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.

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.