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09-16-2013, 11:51 AM   #1
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exposure control and manual lens

i just received a Pentax M lens 50mm f1.7 manual
However when i start my k01 the LCD shows this message : input focal length with a highlighted number that could be changed between 8 to 800
I am expecting that it should be 50
Am I right and what dose this number mean ?
another question : what to do other than to experiment in order to get the correct exposure ?
Thanks

09-16-2013, 12:02 PM   #2
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You are correct - the number entered should be equal or less than the focal length of the lens. This is for the Shake Reduction. If you turn off SR, it will not ask you for the focal length. Zooms depend on the range you typically will be using.

Please refer to the following threads. They should answer all your questions. And welcome to the joys of using legacy lenses on you Pentax Digital Camera.

https://www.pentaxforums.com/forums/pentax-lens-articles/110657-how-use-meter...k-x-k-7-a.html
https://www.pentaxforums.com/forums/pentax-dslr-camera-articles/100260-manual...ic-lenses.html

PS: Be sure Auto ISO is not selected.
09-18-2013, 10:29 PM   #3
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Not just for SR - It also gets recorded in to the image and video meta-data.

My personal use rule, so I can tell which lens I used when editing, is to set primes with their correct number, and zooms with the wide-end number.
So my 24 and 50 primes are entered as 24 and 50, my 28-70 zoom as 28, and the 70-200 as 70.
My two AF zooms tell the camera which lens they are, so no input needed.
09-19-2013, 12:08 PM   #4
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For my Sigma 50mm macro, I usually put in 75 and it seems to work better that way. But I haven't done any real testing, so it might just be placebo/my imagination.

09-19-2013, 12:30 PM   #5
mns
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QuoteOriginally posted by Not a Number Quote
You are correct - the number entered should be equal or less than the focal length of the lens. This is for the Shake Reduction. If you turn off SR, it will not ask you for the focal length. Zooms depend on the range you typically will be using.

Please refer to the following threads. They should answer all your questions. And welcome to the joys of using legacy lenses on you Pentax Digital Camera.

https://www.pentaxforums.com/forums/pentax-lens-articles/110657-how-use-meter...k-x-k-7-a.html
https://www.pentaxforums.com/forums/pentax-dslr-camera-articles/100260-manual...ic-lenses.html

PS: Be sure Auto ISO is not selected.
a friend told me that number should be 75 as this number reflects the actual focal length on DX sensor !!?
50 should be OK for FX cams.
09-19-2013, 04:33 PM   #6
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No, it should be the physical (actual) focal length of the lens.What you set the camera is going use to make calculations for moving the sensor. Draw two circles one 50mm the other 75mm in diameter. Take a 5 degree slice from the center of each. The length of the arc or chord of the 75mm circle on the same 5 degree slice is longer. So if set to 75mm with a 50mm lens the sensor would move more than it actually has to for this type of movement.
09-20-2013, 06:15 AM   #7
mns
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QuoteOriginally posted by Not a Number Quote
No, it should be the physical (actual) focal length of the lens.What you set the camera is going use to make calculations for moving the sensor. Draw two circles one 50mm the other 75mm in diameter. Take a 5 degree slice from the center of each. The length of the arc or chord of the 75mm circle on the same 5 degree slice is longer. So if set to 75mm with a 50mm lens the sensor would move more than it actually has to for this type of movement.
Thanks a lot
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