Shutter/Aperture Priority

Some cameras have modes called shutter priority and aperture priority. These are not available in the basic point-and-shoot models but in some of the more advanced models. These modes are usually represented in the mode dial by S or Tv for shutter priority and A or Av depending on the manufacturers wish.

Shutter and Aperture priority modes are a sort of semi-automatic modes. In the fully-automatic mode, the camera will choose the shutter speed and the aperture value automatically depending on the exposure. These automatically chosen values cannot be changed or can be modified using a 'program-shift' feature that some cameras provide.

But now, in these modes, you have the option of choosing the shutter speed or the aperture value on your own and the camera will adjust the other parameter depending on the light and required exposure. For example, suppose the automatic setting requires an aperture of F/5.0 and shutter speed of 1/200s. Now, in the aperture priority mode, I set the aperture to F/2.8. Now the amount of light has increased because of the wider aperture. So correspondingly the shutter speed will be decreased to about, say 1/500s, by the camera, since it is in the aperture priority mode.

Similarly in the shutter priority mode, you are allowed to change the shutter speed value and the camera automatically chooses the best aperture value for a good exposure.

This might not sound very interesting during normal photography, but it makes for very simple shooting when shooting some creative photos.

Focal Length and Angle of View

The last time we had a look at the picture to the left, we learned how to read the aperture specification from how it is mentioned on the lens. As you would remember, there is also another set of numbers on the lens. This represents the focal length of the lens. In this case the figures say 5.8 - 17.4, the readings being in mm.

The layman can translate this directly to the amount of zoom that the camera provides in terms of magnification(2x, 3x etc). The figures indicate that the cameras can zoom from a minimum focal length of 5.8mm to a maximum focal length of 17.4mm. So the magnification that the lens provides is 17.4/5.8 = 3x. This again reflects what is mentioned in the specification of the camera.

Now that we understand the significance of the figures mentioned, we can have a look at what the numbers exactly mean. For this we have to understand what is focal length. Without being too bookish, the focal length is the distance from the lens to the focus of the lens. The focus of the lens is the single point where the light converges after refracting from the surface of the lens. In the case of a fixed lens, this focus point is constant and is determined during the manufacture of the lens and depends on the curvature of the lens. You can read more about focal length and focus from these links - Focal Length and Focus.

Now lets move away from the geometrics and optics and focus on what it means for us as a photographer. As mentioned earlier, each lens has a fixed focal length and this cannot be changed. But cameras have a range of focal lengths as mentioned in the specification. This is achieved by using a combination of lenses. By using multiple lenses, in a straight line, and varying the distance between each of them, one can achieve different focal lengths.

Now most of the focal lengths are compared using the 35mm equivalents. The 35mm film was the most popular film format available and hence all focal lengths are translated into their respective 35mm equivalents. This is because digital cameras have varying sizes of sensors and there has to be a standard for comparisons. In case of compact cameras, the 35mm equivalent focal lengths are mentioned in the cameras specifications and in the case of SLR cameras a focal length multiplier is mentioned. In the case of the W80, the 35mm equivalent focal lengths for 5.8 - 17.4mm are specified as 35mm - 105mm. What this means in other words is that the 5.8mm lens on the W80 provides the zoom equivalent to a 35mm lens on a film camera.

We will take a more detailed look at what this 35mm equivalent means in a later post.

ISO - Light Sensitivity

ISO adds another parameter to the Exposure equation. Till now we had established the relationship of exposure to aperture and shutter speed. From now on we will understand that Exposure is directly proportional to the ISO setting.

ISO is a measurement of the light sensitivity of the camera. Earlier we used to get films with various ISO ratings. A film with a higher ISO is more sensitive to light and on simple point-and-shoot cameras, a high ISO film would be more suitable to shooting in low light than a film with a lower ISO. In essence, that is what a high ISO will help you do - help you shoot in low light.

ISO might sound useful, but, on digital cameras, shooting at high ISO leads to an increase amount of noise in the photo. Noise is nothing but the graininess that creeps into images when you shoot at a high ISO. The noise happens due to various causes, the main reason being the over-heating of the sensing elements in the sensor of the camera. Putting it simply, to increase the light sensitivity of the sensor, more current is applied through the sensing elements. Because of this, the sensory elements are over heated and hence the resulting noise in the final images. Hence high ISO is not all that good for low light photography.

Now why would you need to shoot in low light using a high ISO when you could as well decrease the shutter speed and/or increase the aperture. For one, increasing the aperture is not an option beyond the limits of the lens. You can only increase the aperture up to the maximum value allowed by the lens. So the other option is to decrease the shutter speed to allow more light. This is possible depending on the situation. If you are trying to capture moving subjects(say a moving car) then you cannot keep a long shutter speed as you would end up capturing just a streak of color. Also, a longer shutter speed would give ghosting or blurry images if the photos are taken handheld(as opposed to a tripod). This is due to the induced 'shake' which results in longer exposures. Similarly, if you are capturing a night shot with lights in the picture, then you might end up getting streaks in the image as a result of the light moving around the screen because of camera shake.

