Wednesday, 16 February 2011

Unit 205 Camera Settings

Why do we need and what are apertures, ISO and shutter speeds

Aperture and F Stops
This is an adjustable control that determines the width of the opening that admits light to the sensor. The wider the aperture, the more light that can reach the sensor, making it possible to take pictures in dimmer light. You can think of an aperture as the pupil of your eye. When it's bright outside, your pupils contract (and you squint), letting in less light. When it's dim, your pupils dilate.
A narrow aperture reduces the amount of light that can reach the sensor, letting you avoid overloading the imaging device in very bright light. These lens openings are used in tandem with shutter speed (the amount of time the sensor is exposed to the light) to control the exposure. Your digital camera needs a selection of lens apertures (f-stops) so that you can take pictures in a broad range of lighting conditions.
F-stops aren't absolute values; they're calculated by measuring the actual size of the lens opening as it relates to the focal length of the lens, using a complicated formula. The easiest way to visualize how f-stops work is to imagine them as the denominators of fractions. Just like 1/2 is larger than 1/4 or 1/8, f/2 is larger than f/4 or f/8. The relationship is such that as the amount of light reaching the sensor is doubled, the f-stop increases using an odd-looking series of numbers: f/2 is twice as large as f/2.8, which is twice as large as f/4, and so on through f/5.6, f/8, f/11, f/16, and f/22 (which is just about the smallest f-stop you'll encounter in the digital realm). Figure 1 shows a lens opening that's partially closed.
Lens opening, f-stop, aperture — they all refer to the doorway that light passes through to the sensor.
If you're taking photos in automatic mode, you don't need to know what f-stop you're using because the camera selects it for you automatically. Your digital camera probably displays the f-stop being used, however, either in the viewfinder or on an LCD panel and the information can be helpful. Just remember the following things:


As the f-stop gets larger (the number gets smaller), more light is admitted, the larger the f-stop (the smaller the number), the more light that is admitted (faster). An f/2 lens (small number, large f-stop) is a fast lens, whereas one with a maximum aperture of f/8 (larger number, smaller f-stop) is slow. If you need to take photos in dim light, you want to buy a camera with a fast lens.The smaller the f-stop (larger the number), the more of your image that is in sharp focus. As the f-stops get smaller (larger number), exposure time must be increased to let in the same amount of light. For example, if you take a photo at f/2 for one-half second, you need to double the exposure time to one full second if you stop down (reduce the aperture) to f/4.Typically, you'll find that among non-SLR digital cameras, the speed or maximum aperture of camera lenses is smaller than is common among 35mm film cameras, and the range of available apertures is more limited, too. For example, even an inexpensive  35mm film camera might have an f/1.9 lens (pretty fast), and serious photographers with 35mm SLRs probably own f/1.4 or faster normal and wide-angle lenses. Although zoom lenses usually have smaller maximum apertures, in the 35mm film world, f/2.8–f/3.5 are common numbers.  because the very short focal lengths of the lenses are more difficult to design with large lens openings. So don't be alarmed if your favourite digital camera has an f/4 or f/5.6 maximum aperture. You might even find that the lens is labelled f/4–f/5.6 because the effective widest opening can vary as a lens is zoomed in and out. A lens might have an f/4 opening when zoomed out to 38mm but only f/5.6 at its maximum telephoto setting of 152mm.
ISO
ISO is actually a common short name for the International Organisation for Standardization. The ISO setting on your camera is something that has carried over from film. Remember back in the ‘old days’ when you used to go and buy your rolls of film and you would buy film rated at 100, 200 or 400, maybe even 800 or 1600? Well that number refers to the film’s sensitivity to light. The higher the number, the more sensitive to light the film is. The ISO bit is from the standards for film sensitivity, and the number refers to it’s rating.
So what does sensitivity mean? Well a low sensitivity means that the film has to be exposed to light for a longer period of time than a film with a high sensitivity in order to properly expose the image. With a lower sensitivity you also get a better quality image too which is why you should always try and use the lowest sensitivity you can get away with.
You might remember buying film for a sunny holiday and the shop assistant would recommend using a film rated at 100 or 200. If, on the other hand, you were going to be taking pictures indoors, then you might be recommended a higher sensitivity like 400 or maybe 800. If you used ISO100 film and decided to take some pictures indoors, chances are you would need to use the flash, or your pictures would come out quite dark. This is because the film’s sensitivity is so low that the shutter would need to be open for a long time to let enough light in. Your camera may not have had the features to allow it to keep the shutter open for long enough, which is why you ended up with dark pictures. This was one of the problems with film. Once you’d loaded it into your camera, you were pretty much stuck with that film sensitivity for 24 or 36 shots. Bring on digital cameras and you can now change the ISO setting for each shot you take. That is one of the big advantages of digital photography.

