Hmmmmmm. actually i disagree with Lt. Bob. I play trombone and have my studio and normally i find the opposite when recording my self. First of all, the fact that the sound is coming also from the body of the horn and not just the bell is only true with instruments such as sax's and clarinets. Trombone and French horns are from the conical shaped horns therefore the sound is produced from the bell. If you mic anything but your just gonna get noises like the player breathing and the slide clicking when coming to first position. Depending on the sound you want really depends on the positioning. For my work with jazz 2feet directly infront of the bell creates a nice sound, but although bright. For this it seems like you would need a more orchestral type sound. Put the mic about 3 feet in front of the bell in this case (i wouldnt go much farther than that) and instead if you need a more warmer and less attackfull sound get the mic out of the heavy SPLs by bringing it off axis and pointing it up towards the bell. This will get the warmer edge of the sound but still get a good defined attack. You could expirement between 1-3feet and the most i would possibly go is 6 feet. A 57 will work but a condenser like an AT4033 will probably work better. However like i said, try and keep the mic closer to about 1-3 feet only or the sound will be very distant sounding, but still expirmenting is your best key. Its much easier to mic a quiet trombone player than a loud one but he could get quite loud.
Ive never miced a french horn before. But i do know that most of the deep rich sound the french horn makes comes from the reflection behind it. Try putting a decent foldable table behind the player to let the sound reflect there isntead of reflecting of the wall. Expiriment with placement on this table also, bring it back to where you have a good balanced sound. In this case you could probably use a boundary mic on the table. But is this case where you might not have one of those, mic infront of the french horn a few feet but make sure your getting the reflection sound.
There is an article i found about this very subject. However, me doing a style of music that requires a really bright in your face sound lead me to find out that these techniques dont work as good. However, they might work great for your situation. Let me remind you that i am a very loud player and the style i do is bright. However, i talk to many trombone players that say you cant get a good sounding recording without a ribbon mic, and that if your using anything different requires you to be about 1-2feet directly infront of the bell, and most ofthese players were soft players. Hearing this a while back was the key to finding the sound that i like.
http://www.sospubs.co.uk/sos/jan99/articles/bress778.htm
Also that article has good information on recording woodwinds also and also goes into depth of whats going on with the sound with the instruments.
My hypothesis on how its easier to get a good sound with the mic right up close is this...I believe that most mics besides ribbons cannot handle the serious SPLs. Sure it can take the pressure without microphone overload but the sounds are still too loud and distort. When you have the mic that close, the waves of the horn have not quite formed and thus are actually more quiet and if captured at the right time, has a much less SPLs. If you want a warmer sound then try moving the mic off axis again also.
But it find most of this information being true as i continue to watch great live recordings on TV of great jazz soloists. Infact sometimes i see the trumpets being miced with a 57 practically in the bell, and it sounds good. Watch on Late Night Shows and watch the band, those mics are quite close.
lots of valuable information here, might be overkill but i believe it is something that many people have a hard time with. You will need to expiriment with the sound you like because its not gonna be the same for every trombonist and every type of music being played. But these are some guidelines that should help you in your expirimenting.
Good luck and let me know if you have more questions.
Last note, i decided i should leave a section here on what mikes i recommend. Of course, Ribbon mics are great for this but becarefull, the SPL of trumpets and trombones can actually split the ribbon. SM57 is a very basic one. I have had very good luck with my AT4033 but something with a little more mid would be better suited for this. Also, for loud harsh players, which are generally hard to mic with a good sound, ive heard a Senheiser MD421 works really good for this. But i think most people will tell you to get a Ribbon mic.
Danny