If you are fortunate enough to be able to go to Istanbul as a photographer, you will be totally exhausted running around the city all day long with your camera. Whether your forte is architecture or landscape, there will be innumerable "great shots" to capture.
One fact that visitors may not realize right away is that the city of Istanbul actually sits on two different continents; Europe and Asia. You will be getting photographic shots of the ancient part of the city as well as a very modern section.
If you speak with anyone who has visited Turkey and Istanbul in the past, it is very likely that one of the first things that will come up as a place to visit first with your equipment is the Hagia Sophia. This is also known as Aya Sofya, and the church in Byzantine architecture will provide some stunning pictures. The building itself is currently a museum, but it served as a mosque for five centuries in the past. The structure was first built in the 6th century!
When you are finished with your shoots of the Hagia Sophia, turn around and look across the street. The beautiful building you see is the Blue Mosque, a.k.a. Sultan Ahmet Camii. The building is from the early 1600s and the six minarets make it stand out as one of Istanbul's most gorgeous buildings. This mosque is still operational, so please keep that in mind and use the proper code of behavior.
Another building that is now a museum is the Topkapi Palace. Ottoman sultans made this place their home for hundreds of years and it is another striking piece of architecture. These are just a few of the mosques and ancient sites found around the city, so you will not be running out of places to use as photo subjects.
In addition to snapping pictures at the Grand Bazaar (called the Kapali Carsisi), you may want to do some shopping here, too. There are numerous bazaars like this all over Turkey, and the things the vendors sell are often beautiful. This would include (of course!) Turkish rugs as well as jewelry, etc. The sellers will perhaps follow you in order to make a sale, however, so be prepared. And don't be afraid to haggle the prices, they usually expect it.
For another historical photo shoot, try Sultan Mehmet's "Rumeli Fortress." This place is known for its remarkable acoustics, so if you catch a play or concert here on a video camera, it should be great footage.
Another great place for pictures is Princes' Island. You will need to take a ferry to the island and you will be moving around by foot because no vehicles are allowed. Be sure to shoot a photo of Istanbul's skyline from the island. This is to where princes who were exiled centuries ago were sent to spend their days. If you don't want to spend all of your time walking, horse and carriage rides are offered.
There isn't very much of the ancient Hippodrome left, and you won't be able to see the chariot races that were held there in the 3rd century, but it is still a spot that you will want to shoot some pictures while you are in Istanbul. In its heyday, the Hippodrome was able to hold 100,000 spectators.