a start.<\/span>)<\/li>\n<\/ol>\n\nIts the first point — a black background — that's important\nfor this post....<\/p>\n\n
The Fundamental Difference<\/p>\n\n
I'll explain in detail further on in this post, but in short, the\nfundamental difference is that the normal “black ink on white paper” method\ninvolves taking a photograph<\/span><\/b>, of which shutter speed is an integral\nand important part of a correct<\/span> exposure. On the other<\/span> hand, my method uses\nthe camera along the lines of a device to detect photons<\/b>, where the\nshutter speed merely indicates the duration over which photon detection\noccurs.<\/p>\n\n\nPerfect Situation: No Vibration<\/p>\n\n
Let's consider the perfect case of absolutely no camera vibration while\ntaking a test<\/span> shot. In that case<\/span>, the shutter speed doesn't matter for the\nbackground areas of the test chart, because fast or slow, no photons will\narrive from it. (I use<\/span> “no photons” quite loosely here, but you know what I\nmean.<\/span>)<\/p>\n\nSo, in the perfect case, the shutter speed has an impact on only how\nbright the lighted portions of the test chart show up. Too fast a<\/span> shutter\nspeed and nothing will show up, and too long an exposure and electrons will\nstart to “bleed” to adjoining pixels. That last situation is\nwell beyond simply “blowing out the highlights,” and requires a\nreally<\/i> long exposure, relatively speaking.<\/p>\n\nSo, as long as the shutter speed used for the test is between the “too\nlong” and “too short” speeds, the result should have meaning. At the ISO<\/span>\nand aperture I used<\/span> (100 and f\/9), I could<\/span> see the test image well enough\nat 1\/250th of a second,<\/span> so we'll call beyond that “too short.” I didn't<\/span> test a shutter<\/span> speed longer than 10 seconds,\nbut they were usable, so we'll call anything beyond that “too long.”\nTheir highlights were likely “blown out,” but that\ndoesn't really matter to vibration testing unless electrons start to bleed\nto adjacent photosites.<\/p>\n\nSo, considering that the range I just<\/span> presented (1\/250th to 10 seconds)\nrepresents about 12 stops, and the D200 sensor has almost that much dynamic\nrange to begin with (as described\nhere<\/a>), I think<\/span> most any speed within that range would properly reflect\nthe perfection of our utopian case.<\/p>\n\nImperfect Situation: Vibration<\/p>\n\n
So, if there is<\/i> vibration, how will that show up in the test\nimage? If even the<\/span> slightest vibration causes the lit portions of my\ntest chart to shine onto the area of the sensor where the background\nnormally appears, those photons will be like a flashlight<\/span> in the dark, and\nshow up in the result.<\/p>\n\nVibration is often a back-and-forth thing, so if a shot's<\/span> vibration\nlasts for, say, 1\/10th of a second,<\/span> it'll cause photons to collect on\neither side of a bright<\/span> area (in an area that should be black) for half\nthat time, or 1\/20th of a second.<\/span> That's well long enough to show up.<\/p>\n\nThis is all very different than what happens with a normal<\/span> test chart,\nwhere the background is white and is flooding the sensor with photons\nalmost all the time. In fact, the only way the vibration shows up in such a\nsituation<\/span> is because the vibration caused a reduction<\/i> in photons, a\nreduction<\/span> that indeed can get “washed out” as the exposure time\napproaches a length<\/span> intended to properly record the whiteness of the\npaper.<\/p>\n\nThis difference in the two approaches is doubly noteworthy because of\nthe nature of human perception of light. When a light<\/span> is dim, as the\nbackground of my test chart is, a 10<\/span>% difference in light intensity<\/b>\n(number of photons) results in a much<\/span> larger percentage change in\nbrightness<\/b> (human perception of light). Thus, a little<\/span> vibration\nresulting in a little<\/span> extra photons can be noticed.<\/p>\n\nOn the other hand, when something is perceived as bright (like a\nproperly<\/span>-exposed sheet of white paper), a 10<\/span>% difference in the intensity\nis perhaps not even noticeable. (It might come as a surprise,<\/span> but you may\nnot even notice half<\/i> of the sun being blocked during an eclipse\nunless you happen to first notice the changing shape. You have to<\/span> cut light\nby about 82% to cut the perceived brightness by half. What remains after\n82% is cut — 18% of the light — is where a standard<\/span> 18% gray card<\/a> finds its\norigins.)<\/p>\n\nAn Analogy<\/p>\n\n
I set my camera up in a dark room, and took pictures of myself making a\ncircle<\/span> with a flashlight.<\/span> In each shot<\/span>, I made about<\/span> one revolution with\nthe light (sometimes a bit<\/span> more, sometimes a bit<\/span> less), and took about\nthree-quarters of a second<\/span> to do it.<\/p>\n\nHere are four of those exposures, with shutter times ranging from 1\/2 second to 15 seconds:<\/p>\n\n
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