MIMF

Assuming that the strings can be brought all the way down, or way to high with saddle adjustments, would a shim differently affect the string to fingerboard distance over the length of the neck? I was wondering if there is a difference between the bridge adjustments that take place at the end of the strings, and the shim being sort of a pivot point closer to the center. Maybe some law of physics or geometry that I'm unaware of. If possible, I'd like the strings to be just a little lower at the high end of the finger board without affecting the the lower end.

I assume you are talking about shimming a neck? What kind of guitar is this?

Normally, if you want the action lowered you lower the saddle.

As far as I can see, shimming the neck has exactly the same effect as adjusting the saddles - you only shim it if you don't have enough travel in the saddle adjustment. Assuming you mean a bolt on neck, whether you shim it or adjust the saddles to change the action, it will affect the action all along the neck, though more so as you go up the neck toward the bridge. If you want the action to stay about the same toward the nut, I'd suggest you loosen the truss rod a little to increase the relief, and then lower the saddles or shim the neck if necessary to get the action you want up the neck. It will be a trial and error process.

Fender neck, I presume... I will indeed shim if I want the string height over the body of the guitar to be a bit higher. Last time I did it, a 1/32" shim (I used a washer around the two inboard screws) gained about 1/8" at the bridge, more or less. I just happened to want the bridge saddles higher so the strings were higher, purely a playing comfort thing. Doing a normal setup after is the right thing to do. But to your question - string height at the nut end is set by the nut slot height more than anything else, and after around the third fret, the saddle height makes more and more impact to string height as you go up the neck. Changing the saddle height at the bridge has very little effect on action in the first or second fret, while changing the nut has almost no impact on action at the 12th fret or higher. Shimming the neck is indeed the same thing as changing the saddle height.

Sorry I left out so much info. The guitar I'm referring to is a Carvin Bolt-T that I built from a kit about 18 years ago, and I was asking about a shim in the neck pocket. I learned how to set it up a long time ago, and the string height and neck relief is pretty typical for a Strat type guitar. I just think it would be better if the string height at the last fret could be as low as at the first fret.

Jeff Leites wrote: I just think it would be better if the string height at the last fret could be as low as at the first fret.

That's never gonna happen. If that were the case, when you fret a string on the lower frets and play it, the large excursion of the string vibration will rattle (buzz) against the middle and upper frets. That's why a little relief (forward bow) in the neck is a compromise to let you set the action a little lower, keeping the vibrating string from hitting those middle and upper frets.

Those young guns who shred on their guitars, tapping and hammer-on/pull-offs and all of that, play with very low action all up and down the neck. Where a normal target action at the 12th fret is 1/16", or .065", they will have 3/64" or even .040". But - they can do that because of their technique - it is a very light touch on the strings, they barely get any sound of them at all, and they play with very high gain in their amplifier. Much of the skill in playing a guitar with that ultra low action is in controlling the output of the guitar, and not over-playing the strings so they buzz out.

I would sooner reshape the neck pocket, putting a slight angle on it, than shimming it.
It doesn't take much to lower the leading edge and flatten the rest on the same angle.