Getting Sharp Aquarium Photos By Understanding Exposure
In a correctly exposed image just enough light has reached the sensor of the camera. Modern digital cameras are superb at getting the exposure right. I can't even remember the last time I saw an image online that was grossly overexposed or underexposed. In overexposed images too much light has reached the sensor. They look washed out with large areas of pure white. In underexposed images not enough light has reached the sensor. They look dark with large areas of pure black. However, if a wrong exposure is subsequently corrected with an image processing software, both types of images tend to look grainy and blurry, and they lack contrast. That's why it's sometimes difficult to tell if an image was underexposed or overexposed, but if the exposure is off, it will be very obvious that something is wrong with the image, even for somebody who knows nothing about photography.
However, if digital cameras are so good at getting the exposure right, why not let the automatic do it's job and forget about the whole process? For each photo there are countless ways to achieve a correct exposure, and for the end result of the image and its sharpness, it matters a great deal not only that you get the right exposure, but also how you get it. It is impossible to take control of this when you shoot in fully automatic mode. Modes like macro, sports, portrait, and so on are a crutch to give people some control, but save them from having to understand the basics. As it is, I have never seen a camera that had a mode for 'fish' or 'aquarium', and understanding the basics about exposure is not hard. Once you do, you can put your camera into your own personal fish and aquarium mode!
There are three factors that determine exposure: shutter speed, aperture, and sensitivity of the sensor to light (ISO). Raising and lowering each of these values has certain pros and cons:
Shutter speed: This is about how long the shutter stays open, and is measured is 1/s. Short shutter speeds help to avoid camera shake, and they also freeze motion. If you want to have a sharp picture of a fast fish, you want to use a fast shutter speed, that is one where the shutter stays open for a very short time. To give an example , for freezing motion 1/1000s is better than 1/60s. Unfortunately a fast shutter speed also means you get little light on the sensor. You need to choose the other values so that you still get enough light to achieve a correct exposure.
Aperture: This is about how large the opening in your lens is, and it is measured in f numbers. A large f number like 16 means a small opening in the lens, and a small f number like 2.8 means a large opening in the lens. The smaller the opening in the lens is, the more depth of field (DOF) you get, meaning a wider area of the subject will be in focus. Only the area in focus will be shown as sharp, not blurry. With a small DOF of f2.8, only the eye of one fish might be in focus and everything else looks blurry. With f5.6 you might get the whole fish in focus, but the other fish in the frame is still blurry. Maybe with f8 both will appear sharp. Obviously you want to use a large f number to get as much as possible DOF, but a large f number means a small aperture, that is a small opening in the lens that lets only a small amount of light to the sensor. Again, you will need to choose the other values so that you still get enough light on the sensor to achieve a correct exposure.
Sensor sensitivity: This is about how sensitive your sensor reacts to incoming light, and it is measured in ISO. At ISO100 the sensor is a lot less sensitive to light than at ISO1600. At ISO1600 you need a lot less light to reach the film to achieve a correct exposure than at ISO100. Unfortunately a high sensitivity means you will get a grainier image. At ISO1600 an image will be grainier than at ISO100. If you want to avoid graininess, you must choose a low ISO, but once again you will need to choose the other values so that you achieve a correct exposure.
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.
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.
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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