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11-27-2017, 05:50 PM   #1
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Focal Reducer Availability

I just purchased a Rokinon 8mm "fisheye" lens and will be using it with my K-1 camera. Normally, crop mode would apply (since it's an APS-C lens) but it occurred to me that it would be nice to capture the entire image circle from the lens at times (full-field still cuts off the top and bottom of the image). In the review of that lens, a "Turbo focal reducer" is mentioned but in Googling, I could only find reducers designed to match a K mount lens to another mount type (and reduce the focal length in doing so).

I'm wondering if anyone is familiar with a focal reducer that is K mount to K mount and would work to adapt the Rokinon to the K-1, capturing the full field of the lens in the process?

Thanks.

11-27-2017, 06:36 PM   #2
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I use FRs for a couple of systems...there isnt one for K mount that i know of.
11-27-2017, 07:34 PM   #3
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AFAIK the design of the K mount means a focal reducer isn't possible.

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11-27-2017, 08:46 PM   #4
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I agree with surfar and Adam. A focal reducer is used to adapt a lens for a mount with a longer registration distance, to a camera with a shorter registration distance, e.g K-mount
full-frame lens to Sony E-mount crop body. it is not possible to use a reducer within the same mounting system.


Last edited by audiobomber; 11-28-2017 at 07:09 AM.
11-27-2017, 09:39 PM   #5
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A focal reducer also need an image circle larger than the system adapted to. 645 to K or 6x7 to K for example not that these exist as focal reducers today.
11-27-2017, 09:42 PM   #6
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Ive tried a Apsc Pentax lens on aspsc FR Fuji and it increases the Vignette,with a FF sensor it'd be worse.The Pentax lens is fine on a M43 sensor.

FR is reverse of a TC.
11-28-2017, 05:02 AM   #7
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QuoteOriginally posted by Bob 256 Quote
I just purchased a Rokinon 8mm "fisheye" lens and will be using it with my K-1 camera. Normally, crop mode would apply (since it's an APS-C lens) but it occurred to me that it would be nice to capture the entire image circle from the lens at times (full-field still cuts off the top and bottom of the image). In the review of that lens, a "Turbo focal reducer" is mentioned but in Googling, I could only find reducers designed to match a K mount lens to another mount type (and reduce the focal length in doing so).

I'm wondering if anyone is familiar with a focal reducer that is K mount to K mount and would work to adapt the Rokinon to the K-1, capturing the full field of the lens in the process?

Thanks.
It's not a focal reducer you need to make a APS-C lens cover full FF sensor, but a teleconverter. (a 1.5x TC that support FF would be optimal, making 8mm APS-C lens into a 12mm FF lens). But in real use you probably will not gain much from using a teleconverter over using crop mode on K1.

11-28-2017, 06:16 AM   #8
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QuoteOriginally posted by Bob 256 Quote
In the review of that lens, a "Turbo focal reducer" is mentioned but in Googling, I could only find reducers designed to match a K mount lens to another mount type (and reduce the focal length in doing so).
It should be noted that the reviewer lists the cameras used as Sony NEX-7 and A7II, with no K-mount bodies.
11-28-2017, 07:18 AM   #9
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QuoteOriginally posted by Fogel70 Quote
It's not a focal reducer you need to make a APS-C lens cover full FF sensor, but a teleconverter. (a 1.5x TC that support FF would be optimal, making 8mm APS-C lens into a 12mm FF lens). But in real use you probably will not gain much from using a teleconverter over using crop mode on K1.
I think you are misunderstanding what the op wants. The image of the 8mm isn't perfectly circular on v the k-1 per the OP. They indicated that want to reduce the size to get the full circular image. At least that's how I'm reading this. That doesn't change the fact that no such focal reducer exists.
11-28-2017, 07:43 AM   #10
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QuoteOriginally posted by UncleVanya Quote
I think you are misunderstanding what the op wants. The image of the 8mm isn't perfectly circular on v the k-1 per the OP. They indicated that want to reduce the size to get the full circular image. At least that's how I'm reading this. That doesn't change the fact that no such focal reducer exists.
I think you are right. After reading the question a second time, and with your comment in mind, I now see I misunderstood OPs question.
11-28-2017, 08:12 AM   #11
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Quite correct. The image circle of the 8mm fisheye is too large even for the full-frame K-1, so I would like to reduce it to fit within the FF area. The fact that a poster mentioned using a Turbo reducer to do this is what sent me looking in the first place, but as most posters here have mentioned, the only reducers available seem to be used to convert from one camera type to another (changing the backfocus of a given lens).

Thanks anyway. Could be something I might play around with if I get the time and try a K-mount reducer (front) with a modified rear mount. Probably cheaper though just to get a fully fisheye lens if I want to go that way.
11-28-2017, 08:32 AM - 1 Like   #12
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How big is the image circle?

If the diameter is under 5500 pixels, you could take two images with composition adjust all the way up and all the way down and merge them.

If the diameter is under 6900 pixels, you could take two images (landscape & portrait) and merge them.
11-28-2017, 08:39 AM   #13
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QuoteOriginally posted by photoptimist Quote
How big is the image circle?

If the diameter is under 5500 pixels, you could take two images with composition adjust all the way up and all the way down and merge them.

If the diameter is under 6900 pixels, you could take two images (landscape & portrait) and merge them.
Great ideas - the lens is arriving today so I will give that a shot when I know more about its coverage. Nice to have that feature in the K-1.
11-28-2017, 09:43 AM   #14
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Do you have the version coming that has a removable hood?
11-28-2017, 09:47 AM   #15
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QuoteOriginally posted by UncleVanya Quote
Do you have the version coming that has a removable hood?
Good point. Without the removable hood you won't get a circular image even if you had a reducer. I've used mine on my ZX-L and the hood gets in the way til I take it off.
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