So why do you only get choices like 100, 200, 400, 800, 1600 and maybe 3200 when it’s digital, surely you could set 154 or 958 if you wanted it? It’s only electrical currents and circuits after all, not a piece of film. Well, in theory you could choose any setting you wanted, but imagine how tricky that would be. There are three settings which combine to give you the exposure, these are Aperture, Shutter Speed and ISO. Each one can be changed individually to allow you to set then to what you think will give you the perfect exposure, or you can let the camera set them for you to what it thinks is the perfect exposure for the conditions it can detect. Already with three different options, each having several settings themselves, the combinations are numerous, so keeping ISO to set values, which people will understand makes it a little less confusing.

Now, I mentioned quality too, and that better quality images are achieved with a lower ISO number. If, again, you go back to film days you may remember the sort of grainy effect some images had. Well this grain effect is what is introduced with a higher sensitivity film. Digital has it’s own grain effect with higher sensitivity and is known as Noise. Digital noise can be seen a sort of speckley effect in areas of similar colour, like skies or dark shadow areas. It is a subject of much discussion and the camera is often judged on the amount of noise it produces at these higher sensitivities. This is why you should always try and keep your ISO set to the lowest number, and use aperture and shutter speed to get the right exposure. If you can’t do that with aperture or shutter speed, move up to the next ISO setting and try again.
Why is a high ISO setting needed? Well for indoor work, where flash isn’t allowed and the light levels are fairly low. Or you can use it deliberately to get the grainy gritty feel to the image.


Shutter Speed
Shutter speed is a setting on your camera which controls the length of time the shutter is open, allowing light through the lens to the sensor inside your camera. Shutter speeds can go from very small fractions of a second, to several seconds long on most cameras. So why would you want to change it?
On a very bright day when there is a lot of light, if you allow the shutter to be open for too long then too much light will get to the sensor. When this happens you end up with pictures that are over exposed.
Let’s say, for a simplified example, that to get a perfectly exposed image on a bright sunny day, ignoring all the other camera settings, that you need the shutter to open for half a second. This half a second allows just the right amount of light through to the sensor to get a well exposed imaged.
Now, as the day goes by and you get to the evening, there isn’t as much light about. So if you took a picture and your shutter speed was still set at half a second you would end up with an under exposed image. This is because not enough light got through to the cameras sensor in that half a second. So in order to compensate against lower levels of light, you would need to keep the shutter open for longer.
This may seem straight forward enough, but the longer the shutter is open, the more chance there is of ending up with a blurred image. The slightest of movements while the shutter is open will register as a blurred effect. Sometimes this can be the desired effect, but most of the time you want a sharp image. Using a tripod, sitting the camera on a solid object like a wall or the floor or holding the camera against a solid object like a big tree or wall can help reduce the chances of getting blurry images.
Most digital cameras will have a fully automatic setting where it decides what settings are best, so all you have to worry about is pointing the camera in the right direction and pressing the button. This may be the mode you use all the time, but it’s well worth experimenting with these settings yourself to see what effect they have. Once you start to understand these settings and what they can do to your image you will open up a whole new range of photographic opportunities and much more creative and pleasing photos.