Do FFMPEG H264 compression presets affect the video quality? [closed]

Answer 1

If it helps here is a git diff from slower to veryfast. Even if you just consider the ref value you can see how veryfast could be lower quality.

ref
In short, this value is the number of previous frames each P-frame can use
as references.

source

--- a/slower
+++ b/veryfast
@@ -1,20 +1,20 @@
 cabac=1
-ref=8
+ref=1
 deblock=1:0:0
-analyse=0x3:0x133
-me=umh
-subme=9
+analyse=0x3:0x113
+me=hex
+subme=2
 psy=1
 psy_rd=1.00:0.00
-mixed_ref=1
+mixed_ref=0
 me_range=16
 chroma_me=1
-trellis=2
+trellis=0
 8x8dct=1
 cqm=0
 deadzone=21,11
 fast_pskip=1
-chroma_qp_offset=-2
+chroma_qp_offset=0
 threads=12
 lookahead_threads=2
 sliced_threads=0
@@ -25,17 +25,17 @@ bluray_compat=0
 constrained_intra=0
 bframes=3
 b_pyramid=2
-b_adapt=2
+b_adapt=1
 b_bias=0
-direct=3
+direct=1
 weightb=1
 open_gop=0
-weightp=2
+weightp=1
 keyint=250
 keyint_min=23
 scenecut=40
 intra_refresh=0
-rc_lookahead=60
+rc_lookahead=10
 rc=crf
 mbtree=1
 crf=23.0

Answer 2

Yes, quality may vary slightly depending on the preset used, but it should not be a significant amount. Here's an excerpt from a discussion on #x264. Similar question to yours with answers provided by one of the x264 developers:

verb3k | Do different presets have an effect on quality when used with CRF?
@Dark_Shikari | verb3k: yes, but not too much.
@Dark_Shikari | a 0th-order approximation is that they have no effect.
@Dark_Shikari | The main reason there's a difference is because the preset affects how x264 itself measures quality
@Dark_Shikari | that is, it uses better, more accurate methods of measuring quality
@Dark_Shikari | obviously, this will affect the definition of what -crf does!
@Dark_Shikari | It's just not too much, so we can mostly ignore it.
@Dark_Shikari | specifically, there are three big things that can affect the definition of quality
@Dark_Shikari | 1) AQ being on/off
@Dark_Shikari | jump: ultrafast to superfast
@Dark_Shikari | 2) mbtree being on/off
@Dark_Shikari | jump: superfast to veryfast
@Dark_Shikari | 3) psy-rd being on/off
@Dark_Shikari | jump: faster to fast
@Dark_Shikari | above fast there are no more big jumps.

This means that a slower preset with the same CRF value will improve quality per bitrate, but might make both quality and bitrate higher or lower.