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Vega measures how much an option's price changes when implied volatility moves by one percentage point. An option with a vega of 0.10 gains about $0.10 per share if implied volatility rises from 30% to 31%, and loses about the same if it falls to 29%.

Implied volatility is the market's price for uncertainty, so vega is the option's exposure to changes in that price. Calls and puts both gain value when implied volatility rises.

Vega is largest for at-the-money options with plenty of time remaining, and it shrinks as expiration approaches. The vega shown on Options Band's chain, like the other greeks, comes from Cboe's delayed feed.

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Suppose an option is priced at $5.00 with a vega of 0.10 while implied volatility sits at 40%. If implied volatility climbs to 50% in the days before an earnings report, vega alone adds about $1.00 — ten points times $0.10 — putting the option near $6.00 even if the stock has not moved. After the report, if implied volatility drops to 35%, the same arithmetic takes roughly $1.50 back off the post-announcement price. This repricing around scheduled events is described in earnings volatility.

Time to expiration drives vega's size. On the same $100 stock, an at-the-money option expiring in a week might carry a vega near 0.05, while the six-month option at the same strike carries something closer to 0.28. Longer-dated options simply have more future for a volatility change to act on, which is why they respond so strongly when the market's uncertainty gets repriced. The flip side is that a short-dated option can shrug off a volatility shift that would move a long-dated one by dollars.

Each contract responds to its own implied volatility — the value for its particular strike and expiration — not to a single market-wide number. Different expirations routinely move by different amounts, a pattern covered under term structure, and implied volatility explains where these figures come from in the first place.

詳細説明

Vega is the partial derivative of option value with respect to the volatility parameter, scaled to a one-percentage-point change.

ν = ∂V/∂σ

Despite the name, vega is not a Greek letter; the symbol ν (nu) is usually borrowed for it. By put-call parity, European calls and puts with the same strike and expiration have identical vega, and for a long position it is always positive.

The main limitation is that vega prices a parallel shift in a single volatility, while real volatility surfaces rarely move that way: short-dated implied volatility swings harder than long-dated, and skew reshapes across strikes. Vega itself is also not constant — it changes with volatility, spot, and time, effects captured by second-order terms (volga, vanna) that no chain column displays.

A related misconception is treating implied volatility as an observed market quantity. It is backed out of option prices through a model, so vega is a sensitivity to a fitted parameter — and on Options Band that parameter is computed from Cboe's delayed quotes.