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05-05-2009, 11:10 AM   #16
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QuoteOriginally posted by newarts Quote
PS Shake reduction will help with side-side camera shake, but not with in-out shake. In-out shake is important for macros because depth of field is small.
And that emphasizes why with such macros, the camera should be on a tripod and shake reduction OFF.

But I wonder: lacking a 43mm lens (which would need either 49mm or 58mm front threads), I mount my substantial Kenko fisheye adapter on a Pentax-M 40-80 zoom set to 43mm for a full image circle on the K20D. The resultant assemblage is about 170mm long. Normally it's stopped down to around f/22 and sits on a tripod, but occasionally I use it handheld. What focal length should I input for THIS beast? 40mm or 45mm, the closest value to the host lens? 8mm, the closest to the effective value of 5mm the adapter produces? 180mm, the closest to the physical length of this structure? I'm at a loss, theory-wise.

05-05-2009, 11:35 AM   #17
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QuoteOriginally posted by newarts Quote
PS Shake reduction will help with side-side camera shake, but not with in-out shake. In-out shake is important for macros because depth of field is small.
Now that's a very simple explanation I can sink my teeth into - thanks! DOF might seem small in relative terms when dealing with large apertures and long focal lengths, but still, we're usually talking about an inch or two *at least* - so a tiny bit of forward/backward shake won't matter so much. Whereas at macro distances you might be talking about DOF in the millimeters, and even the slightest forward/backward motion will put the subject out of the focus plane. Of course, that doesn't in itself imply you're better off turning SR *off* for handheld macro attempts, but explanations involving angular motion don't make sense to me either, because angular motion is that same regardless of distance.
05-05-2009, 11:40 AM   #18
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QuoteOriginally posted by RioRico Quote
But I wonder: lacking a 43mm lens (which would need either 49mm or 58mm front threads), I mount my substantial Kenko fisheye adapter on a Pentax-M 40-80 zoom set to 43mm for a full image circle on the K20D. The resultant assemblage is about 170mm long. Normally it's stopped down to around f/22 and sits on a tripod, but occasionally I use it handheld. What focal length should I input for THIS beast? 40mm or 45mm, the closest value to the host lens? 8mm, the closest to the effective value of 5mm the adapter produces? 180mm, the closest to the physical length of this structure? I'm at a loss, theory-wise.
My assumption is the the aspect of focal length that matters to an SR system is field of view, so it can calculate the effect on the image of a given amount of angular motion. So I'd enter whatever focal length seems to best represent the field of view. If it's really only the equivalent of 5mm in terms of FOV (in APS-C terms - no need to further confuse the uissue with 35mm equivalencies!), and the lowest the camera allows for 8mm, I'd be inclined to turn it off. 8mm is closer to *twice* 5mm than it is to 5mm, and I'd be worried about getting into that territory where you are so far above the focal length that the camera overcompensates and *exaggerates* instead of reduces shake. But a few tests might ease your mind about that.
05-05-2009, 11:54 AM   #19
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QuoteOriginally posted by Marc Sabatella Quote
My assumption is the the aspect of focal length that matters to an SR system is field of view, so it can calculate the effect on the image of a given amount of angular motion. So I'd enter whatever focal length seems to best represent the field of view. If it's really only the equivalent of 5mm in terms of FOV (in APS-C terms - no need to further confuse the uissue with 35mm equivalencies!), and the lowest the camera allows for 8mm, I'd be inclined to turn it off. 8mm is closer to *twice* 5mm than it is to 5mm, and I'd be worried about getting into that territory where you are so far above the focal length that the camera overcompensates and *exaggerates* instead of reduces shake. But a few tests might ease your mind about that.
I agree, it seems to be the easiest way to calculate it.

I am curious, however, how one goes about doing tests with the shake reduction. It would require something that shakes constantly with the exact same amount. No a washing machine would not do the trick

05-05-2009, 02:35 PM   #20
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One guy over on dpreview uses his riding lawnmower and posts picture taken from it every time he gets his hands on a new Pentax camera with SR. Not that that gives a very *controlled* test - just a very *entertaining* one.

Anyhow, I woudn't assume you need omething super controlled - just a whole bunch of test shots of the same scene with SR set to 8 and then with the exact same exposure settings but with SR turned off, to see which set looks better on average. If you can't tell much difference, then it probably doens't make much difference. But certainly, it's very obvious there is a difference when doing this using "normal" focal lengths and shooting distances.
05-05-2009, 03:16 PM   #21
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QuoteOriginally posted by Marc Sabatella Quote
My assumption is the the aspect of focal length that matters to an SR system is field of view, so it can calculate the effect on the image of a given amount of angular motion. So I'd enter whatever focal length seems to best represent the field of view. .......


The relationship I gave earlier involving focal length and magnification in effect contains field of view in the form of focal length; the magnification factor is related to how far the sensor is from the lens, or the mathematical equivalent how far the subject is from the camera.

Lens_subject_distance=F(1+1/m)
Lens_sensor_distance=F(1+m)

The angular motion of the camera times the length from the sensor to the lens must be less than one viewing pixel (Circle_of_confusion) to avoid image smear. The angle is (rate)x(time)

(rate)x(time)xF(1+m) < Circle_of_confusion

time < (Circle_of_confusion/rate)/(F(1+m))

This is the basis of the shutter speed correlation with 1/F.

Iowa Dave
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