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05-29-2011, 12:43 PM   #1
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shake reduction questions

If your lens is, say a 50mm, and it becomes 75mm with the sensor multiplier, what do you set the shake reduction with a manual lens... 50 or 75mm? (think it would be 75mm but am not sure if the camera takes the 1.5 multiplier into account)

Also, how do you figure out shake reduction when using a manual lens and a macro tube?

cheers and thanks

05-29-2011, 12:49 PM   #2
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Focal length is focal length- you can ignore the crop factor entirely. Not sure about tubes, but if you're using them you should probably have SR disabled (AFAIK the focal length increases by the length of the tubes, though).

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05-29-2011, 03:00 PM   #3
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QuoteQuote:
If your lens is, say a 50mm, and it becomes 75mm with the sensor multiplier,
It doesn't "become" 75mm, but the angle of view with a 50mm on a crop sensor looks like a 75mm on film. SR should be for 50mm.

QuoteQuote:
Also, how do you figure out shake reduction when using a manual lens and a macro tube?
As Adam mentioned, the camera should be on a tripod, therefore SR should be off. I have no idea how to calculate that FL.
05-29-2011, 05:11 PM   #4
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As mentioned, focal length is focal length. Forget you ever heard about crap.factor. The lens doesn not change. A smaller sensor chops off more of the projected image than does a larger sensor, that's all. And SR is ineffective at close and macro distances, and MUST be switched off when you're on tripod.

SR *is* effective if you use short macro tubes on long lenses. I may put a 10mm tube on a 200mm manual lens to bring its focus range down from 2m-->infinity to 1m-->60m. This is useful for shoots where I care neither about infinity nor ultra-close-ups. That little extension doesn't confuse the SR'bot; I just tell it I'm using a 200mm lens, and all is hunky-dory.

I say again: FORGET YOU EVER HEARD OF CRAP.FACTOR. It is only meaningful if you're an experienced 35mm film photographer who is transitioning to a smaller-frame camera, and want to visualize the equivalent FOV/AOV (angle/field of view). Otherwise, just become familiar with what each focal length does on your camera. Life is much simpler that way.

05-29-2011, 05:13 PM   #5
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Your lens does not become 75mm. The so-called crop factor is a function of sensor size, and has nothing whatsoever to do with lens focal length.
06-02-2011, 07:54 PM   #6
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I was going to start a thread to ask a somewhat similar question, but I'll just post it here instead. I ordered a couple manual zoom lenses, including an 80-200. I noticed that on the official Pentax site, it says: "Use the rear e-dial to select a focal length which is closest to the focal length of the lens you are using. If you are using a zoom lens, choose a focal length close to the midpoint of the zoom range of the camera."

My question: Should I set the focal length to 140mm per the Pentax site, or is it better to set it to the exact focal length I'll be using? (I'm going to a concert and expect to be shooting at 200mm the whole time; should I set focal length to 200 or 140)?
06-02-2011, 08:22 PM   #7
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QuoteOriginally posted by vegenigma Quote

My question: Should I set the focal length to 140mm per the Pentax site, or is it better to set it to the exact focal length I'll be using? (I'm going to a concert and expect to be shooting at 200mm the whole time; should I set focal length to 200 or 140)?
If you are not zooming, set it to the FL you are using. Zoom implies you will be changing focal lengths and is why the recommendation is to put it in the middle.
06-02-2011, 08:31 PM   #8
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Repeat after me: APS-C cameras *don't* multiply or magnify images, they *crop* images.
50mm = 50mm, always.
The SR mechanism takes that into account.

06-02-2011, 08:31 PM   #9
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QuoteOriginally posted by SpecialK Quote
If you are not zooming, set it to the FL you are using. Zoom implies you will be changing focal lengths and is why the recommendation is to put it in the middle.
Thanks. I suppose it just seems odd to me that shake reduction would still be helpful if I were to set focal length in the middle (140) and take some shots at one of the extremes (80 or 200). It seems odd to me that SR would still be useful when the focal length is off by 60mm; I was surprised Pentax didn't instruct us to set it to the focal length at which we're shooting or disable SR. But hey, if it still offers some benefit, that's pretty neat. But yeah, I'll definitely set it to 200.
06-02-2011, 09:32 PM   #10
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The SR quandary is the main reason I don't much use old manual zooms. But some are so great that I *must* use them. So I often either shoot in good light with SR off, or I re-enter the focal length when zooming.

That was before someone here posted a formula for the optimal focal length to tell the SR'bot. The formula:

(2 * Min * Max) / (Min + Max)

This gives me a table for my manual zooms. I should tape the number to each lens, eh?

35-80 ----> 49mm
35-135 ---> 56mm
50-125 ---> 71mm
55-135 ---> 78mm
60-300 --> 100mm
70-200 --> 104mm
70-210 --> 105mm
80-200 --> 114mm

Of course the SR'bot isn't smart enough to handle such accuracy, so I just round to the nearest possibiity.
06-02-2011, 11:08 PM   #11
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The SR system needs to know the focal length to know how much to move the sensor. If you underestimate the focal length, it won't move the sensor enough, but it will still be better than nothing. If you overestimate the focal length, it will move the sensor too much. A little too much won't be too harmful, but at some point, it will become worse than not moving it at all (ie, worse than not using SR). Without doing the math, it seems likely that point would be around twice the actual focal length.
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