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Sharp Photos & Understanding Exposure - Part 2

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Getting Sharp Aquarium Photos By Understanding Exposure
by Frank Mueller (fmueller)

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In all three cases you want to choose values that limit the amount of light that gets to the sensor. If you are taking photos of your family on a sunny day on the beach, that's not a problem, because there is plenty enough light available to get a correct exposure at for example 1/500s, f8, and ISO100. However, in a fish tank there is usually not so much light. If you need 1/500s to stop the motion of your fish, and you need f8 to get everything in focus, you might have to crank up the ISO to 1600, or else your photo will be underexposed. Sometimes ISO1600 isn't even enough.

One way around that is to make more light available, for example by using extra lights or by using a flash unit, but then you need to take care not to create reflections. Also, with the weak built-in flash of many cameras, even with flash use at ISO1600, you might not have enough light to achieve the shutter speed you need to freeze motion, and the aperture you need to get enough DOF. In that case you either need to lower the shutter speed and put up with some motion blur, or increase the aperture and live with a little less of the image in focus.

If you shoot in full auto mode, and let your camera determine shutter speed, aperture, and sensor sensitivity, it can be hard to tell why your pictures come out looking bad, motion blur due to slow shutter speed, blurriness due to insufficient DOF, or blurriness due to too much grain! All you know is that your images are not sharp, you play around with your camera, and you probably move from one problem to the other, but your images still look blurry. That's when people start looking for a new lens, but their lens was never the problem.

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Once you get out of full auto mode, or modes called macro, sports, portrait, and so on, and you start directly controlling shutter speed, aperture, and sensor sensitivity, you will understand what is going on with your pictures, and then you can find solutions for your problems. A good way to start is switching from the green, full auto mode of many cameras to program mode (P). In P you manually choose ISO, and the camera chooses shutter speed and aperture to achieve the correct exposure. Play around with that, and you will see that without flash you will need ISO1600 or more most of the time. Even with flash ISO1600 might be necessary, and with many cameras the grain is hardly noticeable in small prints or images published on the web.

Once you understand P-mode, you can move to shutter priority (S). It lets you choose ISO and the shutter speed, but the camera will still automatically choose the right aperture to achieve a correct exposure. Alternatively, you can use aperture priority (A). It lets you choose ISO and the aperture, but the camera will still automatically choose the right shutter speed to achieve a correct exposure. Only in manual mode (M) you need to choose all three, but I rarely - if ever - I see a need to do that.

When I shoot fish, I very often use P-mode and set the camera to ISO1600. A lot of the time that gives me very pleasing results. Only if I need more DOF, I will switch to A-mode and select something like f8. In that case I usually also have to get out the tripod to avoid camera shake, because the shutter speed will be quite slow. In addition, many of my images will then suffer from motion blur, but I can look through them and select those where there is none or very little, because the fish just happened to stand still during the time when the shutter was open.

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Those are my fish and aquarium modes. Depending on your own tanks, fish, lighting, and personal preferences, you can now tailor your own set of values for shutter speed, aperture, and film sensitivity. They will help you to get images that are not only correctly exposed, but also look sharp.
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