[FFmpeg-user] Selecting/dropping frames without re-encoding

[Mikhail V]

Hello,

I have a camera recording which is an MXF file and the codec is DNxHD
I suppose it is a single-frame codec so literally it is a container of
images which are independently stored in the file.
Recordings are taken at 50 fps progressive.
Now what I want is to extract only half of the frames, so that it becomes 25 fps
Note that I need to do it _without re-encoding_ for two reasons:
1. Conversion must be as fast as possible, files can be really huge
2. No changes to original information should occur

Theoretically this is an elementary operation, it should just take
every second frame from source and write to output
leaving everything else as is.
I have tried several options whith "-r ..." and added "-c:v copy"
These commands I have tried:

ffmpeg -i Capture0006V_720p.mxf -r 25 -c:v copy out1.mxf
ffmpeg -r 50 -i Capture0006V_720p.mxf -r 25 -c:v copy out1.mxf
ffmpeg -r 2 -i Capture0006V_720p.mxf -r 1 -c:v copy out1.mxf

But they all produce again 50 fps video, so I suppose stream copy just
ignores all "-r" options so I don't know what can I do to achieve
what I want. I have searched a lot, so I've registered here
hoping for solution.

Is it possible that ffmpeg cannot do this without re-encoding?
If no, how can one do this?
Thanks in advance.