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03-17-2017, 07:00 AM   #1
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K-1 asking for focal length input w/ Rokinon 14mm lens

Have a weird issue. I recently bought a Rokinon 14mm F/2.8 lens. Image quality is great but whenever I mount it on my K-1, it asks for the focal length. I've used other prime lenses & manual focus lenses but never seen this. I know there are alot of these lenses floating around out there so I'm hoping someone can chime in if this is normal or not.

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03-17-2017, 07:11 AM   #2
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Quite normal. KA-mount lenses don't indicate focal length to the camera. Shake reduction depends on the current focal length to compute movement compensation.
03-17-2017, 07:26 AM   #3
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Virtually all manual focuses lenses should ask for the focal length, with very few exceptions. 15mm should work fine for this lens.

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03-17-2017, 07:27 AM   #4
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Non-auto focus primes will require this as they don't communicate their focal length which is needed by the stabilization system.

03-17-2017, 08:15 AM - 1 Like   #5
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Due to the relatively large barrel distortion of this lens design, actual focal length is probably closer to the 13-13.5 range. In fact, the rebadged Vivitar version lists the lens as 13mm. Users of the 15-30 have indicated that the difference in view at 15mm is substantial. My preference has been to designate the 12mm setting, as I would rather be conservative instead of over-correction. It's unfortunate that Pentax hasn't implemented the ability to manually enter an exact setting as was done on the Q bodies.
03-17-2017, 08:43 AM   #6
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QuoteOriginally posted by ScooterMaxi Jim Quote
It's unfortunate that Pentax hasn't implemented the ability to manually enter an exact setting as was done on the Q bodies.


especially with the number of DSLR bodies they've introduced over the years - it would be nice just to enter the data manually (if needed).....
03-17-2017, 09:17 AM - 1 Like   #7
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QuoteOriginally posted by pepperberry farm Quote
especially with the number of DSLR bodies they've introduced over the years - it would be nice just to enter the data manually (if needed).....
I would prefer that we be able to add additional lengths to the "menu".


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03-17-2017, 10:35 AM   #8
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Probably the reason Pentax doesn't add addition focal lengths is because they are not needed. Really, how much difference in the shake resistance do you think there would be between a14mm and 15mm lens?
03-17-2017, 01:59 PM   #9
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QuoteOriginally posted by aremmes Quote
Quite normal. KA-mount lenses don't indicate focal length to the camera. Shake reduction depends on the current focal length to compute movement compensation.
Does it solely depend on focal length? I'm thinking about zooms; I've been setting this to the (approximately) longest focal length for the lens, not the length used for a specific exposure. I think I read this was due to weight of the lens. Is this the correct approach for zooms, to use the longest focal length of the zoom's range?
03-17-2017, 02:15 PM   #10
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QuoteOriginally posted by EssJayEff Quote
Does it solely depend on focal length? I'm thinking about zooms; I've been setting this to the (approximately) longest focal length for the lens, not the length used for a specific exposure. I think I read this was due to weight of the lens. Is this the correct approach for zooms, to use the longest focal length of the zoom's range?
The distances that the sensor has to move to compensate for camera movement are proportional to the focal length -- longer lens, longer sensor movement distances.

Setting the focal length to a zoom lens's maximum results in SR overcompensation for any length shorter than the maximum, resulting in blurry images. From what I've read here on these forums, people usually choose one of three methods: set to midpoint, set to minimum, or disable SR. it all depends on how you choose to use the lens.
03-17-2017, 03:06 PM - 1 Like   #11
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QuoteOriginally posted by aremmes Quote
The distances that the sensor has to move to compensate for camera movement are proportional to the focal length -- longer lens, longer sensor movement distances.

Setting the focal length to a zoom lens's maximum results in SR overcompensation for any length shorter than the maximum, resulting in blurry images. From what I've read here on these forums, people usually choose one of three methods: set to midpoint, set to minimum, or disable SR. it all depends on how you choose to use the lens.
I've seen quite a few set to max also. Manual zooms are just problem children with SR.
03-17-2017, 04:51 PM   #12
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QuoteOriginally posted by ScooterMaxi Jim Quote
Due to the relatively large barrel distortion of this lens design, actual focal length is probably closer to the 13-13.5 range. In fact, the rebadged Vivitar version lists the lens as 13mm. Users of the 15-30 have indicated that the difference in view at 15mm is substantial. My preference has been to designate the 12mm setting, as I would rather be conservative instead of over-correction. It's unfortunate that Pentax hasn't implemented the ability to manually enter an exact setting as was done on the Q bodies.
I thought this setting was only for in body stabilization to know how to behave. I don't think it corrects anything? Also, 15mm vs 14mm in Uwa is very substantial regardless, look up 12mm laowa and notice how much wider it is. The wider you get, the more each mm means, at least that's my understanding of it.
03-18-2017, 03:29 PM   #13
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QuoteOriginally posted by awscreo Quote
I thought this setting was only for in body stabilization to know how to behave. I don't think it corrects anything? Also, 15mm vs 14mm in Uwa is very substantial regardless, look up 12mm laowa and notice how much wider it is. The wider you get, the more each mm means, at least that's my understanding of it.
Yes, the setting is for IBIS, and EXIF. You have to let the sensor adjustment mechanism know how much correction to apply. So, for the Samyang/Vivitar design which is somewhere in the 13-14mm focal length range, the settings of 12mm and 15mm available yield incorrect correction. I would rather under-adjust than over-correct. Down at those focal lengths, every mm counts (even more so for FF bodies).

I should mention that I have enough UWA lenses that it is pretty clear to me that uncorrected for distortion Samyang 14mm seems wider than indicated when I compare it to others. (I tend not to correct barrel on UWA, as I find rectilinear projection an unpleasant type of distortion for most of the scenes I shoot.) When corrected to rectilinear, I suspect the lens is 14mm or even a little less wide. However, the proper correction applies to the real focal length, not any adjustments.
03-19-2017, 05:05 AM   #14
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QuoteOriginally posted by UncleVanya Quote
I've seen quite a few set to max also. Manual zooms are just problem children with SR.
What does an IBIS camera do with non-manual zooms? Should we try to do the same manually . . . if it's possible? Or should we turn off IS altogether for manual zooms?
03-19-2017, 05:49 AM   #15
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QuoteOriginally posted by EssJayEff Quote
What does an IBIS camera do with non-manual zooms? Should we try to do the same manually . . . if it's possible? Or should we turn off IS altogether for manual zooms?
read the post 4 up from yours
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