February 17, 2016 at 3:11PM


4K Equals What MM Cropped?

Howdy friends. I have a quick question that I've been trying to figure out for a shoot. I have a Sony A7R II and am filming wildlife for a documentary and am trying to figure out what focal length I am actually shooting at. When using an A7R II with a Metabones adapter (non-speed booster) on a Canon 100-400mm lens mounted on a 1.4X extender in Super 35 APS-C crop mode, I should be filming at 840mm on the long end. I get this estimate by taking 400mm and multiplying it by the 1.4X extender to equal 560mm. Then, I multiply that number by the 1.5X crop factor of the Super 35 mode and it equals 840mm.

Now, before anyone says it's not possible or even a good idea to mount a Metabones adapter to an extender and then to a Canon lens and film with it, I want to just say that I've been doing this and it works great. I built a rig to support it and am very happy with the results. What I am trying to figure out from here is when I crop into 4K on a 1080p timeline in post editing, roughly what is the equivalent focal length to that? In other words, 4K (really it's UHD) has to be scaled down by 50% to show the full image on a 1080p HD timeline. So, if I don't scale it down and instead I leave it cropped in at 100%, does this mean that I am essentially cropping in twice as much as it was really shot? Would that mean that the 840mm I filmed in Super 35 is actually equivalent to 1,680mm when cropped in?

What if I am using a 28-300mm Canon L lens in Super 35 without a tele? 300mm multiplied by 1.5 for Super 35 is equal to 450mm. If I shoot that in 4K (UHD) and leave it cropped in when editing, does that equal 900mm? I am sorry if I confused anyone! I am piecing together a rig for an upcoming shoot and am trying to figure out what the equivalent focal length is when cropped in on Premiere using 4K on a 1080p timeline. I am trying to make sure my math is correct in that cropping a 1080p section out of a 4K video with no downscaling would be equivalent to doubling your focal length, thus allowing me to see exactly what I would see at 1,680mm, if there was such a lens, only doing so with 840mm of actual lens at 4K.I hope this makes sense!


Yes, you are correct, cropping 1080p part of 4K original will get you 2x focal length equivalent.

Best Regards,

February 17, 2016 at 5:55PM, Edited February 17, 5:55PM


Thank you Andrey!

Joshua Milligan

February 17, 2016 at 9:20PM

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