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05-07-2011, 02:06 PM   #16
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QuoteOriginally posted by jlaubza Quote
I follow the point of the FL being what it is but the old rule of thumb of 1/FL for a speed guide should surely be 1/1.5FL for an APS-C DSLR because the FOV on the sensor is as if one is using a lens that magnifies an image 50% more than if that lens is being used on a 35mm frame size camera.
Yes, true. That's because the old rule of thumb was not really about focal length but about field of view; it just happens that focal length serves as a nice standin if you know the right conversion factor. Of course, the old rule of thumb was never more than a rough guess; I think you could easily make a case that it was far too lenient, depending on people not making large prints and noticing how much blur there really was in their shots. I believe Ansel Adams was known to have weighed in on that subject. So I'd actually recommend 2X focal length. But then, SR also helps, at least with respect to camera shake if not subject motion (which is subject to the same dependence on FOV). In the blender of my mind, I sort all these factors out and still decide that 1/FL is about what I prefer to shoot if possible.

Assume for the sake of argument that a 1mm sideways displacement of the camera takes place during a shot. On a FF sensor the movement blur is x% of the width of the sensor but on an APS-C sensor, the movement blur will be 1.5x% of the width of the sensor. To reduce the blur width to the same % as on a FF sensor, the shutter speed needs to be 1.5x faster than what is being used on the FF camera.
Yes again, which is why I pointed out the the designer of the SR system probably needs to know the size of the sensor. But he'd also take that into account when having you input the focal length. If the input were "FOV" rather than "focal length", then indeed, it would be the photographer's responsibility to know the FOV for all his lenses on all his cameras.

05-07-2011, 11:01 PM   #17
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I don't think it's necesary for sensor size to taken into account - the sensor needs to be moved a certain distance in order to counteract image shift due to shake. It's not moving the sensor as a percentage of sensor width. Image shift is purely a function of FL and the detected shake.

As Class A has pointed out on these forums, the camera can only sense shake as accelerations in pitch, yaw and roll. It doesn't sense movement along the lens axis (and can't move the sensor in this direction either) and it won't sense translational movement up/down or sideways either. As these are only important (dominant) in close-up and macro photography, this is why you're advised to turn off SR in these situations.

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