When I run ffmpeg -r 20 -b 1800 -i bloch_%01d.png bloch.mp4
from the User Guide's Generating Images for Animation
section I get the following error:

Option b (video bitrate (please use -b:v)) cannot be applied to input url %04d.png -- you are trying to apply an input option to an output file or vice versa. Move this option before the file it belongs to.
Error parsing options for input file bloch_%01d.png.
Error opening input files: Invalid argument

What works for me instead is this:
ffmpeg -r 20 -i bloch_%01d.png -pix_fmt yuv420p bloch.mp4

Note: personally I print files as zeropadded 4-digit number, so I have %04d.png instead of bloch_%01d.png.