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!