In such cases, a compromise over the noise introduced has to be worked out and you would have to decide between decreasing the shutter speed or increase the ISO at the cost of higher noise.

Exposure

Loosely an exposure is defined as the process of light entering the camera and forming an image on the lens/film.

Exposure is a term that is used widely in photography. An exposure is something which indicates that light is passing into the camera through the lens. Usually exposure is quantified by an other term depending on the quality of the photograph.

For example, one of the usages of exposure is in qualifying the image. Consider the set of photos below. Assuming that the photo in the center is how we want our image to look like, you can easily make out that the photo on the left is darker than desired and the photo on the right is brighter than desired. This means that less light than necessary entered the camera to form the image in the left case. Hence the photo was not exposed correctly, in this case the photo is 'under'-exposed. Similarly in the right photo, too much light found its was into the camera and hence the photo is 'over'-exposed. Since the photo in the center is what we need, we would say that this photo is exposed correctly.


So we have now come across the different exposure qualities. Lets say we consider the under-exposed photo. Why is the photo under-exposed? It could be because the shutter speed was too fast, or the aperture was too small or a combination of both. Similarly the over-exposed image was a result of the shutter speed being too slow, or the aperture being too wide or a combination of both. From either cases we realize that exposure is a resultant of the shutter speed and aperture. Exposure is directly proportional to aperture and inversely proportional to shutter speeds. This is the mathematical definition of exposure.

Mathematically,
Exposure = Aperture / Shutter Speed.
[Note: There might be other factors affecting exposure, but this is the relation of exposure with aperture and shutter speed]

To achieve good exposure one must choose an ideal combination of aperture and shutter speed. In the point-and-shoot mode, the camera itself has an automatic control mechanism where it would choose the aperture and shutter settings for correct exposure. In little more advanced cameras there is a shift mechanism whereby you can set an increase in shutter speed and corresponding decrease in aperture or a decrease in shutter speed and corresponding increase in aperture. You might wonder where is the necessity to modify the automatically chosen shutter and aperture settings. This part we will cover in the posts on how to get creative with aperture and shutter. For now its important to understand the relation of Exposure to Aperture and Shutter-speed.

The other usage of exposure is 'long'-exposure and 'short'-exposure. Long-exposure refers to taking a photo keeping a longer shutter speed duration while short-exposure is one where the shutter speed is very high. The reasons for long and short exposures will be looked at in the posts that deal with creativity.

Speed of Lens

You might have heard from some friends about slow and fast lens. These terms result from the property of the aperture of the lens. The lenses with larger apertures are faster than lenses with smaller apertures. So that would mean that a 'f/4.6' lens is 'faster' than a 'f/8.0' lens. Let us try to understand why aperture is measured in terms of its speed.

You would have understood that the larger the maximum aperture of the lens, the more the light that enters the camera. This should be obvious by now. Also from the previous post, you would remember that the slower the shutter speed, the more the light that enters into the camera. When we take the photo of an object then the image is formed as a result of the light entering the camera. This amount of light has to be regulated and should be as accurate as possible for a good photograph. Too much or too less light and the image will not look as good as it is meant to be. Of course there are creative techniques which make use of these less-than-perfect images and we will have a look at it later.

So, amount of light is directly proportional to aperture and indirectly proportional to shutter speed. Since both apertures and shutter speeds are the property of the camera, the image that is formed depends on both the aperture setting and the shutter speed setting. Let us fix two camera/lens at the maximum aperture value and compare them. One lens is F/2.8 and the other is F/8.0. In the case of the F/2.8 lens, a lot of light is entering the camera because the aperture is wider. Now we will set the shutter speed to a desired value so that we get a good image, say 1/500s. At F/2.8 and 1/500s we get the image that we desire.

Now let us use the other lens - the F/8.0 one. Since this has a smaller aperture value than the F/2.8 lens, it would be transmitting lesser amount of light into the camera. Now since the amount of light has reduced, the image will look darker. As this is the widest aperture of the camera, we cannot increase the aperture any further to allow more light into the camera. So we are left with only one other choice(as of now) - to decrease the shutter speed. Since the shutter speed will now be open for a longer time, it will allow more light into the camera. If we decrease this setting enough then we will have the same image quality for the F/8.0 aperture as we had for the F/2.8 + 1/500s combo. This would be approximately at a shutter speed of 1/2s. So, the same image results at a slower shutter speed.

So, when taking photographs, you are limited by the aperture and hence, while you can shoot fast on a lens with a wider aperture, you will have to shoot slow using a slower shutter speed on a lens with a smaller aperture. It must be understood that the F2.8 lens can be stopped down to F8.0 since it is only a matter of making the aperture narrower. On the other hand the slower lenses are limited by the smaller maximum aperture and hence you cannot increase it beyond the maximum possible value to make it faster.

Do drop in your comments and do let me know if the reason for labeling a lens slow or fast is clear.

