KineMINI 4K goes full frame with modified Speedbooster

[Brian Caldwell]

Mr. Brian, I'd like to take the opportunity to ask and I hope you'd give up insight on a certain technical subject. Would it be technically possible optically to produce a focal reducer that reduces the image circle of a Medium format lens to a Full Frame format? Especially on a camera with a small 18mm flange distance like Sony A7 series. Or there a technical limitation on how large image circles can be reduced? 

​Hi Ebrahim:

It's certainly possible to do a 1-stop reducer from medium format to 24x36mm mirrorless, and likely a bit more than 1-stop.  However, the real question is "why"?  After all, if you offered a device that could convert an 80mm f/2.8 lens to a 56mm f/2.0 lens I think most people would not get very excited.  Simply put, medium format optics are too conservative in their specs, and IMO you're better off purchasing native or adapted 35mm SLR optics for the A7.

[Guest Ebrahim Saadawi]

​Hi Ebrahim:

It's certainly possible to do a 1-stop reducer from medium format to 24x36mm mirrorless, and likely a bit more than 1-stop.  However, the real question is "why"?  After all, if you offered a device that could convert an 80mm f/2.8 lens to a 56mm f/2.0 lens I think most people would not get very excited.  Simply put, medium format optics are too conservative in their specs, and IMO you're better off purchasing native or adapted 35mm SLR optics for the A7.

Highly appreciated that you'd answer Mr Brian. You changed my mind, I agree, it wouldn't be a very useful product but for a very niche market. 35mm lenses are being produced on higher volumes and with much newer technology so it's just better using those. 

Anyway I hope you stick around as we have so many questions that always remain unanswered, so we'll abuse your presense if you don't mind!

1- A second question always circeled the forum is: is there a maximum limit to focal reduction? 

I mean, with the right optical design or stacking of focal reducers, could one theoritically reduce a FF image circle to a 2/3" phone-sized circle, just as an extreme example? What's the limit there?

(Given that we have a 1" or a 2/3" sensor with no mount, just air in front and you can put a mount at any flange distance needed for the optics, because we've seen higher reduction you achieved with the Pocket speed booster but you seemed to be limited by the flange distance)

[Nikkor]

I still want one

[Brian Caldwell]

Highly appreciated that you'd answer Mr Brian. You changed my mind, I agree, it wouldn't be a very useful product but for a very niche market. 35mm lenses are being produced on higher volumes and with much newer technology so it's just better using those. 

Anyway I hope you stick around as we have so many questions that always remain unanswered, so we'll abuse your presense if you don't mind!

1- A second question always circeled the forum is: is there a maximum limit to focal reduction? 

I mean, with the right optical design or stacking of focal reducers, could one theoritically reduce a FF image circle to a 2/3" phone-sized circle, just as an extreme example? What's the limit there?

(Given that we have a 1" or a 2/3" sensor with no mount, just air in front and you can put a mount at any flange distance needed for the optics, because we've seen higher reduction you achieved with the Pocket speed booster but you seemed to be limited by the flange distance)

​Phone-sized image circles are typically a lot less than 2/3" these days!

Four things limit the ability to arbitrarily lower the reduction ratio of a focal reducer:

1) Inability to get really close to the sensor due to shutter, filter pack, mechanical junk, etc..

2) limited space on the master lens side due to a short working distance of the master lens

3) Image quality requirement

4) f/0.5 ultimate speed limit for any well-corrected lens

In practice, its possible to go down to 0.5x for certain special applications, with an aperture as large as f/0.66 or even f/0.63.  In the case of the Blackmagic Pocket, the extra space and smaller sensor permitted 0.57x with great quality even at f/0.74, but all my attempts to get a smaller ratio resulted in unacceptable aberrations.

In your example of a FF-2/3" reducer the magnification would need to be ~0.25x.  So, an f/2 lens would be reduced to f/0.5, which is the fastest speed allowed by physics.  For various reasons a focal reducer capable of 0.25x at f/0.5 is an unattainable goal.  You might be able to get 0.25x at a slower speed, so long as the attached lens has a very large working distance (e.g., telescope objective etc.)

[silvertonesx24]

Well I just ordered my EF speed booster for the KineMini. Excited to eliminate two of the downsides of this camera- low light and crop factor.

And now I have this non-speed booster EF mount lying around...

[Nikkor]

Brian, if we take mamiya rz67 lenses which have a 110mm flange distance, how much reduction would be possible on a sony A7 type camera? Those modern mamiyas are very sharp and cheap, it would be nice to use them on a a7rii in highres pixel shift mode.

[Guest Ebrahim Saadawi]

​Phone-sized image circles are typically a lot less than 2/3" these days!

Four things limit the ability to arbitrarily lower the reduction ratio of a focal reducer:

1) Inability to get really close to the sensor due to shutter, filter pack, mechanical junk, etc..

2) limited space on the master lens side due to a short working distance of the master lens

3) Image quality requirement

4) f/0.5 ultimate speed limit for any well-corrected lens

In practice, its possible to go down to 0.5x for certain special applications, with an aperture as large as f/0.66 or even f/0.63.  In the case of the Blackmagic Pocket, the extra space and smaller sensor permitted 0.57x with great quality even at f/0.74, but all my attempts to get a smaller ratio resulted in unacceptable aberrations.

In your example of a FF-2/3" reducer the magnification would need to be ~0.25x.  So, an f/2 lens would be reduced to f/0.5, which is the fastest speed allowed by physics.  For various reasons a focal reducer capable of 0.25x at f/0.5 is an unattainable goal.  You might be able to get 0.25x at a slower speed, so long as the attached lens has a very large working distance (e.g., telescope objective etc.)

Thank you for answering Mr, Brian. Puts things into perspective. Sad to know the maximum you found usable was the Pocket Speed booster, we've always had fantasies of stronger focal reductions.