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11-26-2010, 12:11 AM   #1
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Inputting focal length of a variable lens on my K-x. / Shake reduction problems?

Hi everyone,

Here's the deal. Whenever I put my variable zoom Vivitar lens on my K-x (and each time I turn it on with it mounted), it will ask me to input the focal length every time. I checked everywhere in my manual about variable zoom lenses and it only talks about the auto lenses when it comes to zoom. Do I have to keep my lens extended to one focal length while I shoot? I read elsewhere in this forum that it's for the shake reduction, but what if my subject moves and I have to adjust the focal length? Will it ruin my shot? With my hands tied with writing final papers and not being able to test the combination some more (I have taken some fantastic pics with the old Vivitar on my K-x). I weep at not being able to play with my new toy some more, but I should be done with all that next week.

I was just wondering because some of my shots are a bit out of focus. It might be that I'm just not used to using manual focus yet, but I was wondering if shake reduction could be playing a factor, too.

Thanks,

Tim

11-26-2010, 12:29 AM   #2
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You are right about it being for the SR system and you do not have to keep your lens to one focal length. I do not believe there is anything in the manual about inputing the focal length of zoom lenses but I have heard from some others on here that a good rule of thumb is to do the longer 2/3 of the lens or at least around there. So for example, on a 100-300 you would input something close to 233mm (you would obviously have to round up or down). You can also do some tests, input the focal length in intervals of 10-20 down the entire focal length, take pics and see if there is any difference in IQ.
11-26-2010, 01:14 AM   #3
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I'm sure this subject will keep cropping up, and it's a very valid question, and the answer isn't totally straightforward.

You're right, it is all about SR - the camera has sensors which detect motion, and it has the means by which it can move the sensor to counteract any motion caused by camera shake. But it needs to calculate the right amount of sensor movement, and for this it needs to know the lens focal length.

With a modern lens, the lens itself passes FL information to the camera. With an older lens, the user needs to provide it, which is why the camera prompts the user at power-up (assuming SR is set to ON).

Obviously, with a zoom lens, you're going to have to find a good compromise FL to enter, unless you want to keep changing the value whenever you zoom in/out. Well, I did look into this fairly recently, and the outcome was documented here:

https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/119297-what-fo...al-zoom-2.html

Assuming your zoom has a range of around 3x, by choosing the recommended FL value you'll get around 1 stop's worth of SR at the zoom extremes (i.e. a shake reduction of x2), rising to max SR when the zoom is set to the FL value.

Please note that it is very counterproductive to use focal length values greater than 2x the lens's minimum FL. If you do, and you set the zoom to the wider end, you'll actually magnify any camera shake (i.e. you'll get more blur than if you had SR turned off). (Sorry to contradict you KxBlaze - it is a somewhat tricky subject!)

Note that the above will certainly apply for the slower shutter speeds, where SR is operating at its maximum effectiveness, but please be aware that SR may be drastically reduced at the faster shutter speeds - this article seems to be saying that:

Falk Lumo: Pentax shake reduction revisited

Good luck with your new toy, TexasLangGenius!
11-26-2010, 01:30 AM   #4
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Personally I would just switch it off

11-26-2010, 02:55 AM   #5
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switch it off or use the minimum focal length of the zoom would be safe.

any focal length below what you set the camera to would blur more.

if you set it to the minimum you'll get some reduction at all lengths but only the right amount at the minimum length... at the same time you wont make it worse in any situations.
11-26-2010, 03:31 AM   #6
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QuoteOriginally posted by WerTicus Quote
switch it off or use the minimum focal length of the zoom would be safe.

any focal length below what you set the camera to would blur more.

if you set it to the minimum you'll get some reduction at all lengths but only the right amount at the minimum length... at the same time you wont make it worse in any situations.
I'm sorry, but you're wrong. You need to set a value greater than 2x the lens focal length in order to get more blur than with SR off.

Let me give an illustration.

Assume a 100mm lens:

Set value to 25mm. Sensor moves 25% of ideal, you get 75% blur.
Set value to 50mm. Sensor moves 50% of ideal, you get 50% blur.
Set value to 75mm. Sensor moves 75% of ideal, you get 25% blur.
Set value to 100mm. Sensor moves 100% of ideal, you get no blur.
Set value to 125mm. Sensor moves 125% of ideal, you get -25% blur.
Set value to 150mm. Sensor moves 150% of ideal, you get -50% blur.
Set value to 175mm. Sensor moves 175% of ideal, you get -75% blur.
Set value to 200mm. Sensor moves 200% of ideal, you get -100% blur.
Set value to 225mm. Sensor moves 225% of ideal, you get -125% blur.
etc. etc.

So at 200mm, you get the same blur as with SR off (but in the opposite direction).
11-26-2010, 11:11 AM   #7
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I've got and old 70-300mm zoom I use on my K200D. I've given up on using SR with it. I just turn SR off when I use that lens and have been much happier with the results. Here's a couple of examples. Both were shot at ISO 200, F11, 1/125sec. handheld. Try it you may be pleasently surprised.





Last edited by kkoether; 11-26-2010 at 11:29 AM. Reason: Added camera settings
11-26-2010, 11:35 PM   #8
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QuoteOriginally posted by kkoether Quote
I've got and old 70-300mm zoom I use on my K200D. I've given up on using SR with it. I just turn SR off when I use that lens and have been much happier with the results. Here's a couple of examples. Both were shot at ISO 200, F11, 1/125sec. handheld. Try it you may be pleasently surprised.
I think with a lens with such a large zoom range (for a vintage lens) there isn't much hope of getting decent SR at the longer FLs (unless of course you match the FL value with actual operating FL). For your lens, the largest value you can safely input is 100mm, so at full zoom you'd only lose 33% of the motion blur anyway. But at least you would still have the benefit of worthwhile SR at the wider end.

I can imagine you would indeed have problems with SR if you'd tried entering values much above 120mm.

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