Venice is all about light: the dazzle of the midday sun on the canals, the play of watery refractions on a crumbling facade, the pinks of the Doge's Palace deepening in the sunset stretching across the lagoon, the glinting foam in the wake of the water bus at night.
Even the fog of a November morning conjures an atmosphere of mystery rather than murk. But nothing I'd experienced in Venice prepared me for what I was about to witness when I pushed open the door to San Zaccaria, a few hundred metres from the throngs in St Mark's Square.
I had already spent a few moments relishing the tranquility of the small, sunny square in front of the church, and as my eyes adjusted to the dim light inside I could make out only a handful of visitors. Venice churches tend to be a little gloomy. Many are hemmed in by other buildings and often the only windows in the nave are set high up in the clerestory. In San Zaccaria these windows are circular, and on a bright day they produce an extraordinary series of beams that pierce the gloom like searchlights.
I'd come to see a painting by Giovanni Bellini, which has hung in the nave for more than 500 years. Bellini was a master of serenity. His Madonnas seem to hover between the real and the celestial.
Paradoxically, Bellini's quiet, contemplative altarpieces were among the most valued paintings when Venice was most dominant - an aggressive trading city at the height of its economic and social power.
And the San Zaccaria altarpiece is one of his most serene.
It depicts the Virgin and child under a golden canopy and flanked by four saints while an angel plays the viol at Mary's feet. The mood is pensive, melancholic. The picture - still set in its original marble frame, which is repeated on the canvas by Bellini's painted pilasters - feels like a window into another world.
As I approached, I realised one of the sunbeams from the clerestory was falling directly on to the painting. By chance, it was focused with absolute precision on the face of the angel. I sat on the end of a pew and stared. And as the earth turned, the beam followed, scanning away from the angel, across the strings of her viol and finally on to the crimson robe of St Jerome.
It was a transcendental moment, and a vivid reminder of what we miss when a work of art is taken out of its original context. The light in churches is not the controlled and neutral constant of a museum or art gallery. It varies with the time of day, the weather, the seasons, the flickering dance of candlelight, and the way we experience the art varies with it. Colours intensify and shimmer, gold leaf glows, shadows lift and deepen.
To experience it in the raw, in the place it was originally made for ... nothing has ever come close to that miraculous moment in San Zaccaria.
Many key works by top Italian artists are still in the churches for which they were painted. They don't travel to exhibitions, but if you go to Venice you can see them in situ, often at little or no cost and nearly always in an atmosphere far removed from the busy galleries of the Accademia, say.
Here is a selection of 10 Venice churches where you can find great paintings in their original locations.
Admission is (normally) free unless stated otherwise. A Chorus Pass costs €12 (about R196) and gives free admission to 18 churches that charge, including the Frari, San Sebastiano and Santa Maria dei Miracoli, listed below.
1. SAN ZACCARIA
As well as the Bellini, there is an early Tintoretto - The Birth of John the Baptist - a Madonna and Saints attributed to Palma Vecchio, and Tiepolo's later Flight into Egypt.
2. MADONNA DELL'ORTO
The lovely red-brick, marble-trimmed Gothic facade gives on to a small square on the outer edges of the district of Cannaregio. It was Tintoretto's parish church: he is buried in one of the chapels, and several of his most important works hang here, including a Last Judgement and a Golden Calf in the choir.
In the first chapel on the left, there should be a Bellini Madonna above the altar. But it was stolen in 1993 and has never been recovered - a sad reminder of the perils faced by art outside the high security of museums.
3. THE FRARI
The main altar of the Frari, one of the biggest churches in Venice, is dominated by Titian's great Assumption, and there is a second seminal painting by him over the Pesaro altar in the nave.
But hidden away in the quiet of the sacristy is one of Bellini's greatest and most serene triptychs - a Madonna and Child of sculptural quality flanked, as in San Zaccaria, by four saints. The gilded frame may have been designed by the artist. Admission €3 (R49).
4. SAN SEBASTIANO
On the other side of the city, San Sebastiano is known to all universally as Veronese's church. He created most of the frescoes, ceiling paintings and altarpieces - a virtuoso display of visual storytelling and trompe l'oeil effects, which fill the building with life and colour. Admission: €3 (R49).
5. SAN GIORGIO MAGGIORE
One of Palladio's great Venetian churches, San Giorgio stands on its own island across the lagoon from St Mark's.
It has a fine collection of mannerist paintings, including two powerful late canvases by Tintoretto - The Last Supper and The Fall of Manna - which were made for the presbytery and have hung there for well over 400 years.
6. SAN FRANCESCO DELLA VIGNA
On the northern edge of Castello, this Franciscan church is also home to a Bellini altarpiece, Madonna and Child with Four Saints and a Donor.
This is not the church Bellini would have known - it was rebuilt a few decades after his death by Jacopo Sansovino and then by Palladio (who designed the facade) - but the work did not affect the setting of his altarpiece, hidden in a chapel next to a stunning garden cloister.
The church is also home to Veronese's first Venetian commission, and to a great series of sculpted reliefs by Pietro Lombardo.
7. SAN GIOVANNI CRISOSTOMO
San Giovanni Crisostomo is around the corner from the Rialto bridge, right next to one of the busiest tourist routes in Venice, yet hardly any of them venture through the door.
They are missing two of the city's greatest altarpieces - one of San Giovanni Crisostomo by Sebastiano del Piombo, and a late Bellini - depicting Saints Christopher, Jerome and Louis of Toulouse - in a chapel on the south side of the church.
The lighting here is remarkable, almost mystical in effect as it filters across the painting and into the chapel from the side windows.
8. BASILICA SAN MARCO
Unless you attend a service, this is one church in Venice where there is no escape from the madding crowds. But even though you will probably have to shuffle along a guided route in a line of other tourists, and the natural light is drowned out by spotlights, it's impossible not to be dazzled by the fields of gold above your head - a panorama of 8,000 square metres of glittering mosaics. The most ancient are approaching 1,000 years old.
9. SANTI GIOVANNI E PAOLO
Also known as Zanipolo and the great rival to the Frari, this spectacular Gothic church lost several masterpieces in a fire in the 19th century, but it still has works by Veronese, Vivarini and Lorenzo Lotto, and a polyptych by Bellini depicting St Vincent Ferrer, which is still in its original frame. Admission €2.50 (R41).
10. SANTA MARIA DEI MIRACOLI
This church is unique in Venice because it was built to house a particular painting - Niccolo di Pietro's Madonna of 1409 - which, it is said, miraculously started to weep in about 1480. The Lombardi brothers were commissioned to build this exquisite jewel box of a church, faced inside and out with decorative marble. Today it completely outshines the painting it was made for. Admission: €3 (R49). - The Sunday Telegraph..