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05-08-2012, 12:10 AM   #31
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QuoteOriginally posted by newarts Quote
There is only one SR setting which ends up with the same amount of SR for the zoom lens at max and min. Thats when:

(SR-max) = -(SR-min) which is the same as SR=(max+min)/2 in complete agreement with your above requirement.

Further this choice of SR parameter is the one that minimizes the average error for random focal length settings between max & min.
That's completely incorrect, I'm afraid. I'll give you a quck example: say you have a 50-200mm zoom, your compromise FL value would be 125mm. With the zoom set at 200mm, the camera will move the sensor to follow the movement of the image due to shake, but will only move it 125/200 as much as it needs, so the blur is reduced to 1 - 125/200, or 37.5% of the amount you'd get with SR turned off.

Now you set the zoom to 50mm. The camera will move the sensor 125/50 or 2.5 times as much as it needs. What this means is that the camera has overshot by 150%, so it's actually added 50% extra blur, over and above what you'd get with SR off.

It's crucial you fully understand this, otherwise we can proceed no further. I'll give you this link, because it illustrates what's going on in these situations:

https://www.pentaxforums.com/forums/pentax-dslr-discussion/132496-shake-reduc...ml#post1380845

QuoteOriginally posted by newarts Quote

I don't follow your logic in dividing (1-upper.error) by max (H), ie. x/H and (1-lower.error) by min (L),ie (2L-x)/L. If these weights are not used you and I get the same result.
Those equations appearing in post 10 of

https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/178019-tamron-...ml#post1859602

reflect the model I've described. If you don't fully understand the model, then you won't understand the equations.

As I've said before, my aim is to ensure some degree of SR effectiveness whether you're at the min or max end of the zoom range. Having the same SR effect at both ends is a good way of ensuring you don't run into problems at the wide end, and therefore you know you'll always be better off than you'd be by turning SR off.

05-08-2012, 03:07 PM   #32
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QuoteOriginally posted by m42man Quote
That's completely incorrect, I'm afraid. I'll give you a quck example: say you have a 50-200mm zoom, your compromise FL value would be 125mm. With the zoom set at 200mm, the camera will move the sensor to follow the movement of the image due to shake, but will only move it 125/200 as much as it needs, so the blur is reduced to 1 - 125/200, or 37.5% of the amount you'd get with SR turned off.

Now you set the zoom to 50mm. The camera will move the sensor 125/50 or 2.5 times as much as it needs. What this means is that the camera has overshot by 150%, so it's actually added 50% extra blur, over and above what you'd get with SR off.

It's crucial you fully understand this, otherwise we can proceed no further. I'll give you this link, because it illustrates what's going on in these situations:

https://www.pentaxforums.com/forums/pentax-dslr-discussion/132496-shake-reduc...ml#post1380845



Those equations appearing in post 10 of

https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/178019-tamron-...ml#post1859602

reflect the model I've described. If you don't fully understand the model, then you won't understand the equations.

As I've said before, my aim is to ensure some degree of SR effectiveness whether you're at the min or max end of the zoom range. Having the same SR effect at both ends is a good way of ensuring you don't run into problems at the wide end, and therefore you know you'll always be better off than you'd be by turning SR off.
But it isn't necessarily the optimum way of decreasing error on the image, which is our goal.

I have completely followed your links and understand what you say; you are finding how the SR parameter chosen affects the fractional error with SR compared to no SR relative to the end points of the span. Then you find the SR parameter that gives the same error at both ends of the span.

But the goal should be to minimize the absolute error on the sensor, not to minimize the fractional error in the SR engine. Say for example we have a 10% error at both ends of a range from 50 to 150mm. The motion of the image on the sensor at 150mm is 3x larger than at 50mm, although the percentage error is the same at both 50 and 150.

