessel function with the FT of the exponential in the frequency domain.
Christoph Breitkopf says
What about using the fully electronic shutter (silent mode) of the A7Rii to avoid the trailing curtain problem? Slowness should not be a problem in this use case, or am I missing something?