Aperture

Above photograph is the copyright of http://web.uvic.ca/ail/techniques/

The aperture of the camera is the opening of the lens through which the light enters the camera. In the camera the aperture of the lens is controlled by a diaphragm which opens wide or contracts as per the input give by the user. The aperture can be controlled either from the controls on the body of the camera or from the lens itself(as in the case of SLR lenses) using the aperture ring on the lens.

Aperture is measured as an f-number and hence you will find the cameras displaying the aperture values as f/3.6 where sometimes even the '/' is omitted and the aperture is represented as f2.8 only. This f-number is calculated as the ratio of the focal length to the
aperture diameter. It should be fairly obvious that the larger the opening of the lens, the more the light that can come in. The increases the exposure of the image - in other words the image will appear brighter if all other parameters are kept the same. We will have a look at exposure and how it affects the other parameters of the image in a later post. When the aperture is narrower then less light enters and hence the image is darker if all other parameters are kept the same.

It should also be remembered that the larger value denotes a smaller aperture. For example, the aperture opening in f/2.8 is larger than the aperture opening in f/5.6. This is easy to understand as the number measured is inversely proportional to the aperture value.

How to read the aperture specifications:
Many of the smaller cameras would have some specification numbers written on the lens. As an example I am taking the Sony W80. As you would see from the image alongside, the specification mentioned is 2.8-5.2/5.8-17.4 on the lens. The first part is the aperture which we are looking at now whereas the second pair is the focal length or zoom range available on the lens.

2.8-5.2 essentially means that the lens has a maximum aperture of f/2.8 at the wide(minimum) end of the zoom and f/5.2 at the tele(maximum) end of the zoom. In between the two ends of the zoom range the widest possible aperture gradually reduces from f/2.8 to f/5.2 as you zoom in. The feature of a lens is maximum possible aperture value and hence this is the reason why the maximum apertures are mentioned and not the minimum ones. Generally in the compact cameras the minimum aperture value is f/8.0. At this aperture the least amount of light enters the camera.

A rule of thumb when deciding on a camera is to go for a camera which has the widest possible aperture range. For example - prefer a camera with a lens that is 'rated' F/2.8 - F/3.6 over a camera with a lens that is rated F/2.8 - F/5.2. I am not referring to aperture range here, but to the widest aperture-range. In other words, the aperture should not change too much over the entire zoom range and should try to remain as wide as possible. So, in the first case it change by a value of 0.8, whereas in the second case it changes by 2.4, and hence the first lens is the better one. Hope this is clear.

Why and how the aperture is important will be covered in the next post.

Shutter Speed

Whenever an amateur photographer wishes to break out of his shell and get into some serious photography then he would have to start learning about 2 important features of the camera. The shutter and the aperture. In this post we will have a look at the shutter of the camera.

So, what is this shutter?
The shutter is something which covers the opening to the sensor of the camera. If the shutter is closed then there will be no light going to the camera and if the shutter is open then the light will reach the camera and it will be able to capture the image of the object that you are pointing the camera at. You can think of the shutter as something similar to your eye lid. When you open your eye lids, light enters and you are able to see. When you close your eye lids, no light enters and you are hence not able to see anything.

Shutter Speeds:
So, what can you do with the shutter to influence the photos that you take? You can set a parameter in the camera called shutter speed. The shutter speed dictates the duration for which the shutter will remain open hence allowing light into the sensor/film of the camera. Shutter speed is measured in seconds. If you set the shutter speed value as 1s, then that means the shutter will be open for 1 full second. If you set it to 1/500s, then that means the shutter will be open only for 1/500th of a second.

How do you think the shutter speed can influence the image formed? Think of when you were a kid and you used to burn leaves/paper using a magnifying glass. You had to focus the rays of the sun onto the paper for a good number of seconds before the paper caught fire. If you had held it for just a few seconds then it wouldn't have made a difference to the paper. If you had held it a little longer then you would have noticed that the paper start to get charred. What I am trying to say is that the intensity of light that the paper receives, is directly proportional to the amount of time for which light falls upon its surface. Similarly, if the shutter is open for a short duration then less light enters the camera, than if the shutter is open for a longer duration.

Usually shutter speeds are not mentioned anywhere on the body of the camera as a major specification. This is very much unlike the other parameters of the camera like aperture, zoom, megapixels etc. This is because not many people look for the shutter speeds when buying a camera. Usually the compact cameras have a fastest shutter speed of 1/2000s, larger(and costlier) ones have it at about 1/4000s. Semi-pro SLRs have much faster shutter speeds at 1/8000s. On the other hand the slowest shutter speeds can range from 2s in some compacts, to 8s in the majority of the compacts. Some more advanced compacts have slowest shutter speeds in the range of 30s - imagine the amount of light entering the camera at this setting. SLRs have a mode called bulb mode, where the shutter is open for an indefinite time till a pre-defined button is pressed.

We shall see how to be more creative at taking photographs using the shutter speeds in a later post. For now this is all you would need to know about shutters in cameras.