Eyeglasses with three divisions of correction in the lenses are called trifocals. These are considered to be better than bifocals even if it sounds harder to see through. The underlying principle is simple – when a person is progressing from one step to another, an intermediate step between the two steps makes it easier for the person. So do not think that having three different corrections will make tougher to see through.

People with acute presbyopia are usually prescribed trifocals. These people require vision correction at all three steps – long distance, intermediate and near/reading distance. The calculation for the intermediate correction is usually made by halving the reading addition. For example, if a presbyopia patient has -4 diopters with a +3 reading addition; the net power is -1 and hence the intermediate correction would be -2.5 diopters.

As can be understood, trifocals are different from bifocals. Bifocals only have two different corrections on each lens. Trifocals on the other hand have three different vision correction stages. So, trifocals offer more clarity in vision and less strain on the eye.

Trifocals belong to the multifocal classification on glasses.

Trifocals are similar to bifocal glasses. The only difference is that they are equipped with a three level vision correction lens on each lens instead of two level vision correction lenses.

In addition to the distance vision correction and near/reading vision correction like in bifocal lenses, trifocals have an intermediate correction field in each lens to give a better viewing experience.

The calculation for the intermediate area is simple enough. Let's say that a person has been found to have a power of -4 diopters and +3 addition for reading correction. With these numbers, we know that the net power is -1. The rule for finding out the intermediate is halving the reading addition. So with the given numbers, it would be -2.5 diopters.

The trifocals have the usual semi-circular area with the reading addition like bifocals. There is also the area for seeing things in the distance. And in addition to these two, there is a narrow band of intermediate correction viewing area in the middle of the lens for seeing things that aren't very far but are further than the reading distance. This is immensely useful for many people. For those who need this area more, like those who are constantly working on computers, it can be made bigger on demand.

People who suffer from acute presbyopia are usually prescribed trifocals. The term has been derived from Latin and it means ‘ageing eyes'. This means that has the body ages, the eyes lose their capacity to adjust and focus on different things. This usually happens after a person reaches 40. Usually this can be countered with corrective glasses, refractive surgery or contact lenses.

But sometimes these measures are not enough and the person develops a condition where all three stages of vision need corrective glasses. So instead of using three different pairs of glasses all the time, people go for trifocals.

Now there are progressive lenses that have no lines to distinguish them from the usual lenses. These are called PALs (progressive addition lenses), progressive power lenses, graduated lenses, no-line bifocals and varifocal lenses.