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06-30-2012, 04:17 PM   #1
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SR with macro extensions

I was wondering how to adjust the SR settings with macro tubes? lets say, for example, I put on a M 50, and I get to choose the SR (which normally would be 50mm) do you add to the 50mm as the tube makes the lens longer? if so, what compensation are you using?
Only hand holding, of course

thanks

randy

06-30-2012, 05:32 PM   #2
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Good question, but I bet you'll get a variety of answers. I usually add the rough amount of extension to the focal length of the lens when dialing it in. Sometimes I'll go slightly conservative with it, so if focal length and extension totaled say 120mm I might dial in 85 instead. I see very little, if any difference between using 85 and 120 in that scenario.

The way the SR works is that it has to make stronger corrections for longer focal lengths, due to the reduced angle of view. You can see this effect yourself, if you compare hand holding a 300mm or greater to say, the kit lens or a 50mm. It's much harder for our hands to stabilize the longer focal lengths. So, it follows that when doing higher magnification macro work, it might pay to try stronger SR settings. I'd say just experiment, have fun, and find out what works best for you.
06-30-2012, 06:21 PM   #3
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Add in the whole extension. The theory and algorithm use the distance from the lens to sensor.

For 50mm lens with 35m total extension, tell the camera 85mm.
06-30-2012, 08:45 PM   #4
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Have a look at the discussion than ensued when I asked the same question:
https://www.pentaxforums.com/forums/pentax-dslr-discussion/182304-shake-reduc...ion-tubes.html

I'm not going to try and summarize an answer...

07-02-2012, 06:11 AM   #5
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I am in the school of thought that no matter what the original focal length is, when ever you increase the length of something, the motion is going to increase at the one end due to the law of physics. If anyone has taken an ocean cruise would know how the bottom of the boat sways a lot less than the top
07-02-2012, 09:31 AM   #6
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QuoteOriginally posted by slip Quote
I am in the school of thought that no matter what the original focal length is, when ever you increase the length of something, the motion is going to increase at the one end due to the law of physics. If anyone has taken an ocean cruise would know how the bottom of the boat sways a lot less than the top
@SLIP is exactly right. In fact using the lens' true focal length for SR for anything other than infinity focus is incorrect.

Shake.Reduction.Focal.length = f(1+m) = f(1+sensor.width/scene.width)

Ignoring subject distance for SR gives a fractional error = sensor.width/scene.width

for example with an asp-c camera, using an uncorrected focal length for a subject 10 feet from the camera results in a shake reduction error of about 1"/120" ~ 0.8%.

Dave in Iowa

Last edited by newarts; 07-02-2012 at 09:46 AM.
07-02-2012, 10:34 AM   #7
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FYI-- The reason why the focal length increases in close up work is that the rays entering the lens are not parallel (like infinity work) but are diverging. These diverging rays force the focal point further back, requiring tubes to be added to compensate.
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