Do you have the acro "red bugs" or flat worms
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Why can't you make things simple and always shoot at 1/500 or even faster? Because those faster shutter speeds don't always give you time to collect enough light, and the picture turns out too dark. You can compensate by opening the aperture wider (which lets in more light but restricts the depth of focus), or increasing the ISO (which effectively lets in more light but makes the picture grainy). The photographer has to balance the combination of shutter speed, aperture, and ISO to create an appealing photo where the subject is sharply focused and not too dark or too light.
The aperture setting controls the depth of field, or the amount of your picture that can be in focus if everything else is correct. Large diameter apertures let in a lot of light, but the background will be out of focus. You may want the background out of focus to make a fish stand out nicely from a cluttered background, but too large an aperture setting and much of the fish will be out of focus. The bottom line is that you have to have at least one of the speed/aperture/ISO trio be in the "Provides more light" column to avoid a crummy picture.f/1.8, 1/80sec. The large aperture lets in more light, but only a small range of distance is in focus. Everything outside of this narrow band is blurredf/10, 1/60sec. The smaller f/10 aperture limits incoming light, but more of the picture can be in focus.Here's some example pictures and how I prepared to take them. My copperband butterfly is constantly moving, so I needed a relatively fast shutter speed to keep her from being blurred. I set the camera to shutter priority, speed 1/160sec, which locks in the shutter speed and lets the camera decide on how to adjust aperture and ISO. I knew it was ok for the camera to pick a larger aperture, because I wanted the background to be blurry to make the fish stand out more. Plus I was taking a profile shot, so the whole fish was at the same distance from the lens and could all be in focus. If I had been using a point and shoot camera, I would have selected Action, Sports, or Kid mode because they typically have a fast shutter speed preselected. f/1.8, 1/160sec, ISO 200 with Nikon D40 w/ 50mm f1.8 lensAn f/1.8 is a pretty large aperture setting, and not all lenses are able to get that large. If that's the case, the camera will likely make the aperture as large as possible, and then raise the ISO (making the picture grainy) until it gets enough light to take the picture. If you run into this problem, try slowing the shutter speed a little bit and see if you can still get a sharp picture. You may be able to slow down to 1/125 or even 1/60, so that your camera doesn't have to raise the ISO. If the fish had been a blennie, goby, or something else that stays still, I could have used a much slower shutter speed and the camera would have been able to use a smaller aperture, making more of the fish in focus without raising the ISO and making the picture grainy. If your camera doesn't have Aperture priority mode, see if there's a Portrait mode available as it is likely preset to a large aperature.Taking a closeup of the gorgonian required a different strategy. Because the coral is almost completely stationary, I could use a much slower shutter speed, giving me the freedom to select a much larger aperture setting. In this case I used aperture mode and set f/22, which lets in very little light but gives a larger depth of field. This allows me to have more of the coral in focus, while still nicely blurring the background coral and not having to increase ISO much and get grainy. On a point and shoot camera you can try using macro mode, or landscape mode if you don't have macro. f/22 1/2sec ISO 400, Nikon D40 w/ 50mm f/1.8 lensDon't have a fancy digital SLR camera and a bag full of lenses you say? Fear not, because these days most point and shoot cameras give you at least some control over the camera. I've taken some very satisfying shots with my old Nikon 4500 4MP camera. Most cameras now have preset modes or "scenes" like Action/Sports, Landscape, and Portrait. You'll have even more control if your camera has Aperture priority, Shutter priority, and Macro modes.
Other important factors affecting focusNow you have a basic recipe that allows the camera to take a focused picture