Regarding your example,
QuoteQuote:
I'll give you a quck example: say you have a 50-200mm zoom, your compromise FL value would be 125mm. With the zoom set at 200mm, the camera will move the sensor to follow the movement of the image due to shake, but will only move it 125/200 as much as it needs, so the blur is reduced to 1 - 125/200, or 37.5% of the amount you'd get with SR turned off.
It will move the image on the sensor in proportion to (200-125) = 75mm

QuoteQuote:
Now you set the zoom to 50mm. The camera will move the sensor 125/50 or 2.5 times as much as it needs. What this means is that the camera has overshot by 150%, so it's actually added 50% extra blur, over and above what you'd get with SR off.
It will move the image on the sensor in proportion to (125-50) = 75mm

Exactly the same image shift in each case - the % of what it would have been had shake reduction been off is not the question (at the low end the % error is larger because the denominator is smaller) - but that's not the concern.


What we want for a sharp photo is for the absolute motion of the image on the sensor to be a minimum given a random selection of focal lengths within the range. That's different from equating the fractional error at both ends of the focal length range.

I believe your point that when the focal length range is large enough, there is range where no SR is better than SR. Therefore there is a strategy where, if the chosen focal length is short enough, one should shut off SR - but that means you know what the focal length is, in which case you are best off to just tell SR the known focal length and have no error.

Dave
05-08-2012, 11:46 PM   #33
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Dave, I'm afraid you still haven't got how the SR works. I'll go into some detail this time:

The camera uses info from its accelerometers, coupled with knowledge of the lens FL, to know how much, and how fast, to move the sensnor so that it exactly follows the movement of an image across the plane of the sensor. So, using the example of the 50-200mm zoom, set to 50mm, we'll first assume the camera knows it's set to 50mm. Say you're taking a shot with a 1sec shutter, and and the camera works out that the image will be moving at a constant 1mm per sec (due to shake).

While the shutter is open, the camera will move the sensor at 1mm per sec to track the image. When the shutter closes, both the image and the sensor will have moved 1mm, there is therefore no relative movement, so there's no blur.

Now you tell the camera the FL is 125mm. The camera will now move the sensor at 2.5mm per sec. At the end of the 1sec exposure, the image will still have moved 1mm, but now the sensor has moved 2.5mm. There's now a difference of 1.5mm, so there'll be 1.5mm's worth of blur. Compare this with only 1mm of blur if you'd turned SR off. You've actually increased the blur by 50%. This is NOT a good compromise FL setting for this lens!

Note that if you do a similar analysis for the 200mm end you'll merely end up with 0.375mm of blur, obviously an improvement over the SR-off case.

Because of this model, I came up with two equations, one for when the actual FL is less than the value you entered, one for when it's greater. I use the term "SR effectiveness" to indicate how much the sensor moves, as aproportion of the optimum movement (giving zero blur).

1. When it's greater: SR effectiveness = x/actual

2. When it's less: SR effectiveness = (2*actual - x)/actual

(x is the FL value you input to the camera, actual is the actual FL of the lens at the time)

Equation (2) is harder to understand than (1), but do try putting a few values in and seeing how it works out.

Anyway, you can substitute values for min and max in place of actual, and you end up with simultaneous equations which yield a useful value for a compromise setting. I happen to have chosen that you get equal SR effectiveness at each end of the zoom range, but it would be easy to arrange for a different criterion. In fact, I did consider biasing this towards the max end (which would be minimising absolute error), but I quickly realised that what you'd gain at the long end would be outweighed by the shortfall at the wide end. SR performance at the short end is very sensitive to small changes in the value you give to the camera.

In other words, minimising absolute error results in drastic reduction in performance at the wide end for very small gains at the long end. In addition, you run the risk of actually adding to blur - don't forget a nominally 70-210mm zoom may actually be more like 65-205, and you'd be in trouble.

The whole object of the exercise is to come up with a value which enables you to use any zoom setting, without fear of the SR screwing things up, and that's what my formula does. Your formula trades small gains at the long end for blur amplification at the wide end. I'll compare the two, againg using the example of the 50-200m zoom:

Long end net blur reduction: my formula 1.7x; your formula, 2.6x.

Wide end net blur reduction: my formula 1.7x; your formula, 50% increase in blur.

Most vintage zooms will be of a smaller zoom ratio. My formula yields minimum blur reduction figures of 3x for a 2x zoom and 2x for a 3x zoom.
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