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.
posted by Kuku @ 4:16 PM
6 Comments
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6 Comments:
dude,
as u know, i have very lil knowledge on photography :) so this could be a stupid question...but neways here it goes...
U say exposure is iversely proportional to the shutter speed. I gather from this that the faster the shutter closes, the lesser is the amount of light that enters the camera. However, unlike lets say water coming in a falls, if there is increase only in the time of exposure, there is no increase in the anount of light that falls on an object....amount of light that falls depends only on the surface area that is exposed rt?
I am not sure whether lights are made of waves or particles but this is what i felt :) do let me know ur thoughts.
Ok, don't look at it as the light that is coming into the camera. The light as perceived by the camera is different from the light as perceived by the human eye. If we keeping seeing an object it doesn't make any difference to us if we see it for 1 second or 100 seconds. Whereas it is different for the camera.
Going off-topic: The way the eye sees light can be thought of as a way of sampling. The nerves in the eye send the information of the light received by the eye to the brain. The eye doesn't have any concept of shutter speed. The eye only has a concept of aperture(like when your pupils open up in dark light and like when you contract your eye in bright light). The work of the shutter is done by the 'software' in the brain. The brain adds images to make a brighter image or reduces the image intensity to make it darker - as need be. In fact you gave me a new idea for a post on metering in cameras. Thanks.
Now the camera receives the light as intensity. I cant comment on whether light is made of waves or particles in this perspective. Light is of course made of photons, i.e particles, but that wont make any sense in this discussion. So, we can look at it this way. How light receives information is in the form of progressively increasing intensity.
Eg: Consider a fire/flam(from a, say, candle). Move your hand over the flame quickly. Nothing happens - you dont feel anything. Now hover over the flame for a second. It feels hot now. Hover over it for a few seconds now. Chances are you cant bear the heat. Do the same experiment with a paper and you will see the difference it makes on a paper. First nothing happens. Then for longer duration the paper starts to get charred. Hold it over longer and it catches fire and burns.
This is how the camera sensor sees light. For very short durations it doesn't make any difference to the camera. Over longer durations, the photo forms and then progressively gets washed out(overexposed).
In fact, you can compare the features of a camera and light in the following way.
Camera = Any substance that burns.
Light = Fire
ISO sensitivity = wood(iso 80), paper(iso 200), dry grass(iso 400), cloth dipped in diesel(iso 1600)
Very well explained, Kuku. Simply put, the longer the sensor (in the camera) is 'exposed' to the light reflecting off the subject, the better it captures the detail (Hence makes semse to have longer exposures in darkness). But when there is sufficient light or too much light, use the Aperture priority mode of the camera. This allows the camera to decide the aperture size (and thus the amount of light hitting the sensor). However during daytime when the sun is really bright, under-exposing a picture can give stunning results.
simply put take small aperture RAW pictures, and then pull up the levels using photoshop...
It works well for museums, sunsets etc.
note that the picture will appear visibly dark on screen.
Thanks for the comments kuku. I agree with the end result and am aware of the difference in quality of the photo over various exposure rates. However, the candle example is a bad one dude :) the heat part is infra-red part nothing to do with light and the absorption of infra-red is very different from absorption of light in a material. I would say if we really understand how the camera captures the image when light falls on it, we will understand this better.
Thanks,
Sandy
@sandy: Hmmmm... I cant find me ever mentioning anything about the light of a candle. I was creating an analogy between light and heat. The camera sees light in the same way we feel heat. I seem to have confused you somewhere. :)
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