Minecraft – Did minecraft 1.4.2 nerf high level enchantments in favour of the new Anvil

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Step One - Applying modifiers to the enchantment level

The first thing that Minecraft does is apply two modifiers to the base enchantment level. Each modifier is restricted to a certain range, with numbers close to the middle of the range more common than those near the ends.

The first modifier is based on the item's "enchantability," which depends on the material and the type of the item (see the table below). Minecraft picks a number between 0 and the enchantability, then adds that number plus one to the enchantment level. Bows have an enchantability of 1 for this purpose.

Shoot...SCORE!

Source

Enchantments are completely random. There is no way to guarantee which enchantment you will get.

From the wiki

The level requirement influences the quantity, type, and level of enchantments instilled in the item, with a higher experience level generally resulting in more and/or higher-level enchantments. Nevertheless, there is a heavy random factor, and even a level 30 enchantment (the maximum) doesn't guarantee more than one enchantment, nor even that enchantments will be "maximum strength" — a level 30 enchantment can still yield Fortune II or Efficiency III alone, for example.

If you don't want to waste enchantments on equipment, I would suggest trying to enchant books instead. This is slightly more resource intensive, but that way, later you can combine books in an anvil to get stronger enchantments, and even apply a specific set of enchantments of your choosing to the equipment you want to enchant.

Post-Note: I have actually found that if you are looking for a bow with the "Infinity" Enchantment specifically, you will actually have better luck with fishing.

• You can start off with an un-enchanted fishing rod; with luck you will eventually find a rod with the "Luck of the Sea" enchantment (Lowers chance of "junk" catches by 2.5% per level and increases chance of "treasure" catches by 1% per level)
• If you then use this, you have an increased chance of finding an enchanted bow - I personally have had much better luck of getting a bow with the "Infinity" enchantment this way.