I am fairly happy with my buttonholes on the whole, but there is a fundamental difference that I see with professional ones and I can't work out what I am not doing quite right.
Pictured below are 1. One of mine and 2. One on a velvet Savile Row DJ I have (not made for myself sadly):
The stitches seem much tighter, closer together. It's also much stiffer. The thread might also be finer I think?
If I try to pull stitches tighter it just makes the hole deformed. Any ideas?
The first I learned about hand made buttons is to use two strands of buttonhole twist. Lay them side by side. On trousers they are more flexible. A business suit gimp might have the best appearance, sometimes. A sports coat is very different coat. I would think for most a softer buttonhole.
Whatever the customer wants. It's theirs.
For anyone looking again at this, I found using stiff gimp (rather than winding my own) made a big difference. Not sure why though!
Practice makes perfect. Sometimes it never seems true, at least, not soon enough.
The knots (pearls, purls) pulled to latch- is tight enough. The knots setting a little higher look best. Pulling the thread, not the needle, up and whatever direction, getting them in a nice straight line and setting the proper height. With practice you will be making fewer and fewer movements as your skills develop.
Clothes that go in the regular wash, jeans for example, I won't use silk twist, but sewing thread, two strands, not twisted, but side by side. I like the look.
When I made my first suit at Henry Pooles I asked Jinny to put in my buttonholes because she was considered the best.
That coat now hangs in my classroom as an example of what can be achieved. I am sure I could show at least one or two holes of that quality, but 13 on one coat would be a stretch.
It's most likely a finer thread, as you suspect.