In fly-fishing, the rod, reel and line is chosen according to the size of the fish you'll be catching; combined with the conditions under which you will be fishing. For example; the rod for trout fly-fishing is going to be different than that of bonefish fishing, and a bass fly-fishing rod is going to be different yet again. There is a science and logic to matching up a rod, reel and fly line. Weight class is used to categorize fly-fishing rods, reels and fly-lines. The theory goes, the higher the weight number of the rod the bigger the fish the rod can handle. For example a 2wgt would be ideal for small brookies, a 4, 5, or 6 weight for regular trout, and 8, 9, or 10 weight for steelhead or salmon; depending on which species of course.
I'm going to assume the rod to be chosen will be used to fly-fish for trout in the mountain west; mainly in Colorado, Wyoming, Montana, Utah and or New Mexico. I have been fishing those waters for over 40 years, and can confidently say; most of the fish that are caught will be in the range of 13 to 18 inches; (one to three lbs), and more often than not the wind will be blowing. There are places where you can catch trout (and lots of them) from 20 to 26 inches;( three to eight lbs). Once in a while, if you're really lucky; you might even hook up with a trout over 10 lbs. However, to do that would mean a guided trip and/or a specific location where the fish are known to reside (a private lake or river section for example).
There are three types of fly rods: bamboo, fiberglass, and graphite.
Bamboo rods are wonderful for spring creek fishing. They were the first of the fly rods and are still highly prized as fly fishing rods today. A good modern day bamboo fly rod has the most incredible feel for delivering a dry fly of any of the fly rods sold today. However, bamboo rods are the most expensive of all the fly-fishing rods ranging in cost from $1200 to $3000 per rod. It's not the kind of rod you want to loose in a river, or accidentally break by stepping on it or getting it caught in a car door. I could be wrong but I don't think a bamboo rod should be your first choice as a fly rod. However, if you're so inclined great for you.
Fiberglass rods on the other hand, are still being made. However, the market for fiberglass fly-fishing rods is relatively small. Fiberglass has some advantages over graphite: they're less expensive, and they're very strong rods. You can actually lift a heavy fish out of the water with a fiberglass rod, but I wouldn't try it with a graphite rod. A good fiberglass rod will cost about half as much as a comparable graphite rod. Fiberglass rods made today are considerable lighter than the first rods sold years ago. However, fiberglass rods are heavier than graphite rods. And weight will make a difference at the end of the day.
Graphite. As a rod type, graphite rods are the rods of choice for most fly fishermen. They're exceptionally lightweight and powerful. For example; The Helios rod from Orvis weighs just one ounce. Think about that for just a moment. One once! How can a fly fishing rod that weighs one once catch a 10 pound fish you ask? Well it's the material properties of graphite that make it strong and superb for long distance casting. Graphite rods are made for delivering fly line and controlling a fighting fish but not for lifting fish out of the water, else they will break.
Rod performance is described in terms of "action" and/or flex and relates to how the fly-fishing rod bends.
Rod flex and rod action are one in the same. You can use the terms interchangeably. I'm going to refer to the bend of the rod as rod flex because I think it's more intuitive to understand as apposed to rod action.
There are three basic flex's in fly-fishing rods": slow, medium and fast.
"Slow"rods flex from the butt to the tip. They load slower and because of this they are very easy to learn to cast for the beginner. The disadvantage of a slow flex rod is they do not generate the power needed to cast into wind. However, they're great for short and delicate cast like those needed dry fly fishing.
Medium flex rods flex from the center of the rod to the tip. It loads faster and is adequate to cast in the wind. A medium flex rod is very versatile. You can make delicate presentation and/or generate the power needed to cast into a stiff breeze. Medium flex rods are a good choice for beginning fisherman.
Fast action(flex) rods, are powerful and load very quickly. The flex comes in the upper 1/3 of the rod with the lower 2/3's being very stout. This combination allow for easy casting into the wind. Additionally, fast actions rods are the rods of choice for fishing sinking lines.
Rod weight. Rods are classed by "weight"; not their physical weight but the weight of the lines they cast. Fly lines are measured in terms of grains. Hardly anybody knows how things are measured in grains so manufactures devised a system to apply to their fly lines beginning with the lightest lines to the heaviest.
Matching the fly line to the fly rod.
Manufactures number the line according to the grain weight; that is 1 through 14. A #1 line begins the lightest line weight; and onward and upward to the heaviest #14 line weight. The heavier the line weight the more powerful the rod has to be to deliver it. That means, the bigger the fly being fished; the heavier the line needed to cast it. Additionally, the stronger the wind the more powerful the rod has to be along with a heavier weight line. To repeat, the rod is matched to the line according to the line weight. For example, a #5 weight rod would take a #5 weight line.
Then there's the question of rod balance. When you pick up a well balance rod, you can feel it. How? Well, because it's balanced. For me this has to do with the "feel" of the rod in combination with the reel (loaded with the line). Rod balance has nothing to do with the flex of the rod but more with the weight distribution along the rod's length. If the rod is bottom heavy because of the reel, you won't be able to feel the load of the line as well, and conversely if the rod is top heavy because the reel is too light; it's going to effect the timing of the cast. The goal of good rod balance then is to feel the performance of the rod from butt to tip.
A word about fly lines.
Fly lines are designed to deliver the fly to its intended target in a specific way. For example: If I were going to fish dry flies, I would use a 4wgt or 5wgt double taper floating line. Why? Because a tapered line will transfer the energy of the cast through the line in such a way as to cause the line to unroll to the end, and deliver the fly softly--on the water (provided of course the leader and tippet combination is correct).
When choosing a fly line you must decide what style of fly-fishing you're going to do. Basically, there are three kinds of fly-lines: floating lines, sinking lines or sink tip lines (front section sinks while the rest of the line floats).
Now, combine a fly fishing style for example; dry fly fishing, nymph fishing, streamer, etc, with various sizes of water such as small streams, medium size streams, or big rivers, and mix those variables with the size of the flies: tiny dries, normal nymphs, big streamers, etc. that you'll be fishing. Then throw in the wind factor along with other weather conditions, and you can see the implication for choosing the right rod regarding the style of fishing. Therefore, there are many combinations of rod, reel and line one can choose from. The problem is you only have enough money to buy one rod; that being your first rod. So which one do you choose?
To sum it all up I'm going to make it simple for you and tell you your first fly-fishing rod should be a 5-weight-fast- action-graphite. Match a reel to the rod loaded with a DT (double taper) high visibility floating line, and, if you have the budget for it, buy an extra spool loaded with a #1 sinking line and another spool loaded with a #3 sinking line. The above suggestion will give you one rod that you can fish all the styles in all conditions.
The rod I have fished for years is a Sage #5 weight, fast action, 8'6" 2 piece rod. At the time that I bought it, it was the state of the art in graphite rods. Today there are many comparable (and less expensive) rods on the market that would be an excellent choice for your first rod. Believe me, if you become a serious fly-fishing, over time you will end up owning more than 5 or 6 rods.
Do your research and choose wisely my friend. Like the pizza man says, "better ingredients, better pizza".