Jacques Cousteau. Without him scuba diving would probably not exist, and my interest in fish might not have become as fanatical as it was.
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Your picture is grainy because of lack of light.
Most digital cameras automatically set the ISO speed to a higher number when you use the flash. The higher the ISO number, the more grey spotting on the picture. The grey spotting is commonly referred to as grain, hence the term "grainy image".
The ISO speed is to do with the amount of light that film can pick up. High ISO numbers (800, 1600, 3200 ISO) can records things even when it's dark and there isn't much light around. Low ISO numbers (64, 100, 200) are only good in brightly lit areas, such as outdoors during the middle of the day.
With film cameras you buy film with a set ISO speed, (100, 200, 400 ISO). Fashion photographers normally use 64 ISO film because it gives a really fine picture with little to no grain. However, they need lots of light otherwise the pictures are blurry. This is why you see studio lights directed at models when they are doing a photo shoot indoors.
At night time, you use a higher ISO speed because it can record things in low light, but you get more grain on the picture.
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When you use a flash on a digital camera, the camera slows the shutter speed down and increases the ISO to let as much light into the lens as possible. The drawback to this is either a grainy image or a blurry image, or both.
The blurry image is caused by a slow shutter speed, which allows more light in but if the subject moves, the picture will be blurred.
When using a flash on a camera, you want the shutter speed to be at least 1/100th second. This is the amount of time the lens will be open and how much time the light has to get onto the film or sensor in the camera. Unfortunately, digital cameras normally drop the shutter speed to 1/25, 1/50 or something lower than 1/100 when you use the flash. If the subject moves or if you move when taking the picture with a slow shutter speed, the image will be blurred. Most professionals use a tripod and cable release to keep the camera still when using slow shutter speeds. However, this doesn't work when photographing fish.
If you can set the shutter speed and ISO on the camera, it will help when using the flash. For photographing fish you want the ISO around 100 or 200 ISO, and the shutter speed at 1/100 to 1/250th.
You should also increase the light on the subject you are trying to photograph. In an aquarium, you can put a couple more lights above the tank, open the curtains and turn the room lights on. Don't do this at the same time because it will stress the fish. But you can open the curtains, wait an hour and turn the room light on, wait another hour and turn one tank light on, wait another hour and turn another tank light on. Wait another half hour or so and start taking pictures. The extra light will let the camera use a faster shutter speed and lower ISO, and you get a better picture. You still use the flash even with the extra light in the room and on the tank.