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04-13-2012, 01:20 PM   #16
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QuoteOriginally posted by RioRico Quote
I think we can establish a rule here: If you add enough extension to need changing the focal length number for the SR'bot, the working distance will be too close for SR to be effective. Use flash or other external light, and/or a fast shutter, and/or a tripod or other immobilization. SR has its limits.
QuoteOriginally posted by Nass Quote
Still confused. Because that particular post actually had nothing to do with extension, just a lens focused at different points.
The appropriate focal length to use includes the extension; SR continues to mitigate camera rotations but does not help in-out, so use it for hand-held shots even though it isn't perfect. try some shots at 1:1 handheld with & without SR at 1/10 s.

Even for magnification without extension the effective focal length to enter is about actual.focal.length(1+magnification); the magnification can be viewed as increasing the sweep of the image across the sensor when the camera moves.

Dave

I said "about" because in at least some cases the "pupilary ratio" should also be considered.

04-13-2012, 01:23 PM   #17
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QuoteOriginally posted by Nass Quote
Still confused. Because that particular post actually had nothing to do with extension, just a lens focused at different points.
I guess I was (sloppily) trying to address both that and the general question. Yes, your knickers were twisted unnecessarily. And yes, there's a general rule we can apply.

Back to focus distances: At any specific focal length, up-down and back-forth wobble should be of the same angles regardless of where the lens is focused. The SR'bot needn't calculate the distance swept by those wobbles; it just needs to estimate the arc|angle from the focal length. Fore-back wobble will be insignificant at far distance, up to very significant at close-up distance. Focusing anywhere closer than infinity extends the lens a bit -- a lot with a longer lens -- and so increases the fuzziness caused by fore-back motion. Adding extension just exaggerates that.

Maybe an ideal SR system would use IR or laser or ultrasonic (SX-70!) rangefinding to pinpoint a subject and lock the sensor onto it, tilting the sensor plane in 3D to counter camera motion in any axis. Or maybe a wee tiny gyroscope would help...
04-13-2012, 01:40 PM   #18
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QuoteOriginally posted by RioRico Quote
The SR'bot needn't calculate the distance swept by those wobbles; it just needs to estimate the arc|angle from the focal length
What SR corrects is the arc|angle times the effective length from lens to sensor. That's why extension is counted in the SR query.

Say the camera swings a small angle "alpha" the distance an image point on the sensor moves is (distance.to.lens)x(alpha). The lens distance acts as a lever arm; wiggle a laser pointer at a wall - then move back & wiggle it the same amount - the pattern on the wall will increase in proportion to how far you move, just as light from the lens sweeps over the image plane when the camera is wiggled.

The accelerometers measure alpha - the lens-sensor distance must be otherwise determined.

Dave
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