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Old May 26, 2008, 10:47 AM   #1
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Hi all, Assuming we are using a 50mm (35mm rated) lens on a Pentax body, should we be setting the shake reduction at 50mm or 75mm to allow for the crop factor of the sensor. Certainly if we added a 1.5X TC we'd then rate the lens as a 75mm, so does the same apply in the crop factor case as we are in effect getting 1.5X magnification in both cases.I doubt there's much difference between the two settings in the case of a short prime, but in the case of a 300mm (35mm rated) prime the setting is either 300mm or 450mm allowing for the crop factor ... here obviously we have a considerable difference, so what's the answer ?, It's not an easy thing to test as it would need to be done manually, if you use a tripod you are told in the manual to turn SR off.
I'd appreciate your opinions on this please, it's something I've never really got my head round.... Jack.

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Old May 26, 2008, 11:06 AM   #2
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First - focal length is focal length - crop factor does not change the focal length of the lens.

Feed the combined lens x TC into the camera.

For instance - using a 35mm (135 format) camera with a 50mm lens. An 10mm object on the film will ------ on a APS-C format Pentax digital camera ------ be a 10mm object on the sensor. Crop factor dealswith the total area of the sensor, not focal length.

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Old May 26, 2008, 11:30 AM   #3
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Hi Jack,

As PDL stated, the crop factor does not enter into the FL for SR value. If you have to manually enter a FL for SR, a 50mm lens is entered as 50mm.

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Old May 26, 2008, 12:06 PM   #4
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The focal length of a lens doesn't change just because you put it on another camera. They are really what it says on them, 50=50. BUT since the CCD sensor is smaller than a traditional negative, when enlarged to a copyit gives you the effect of a lens that is 1,5 times longer than if the same lens is used on a traditional film camera. So it gives you pictures AS IFit would have been75 mmon a 35 mm film camera.

Mostphotographers use this "old" measure because they are familiar with it. Compare with horse powers, it made sense when most people could refer to the strenght of a horse and wanted to know how strong the car engine was. Since we have entered the digital era with the smaller sensors being the common ones for us amateurs, lets get used to the change and skip the "crop factor".

Kjell
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Old May 26, 2008, 12:40 PM   #5
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OK, PDL, Scott, and Kjell, Thanks for the comeback ... what you're all saying makes sense, I think what was throwing me was the apparent gain in magnification, being the same in both the crop factor, and adding a TC, I was looking at it the wrong way, and couldn't see wood for the trees. Thanks again ... Jack.
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Old May 26, 2008, 7:11 PM   #6
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This question comes up every so often and I thought I read something in the owners manual that said to use the focal length of the lens, not the crop value. It doesn't actually say that (I looked it up in both the K100 and the K20 manuals), but it implies that. It says "If the focal length for your lens is not listed above, select the value closest to the actual focal length." It also says (which I didn't remember) "When using a zoom lens, select the actual focal length at the zoom setting in the same manner." - page 49, K100 manual.
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Old May 26, 2008, 8:56 PM   #7
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Heck...Half the time I don't change the setting for the AS anyway...I know it is supposed to make a difference but danged if I see much difference between 135 and 50!

Dawg

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Old May 27, 2008, 3:34 AM   #8
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Hi Harriet & Ed, I have followed the manual most of the time, but not having looked at this correctly, I have experimented with the settings, and to be honest I've never noted much difference either, certainly not with lower focal lengths. It makes you wonder if SR is working. I haven't thought to do it before, but I must run a few test shots with it on or off to see if I can detect the difference ... If it's working I should get a positive result. ... Jack.
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Old May 27, 2008, 8:10 AM   #9
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I've noticed that the K20's SR is more sensitive to focal length - if I use a setting that's not right for the lens I am more likely to have blurry photos. And I've twice had the SR go nuts when I've rested the camera on a wall or something and I've had the SR on. The K100 and K10 never seemed as particular.
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Old May 27, 2008, 5:48 PM   #10
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Looking forward to your results Jack!



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