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05-03-2011, 08:06 AM   #1
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Manual Focal lens length setting

Every once in a while I forget to reset the focal length of the lens when using my older k mount leneses on my K-m. Does this have any effect with the SR system? Other then the fact that I have the wrong length length on the file, does this affect how well SR system will counter small camera shake? I have noted some images blurring a bit but overall they come out ok even with small variations of focal length input compared to the real lens mounted on the camera.

05-03-2011, 08:21 AM   #2
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Everything I've read says that you want to put the right length in the SR system. YMMV
05-03-2011, 08:21 AM   #3
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QuoteOriginally posted by stevbike Quote
Every once in a while I forget to reset the focal length of the lens when using my older k mount leneses on my K-m. Does this have any effect with the SR system? Other then the fact that I have the wrong length length on the file, does this affect how well SR system will counter small camera shake? I have noted some images blurring a bit but overall they come out ok even with small variations of focal length input compared to the real lens mounted on the camera.
Yes, as far as I know it will affect the effectiveness of the SR system. The SR system needs to know what focal length your are shooting at in order to most effectively adjust the sensor position.
05-03-2011, 08:57 AM   #4
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QuoteQuote:
forget to reset the focal length...does this affect how well SR system will counter small camera shake?
Yep, that is why it is there.

05-03-2011, 09:21 AM   #5
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I just need to remember this step when changing my older lenses. There are times when somethings are happening a bit too fast to reset this small function. At least it is not hurting the camera, just the pictures if it is not set right!
05-03-2011, 10:46 AM   #6
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QuoteOriginally posted by stevbike Quote
At least it is not hurting the camera, just the pictures if it is not set right!
Won't hurt the camera, but it can make a decent picture worse by over-stimulating the sensor in some cases.

I try to habitually pre-set a situationally appropriate hyper focal distance and aperture for manual lenses when I first mount them which (sometimes!) helps remind me to check the SR condition too.

H2
05-03-2011, 11:39 AM   #7
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I have gotten into the habit of turning the camera off during lens changes, which supposedly is a good idea from the perspective of dust, but it also means I get prompted to enter focal length when I turn the camera on. So I rarely forget any more.

If you really think it through, you will realize that entering to *low* a value might hamper the effectiveness of SR, but it won't ever be worse than turning it off. Entering too *high* a value could definitely cause the SR to become counterproductive, but only if entering a value *much* larger than the actual focal length. I'm guessing entering 2X the actual focal length would basically nullify SR by moving the sensor twice as far as necessary, which would just reverse the direction of the shake. Anything more than 2X and you'd be getting more blur than with SR turned off. But still, you're not going to hurt anything in the camera; just the IQ of any images taken with the much-too-high setting.
01-25-2013, 03:55 PM   #8
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What if you are using a manual zoom lens? As an example i'm using a 80-200 manual zoom lens. The setting on my k-r only allow you to choose one length setting examply 28, 35, 50 and so on? does this affect anything when zoom from 200 back to 80 or is there a better way of doing this?

01-25-2013, 04:00 PM   #9
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QuoteOriginally posted by maxelkacemi Quote
What if you are using a manual zoom lens? As an example i'm using a 80-200 manual zoom lens. The setting on my k-r only allow you to choose one length setting examply 28, 35, 50 and so on? does this affect anything when zoom from 200 back to 80 or is there a better way of doing this?
This question comes up all the time, and the answers are as variable as the number of posts.

You can set it to the focal length you are using each time you change, you can set it at minimum, so the shortest focal lengths are correct and longer are under compensated, or to the median, so the average error in compensation is minimized if you a constantly changing focal length.

Figure out how you shoot and pick one
01-25-2013, 04:07 PM   #10
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sweet thanks for the advice lowell !
01-28-2013, 01:49 AM   #11
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As has been said, it can be dangerous (to your image, not the camera!) if you enter a focal length greater than 2x the actual focal length, so for a zoom you need to be aware of this. A while back, I devised a formula which not only avoids this situation, but also maximises the amount of shake reduction:

FL = 2*max*min/(max+min)

(Where FL is the value you input, max and min are the maximum and minimum focal lengths of the zoom lens.)

So, for a 28-70, you'd set it to 40mm, a 75-150 to 100mm, a 70-210 to 100mm (being the closest available value to the calculated 105mm).

The formula equalises the amount of shake reduction you get when you set the lens to either extreme. So, for example, in the case of the 70-210, you end up with 50% shake reduction (meaning that you reduce any blur due to shake by a factor of at least 2x), even if you'd set the lens to either 70 or 210mm. Note that, if you'd used an "average" value of 140mm, you'd get zero shake reduction when you were operating at minimum focal length (and in fact you'd run the risk of magnifying the shake).
01-29-2013, 02:01 PM   #12
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SR - on zoom

Keep in mind that focal length changes are essentially factored exponentially. When you go from 50mm to 100mm you are magnifying the image horizontally and vertically, and the sensor adjustment needs to magnify accordingly. When I set the SR on a zoom I put at the number I am sure will be the lowest focal length I will be using, and if I only have a best guess of that length- I'll set one notch lower.

Having used a normal length lens, and changed to an 8mm without remembering to change the SR the effect was rather humorous (as these were not vital photos in any way) due to the extent of magnified sensor shift.
01-30-2013, 02:50 AM   #13
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QuoteOriginally posted by ScooterMaxi Jim Quote
Keep in mind that focal length changes are essentially factored exponentially. When you go from 50mm to 100mm you are magnifying the image horizontally and vertically, and the sensor adjustment needs to magnify accordingly. When I set the SR on a zoom I put at the number I am sure will be the lowest focal length I will be using, and if I only have a best guess of that length- I'll set one notch lower.

Having used a normal length lens, and changed to an 8mm without remembering to change the SR the effect was rather humorous (as these were not vital photos in any way) due to the extent of magnified sensor shift.
When you set a value of 50mm and use an 8mm lens, the SR mechanism will move the sensor by 6.25 times the correct amount. This means you end up with 4.25 times the amount of blur that you'd get with SR off.

I don't blame you for setting the value to the lowest FL that you'll be using (it's a safe option), but you might find you'd get better results by using my formula (though of course, it depends how many shots you take at whichever focal length). And to set a value lower than the minimum FL is just plain wasteful...
01-30-2013, 04:57 AM   #14
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QuoteOriginally posted by m42man Quote
When you set a value of 50mm and use an 8mm lens, the SR mechanism will move the sensor by 6.25 times the correct amount. This means you end up with 4.25 times the amount of blur that you'd get with SR off...
It is not quite as simple as that. If you read the patents you'll realise how much cleverness goes into moving the sensor plate at the right velocity and direction during the exposure
01-30-2013, 05:11 AM   #15
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As a simplified formula for one setting, you might just add 1/3 to the shortest focal length of your zoom - it won't be too far out.

Personally I tend to set mine to something a bit longer than half way between the zoom extremes. This is of course entirely the wrong thing to do to get a compromise between all settings but the shots I tend to take in a hurry tend to be at the long end and for wider shots where I have more time I can reset to the proper value - but thats just me ...
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