Sight-reading is a lucrative skill. If you can do it accurately, you can play a lot of ensemble music and do studio work. I have mentioned this before, but every orchestral soundtrack you have heard in a movie was recorded in ONE SESSION of two and half hours. This is a single "service" under the Musician's Union contracts. Most if not all of the score was played once and once only. Session musicians are people who can walk in, sit down and play whatever is on the stand. Pretty impressive. Mind you, it would be easy to write parts that they simply could not play at sight--but if you do that, they will never hire you to write another soundtrack! Film composers are usually going to stick to things that are reasonably easy to read.
Some instruments are tricky to sight-read on: the guitar for one. Every time there is a chord, which might be three, four or five notes together, it is likely that it can be played in some positions, but not in others. So you have to make some quick decisions--or leave a note out, which is what the really seasoned players do. Today, just for fun, I tried to do something I haven't done in a long, long time: sight-read a vocal and guitar score simultaneously. Sure, it's possible. I picked a very easy little song, "Delicate Beauty" by Henry Lawes, the 17th century English song-writer. If you are reading a vocal part and a guitar part at sight, you are engaged in two quite different things simultaneously. You have to find where the starting note is for the voice and then read the intervals and reproduce them. At the same time, you are reading the guitar part (originally the accompaniment was a bass line, perhaps with some figures to indicate inversions, but I was playing from a version for guitar in three or four voices) and have to decide which finger to put on which note. Oh, and also, there are words so you have to read them as well. Sounds complicated, but if the song is very simple as this one is, it is not too hard. I had a student that took up the lute and got very good at performing Dowland, singing and accompanying himself. As he was playing from the original lute tablature, he would actually be reading three entirely different systems of notation simultaneously: vocal notation, the lyrics and lute tablature which consists of letters on a six line staff (one line for each string or course and letters to indicate which fret). Here is how that looks:
At the top you have ordinary vocal notation on a five-line staff. The clef, though funny-looking, is an ordinary treble clef. There is one flat and the time signature is 3/2. Under that staff are the lyrics in ordinary Roman letters, though since it is the 16th century, they show a lot of 's' letters as 'f's. Underneath that is the lute tablature. This is French tab, with the 1st string on top and letters for the frets. 'a' is open, 'b' is first fret and so on. In Italian tab they use numbers and the 1st string is on the bottom, not the top:
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This is the kind of job you give to a musicologist, of course: transcribe all these nearly unreadable German tablatures into either French or Italian tablature so modern lutenists can read them, or into ordinary vocal notation so guitarists can read them. I once met a musicologist who had obviously done a great deal of this as she claimed to be able to sight-sing from German lute tab. Think about it... Now that would be a very complex mental operation, indeed!
I mentioned sight-singing. This is part of standard training at music schools. Ear training consists of learning to hear and identify intervals, harmonies and rhythms by learning to write them down. This is called "dictation." The opposite process consists of looking at a written-down melody and singing it at sight. These used to be widely practiced skills if you go back a few centuries, but now they are fairly specialized. At McGill first year sight-singing required that you sing simple melodies at sight, like I was doing with the Lawes. Second year sight singing used a text called Quatre-vingt Études de Solfege avec Changement de Clés which translates as Eighty Studies in Solfege with Changing Clefs. This means that every two or three bars, the clef would change: treble, bass, alto, soprano, tenor! Agh! Plus, the melodies were extremely chromatic in the first place. Pretty much a sight-singing nightmare. For third year you had to sing atonal melodies at sight. Oh, yeah!
Here is that song by Lawes: