It isn't the same for all spiders. Some - like the jumping spiders - have extremely acute vision with front-facing primary eyes for images and auxillary side eyes to detect motion. Other spiders - such as tarantulas and many web weavers - are nearly blind, as they detect prey mainly through vibrations and do not require refined vision.

Mechanically, spider eyes are not like compound eyes of insects that transmit hundreds to thousands of micro-images to an immobile inner eye. They are made of single immobile lenses, with mobile retinas behind them that can shift focus to separate lens ports. This is readily apparent in certain species of spider with partially translucent exoskeletons and eye lenses, such as the magnolia green jumping spider. The two front eye lenses remain immobile, but you can see a small black retina shifting behind them. Because their lenses are immobile, the spider's field of view is very narrow, so they must turn their bodies to look around. Not all spiders are capable of such focused sight - the aforementioned tarantulas basically have a teeny tiny eyeport on the top of their bodies with rudimentary eye spots that can only detect light and some motion, but can't produce a focused image.

Generally, spiders that evolved to actively hunt down prey evolved the more sophisticated, forward-facing primary eyes, and those that adopted a more passive method of obtaining prey - like spinning a web and waiting for something to get caught - did not. The diversity of body morphology among spiders is truly mind-blowing.