Look at the two following images. Try to spot the difference.
Both look the same at this size. When you look at the first image up close, the iESP image is over exposed in the centre. The second image is perfectly exposed in the center and under exposed in the darker areas at the edge.
Cameras try to set the correct exposure for an 18% grey scale, which is approximately a medium green leaf. The spot meter gives the correct exposure from the green plants in the middle of the image. I usually take a reading from a plant or grass. For the most accurate measurements indoors where there are no plants, carry an 18% grey scale card, around $20, and measure the exposure from the grey card.
Choose the Part You Want Exposed Accurately
The iESP tries to include more image than the center spot. That means the iESP tries to average in the dark shaded areas at the edge. iESP then increases the exposure and overexposes the middle of the image. If you want the middle of the image correctly exposed, use spot metering. When you want a wider area correctly exposed, use iESP.
When you photograph one person against a very bright or very dark background, use spot metering on their face. When you photograph two people and the center spot will focus on the background wall between the two heads, use iESP or use a manual override.
iESP uses a small number of exposure measurement zones. Expensive cameras use hundreds of zones. Multiple zones do not help unless they choose exactly the same zones as you would choose. If in doubt, use the spot meter on the exact spot you want correctly exposed to a medium tone approximating an 18% grey, which is a very light brown skin.
When you run out of accuracy with the built in metering and are shooting up close, choose an external incidence meter or carry a grey card.
In Black and White
Some expensive cameras, designed for the wedding market, let you take a special reading on the whitest spot and the blackest spot. The exposure is then calculated between the white and black reading. This option could be standard on every camera at no cost because it is just a software upgrade.
Olympus's iESP is acceptable for beginners but everyone should learn to use spot metering as soon as they are over the initial fun of snapping pictures and before they get a disappointing picture.