Oblivion:Feather

Feather decreases your current encumbrance (the weight of your inventory), increasing total carrying capacity. Your encumbrance is decreased by M points for D seconds.

Notes
This effect can be gained for spell making and enchanting via spells only.
A Feather effect's magnitude will never be so high that it would drop your encumbrance below zero. If a new effect would bring your total Feather magnitude higher than your encumbrance at the time it is cast, the new effect's magnitude is instead reduced to what would be needed to bring your encumbrance to zero.
The magnitudes of active Feather effects generally do not change once applied (see Bugs for an exception). Even if you lower your encumbrance by dropping an item, the maximum amount you can carry stays the same.
Feather is one of the few spells where you can create a custom spell with both maximum magnitude and duration and still be able to cast it in-game. In fact, such a spell (100 points for 120 Seconds) is only considered an Apprentice level spell.
Since Feather spells stack, you can even create multiple versions of this same spell with different names for a larger effect. In other words, if you create one 100 point spell named "Feather 1" and a second 100 point spell named "Feather 2", you can cast them both for 200 points of Feather. Add "Feather 3" and get 300 points, etc.
Using Feather makes you run faster, because it lowers your equipment weight (the WornWeight variable). The higher your equipment weight, the slower you run. If you are wearing extremely heavy equipment, Feather can increase your running speed by almost 67%.
It is impossible to add Feather effects using abilities via the Construction Set.  Regardless of the effect's specified magnitude, the in-game magnitude always ends up being zero, presumably since the abilities are resolved before encumbrance. You can, however, add it as a disease.

Fortify Strength
Another method of increasing carrying capacity is with Fortify Strength.  In addition to its other effects, each point of added Strength increases maximum encumbrance by 5 points.
Fortify Strength provides more carrying capacity than Feather for custom enchantments unless using a Grand Soul (where both effects provide 50 points more capacity).
For Sigil Stones, Fortify Strength only outperforms Feather for Descendent Sigil Stones (35 points vs. 25 points).
When comparing Transcendent Sigil Stones, Fortify Strength provides less than half as much capacity as Feather (60 points vs. 125 points).
Fortify Strength is always less magicka-efficient than Feather when casting custom-made spells, assuming you have equal skill in Restoration and Alteration.
The benefit of running faster due to reduced equipment weight is only available by using Feather, and not Fortify Strength.

Warning
On the Inventory Screen, Feather appears to increase your maximum encumbrance (for example, a 100 pt Feather effect causes an encumbrance of 125/200 to look like 125/300).  The game actually is reducing your current encumbrance, though (making the previous example 25/200, not 125/300 as listed in-game).  Although this difference is often semantic, it can lead to unexpected behavior if your total encumbrance reaches 0: all additional points of Feather are not applied.

A typical scenario is that you are trying to bolster up your encumbrance to carry a dungeon's worth of loot, and start casting spells before picking up all of the loot — then realize that your maximum encumbrance is smaller than expected and you can't pick up the loot.  If you look at the Active Effects tab, all the effects will be listed under Feather, but the magnitude next to many of them will be zero.  What happened is that the game reduced the weight of the items you were carrying when the Feather effects were applied to zero; effects beyond that did nothing. The magnitude shown on the Active Effects page is less than it should be and remains fixed at that low magnitude even if you subsequently pick up more loot.

The best way to avoid this problem is to not use Feather effects until you need them - don't apply a new effect if it would make your total Feather magnitude higher than your current encumbrance. This is most important for Potions, since the magnitude of Spells and Enchanted items will be reset if you re-cast the spell or re-equip the item.

If you wish to apply a large number of Feather effects before starting a dungeon, try to collect a large pile of heavy items near the entrance to the dungeon. Pick all the items up at once (going far over your maximum carrying capacity), apply as many Feather effects as you can, and then drop whichever items you aren't going to want later. Alternately, high-magnitude, short-duration Burden on Self spells can be used to artificially increase your weight for the few seconds you need to apply your Feather effects.

This warning does not apply to Fortify Strength effects, since they genuinely increase your character's maximum encumbrance.

Bugs
Applying a new Feather effect when the magnitude of your existing Feather effects is higher than your current encumbrance (because you either Burdened yourself or you removed items) causes the magnitude of existing Feather effects to reset, but their magnitudes listed on the Active Effects tab will remain the same.
Example: If your encumbrance would naturally be 50/400 and you use a custom spell with Burden 100 points for 1 second on Self, Feather 100-points for 120 seconds on Self, your encumbrance will become listed as 50/500 (once Burden wears off), and your active Feather magnitude will be listed as 100. If you equip an item with a 50-point Feather enchantment after this, however, your listed total Feather magnitude will become 150 points, but your listed encumbrance will drop to 50/450 (instead of the 50/550 it would be if you really did have 150 points of Feather active).

See also
Related effects:
Burden
Fortify Strength

Alchemy Ingredients
The following alchemy ingredients can be used to make a potion of Feather: