Exposure Basics

Camera Exposure Basics
Last Updated: 31^th May 2008
Written by: Siegfried Seierlein

This web site always enjoyed readers who are new to the digital camera, photo editing or to photography in general. We also have advanced photographers that are part of our community and we are very thank full for their valuable contribution to this site.

That said, many articles to date has been written to discuss basic aspects of photography or in some cases more advanced subjects like dynamic range. One thing we never did was to write about the most basic and the most important aspect namely, how does it happen inside the camera.

In this article I like to go back to the basics in terms of exposure. What is exposure and why is it so important? If I have to give a short definition applicable to this article then, Exposure can be described as follows.

The picture in front of the camera is for a short moment reflected onto the camera sensor when the shutter is pressed all the way. We as photographers need to ensure that just the correct amount of light (picture) reach the image sensor. If to little light reach the sensor then the picture is under exposed and if to much light reach the sensor then the picture is over exposed.

In the picture below one can see the image sensor and the shutter mechanism. The iris mechanism (The lens pupil) is build into the lens and is placed between the front of the lens and the shutter.

If you carefully study the picture below, then you will see that there are only 3 variables that can influence the amount of light or exposure that will reach the image sensor. These variables are:-

Camera ISO (ISO determines how sensitive the sensor is)
Shutter Speed (Determines the time the shutter stays open - shutter speed)
Aperture (Determine how wide the Iris mechanism open - aperture setting)


	If the iris is open and if the shutter is open then picture signal will go through the iris and the shutter and it will expose the sensor for a set (shutter Speed) time.

On the picture to the left you can see how the iris forms a round circle and how it allows the light to go through.

If the iris had a smaller opening then less light will “expose” the image sensor

Lets consider what will happen when the shutter is pressed all the way. First you need to keep in mind that the iris has already been opened before the shutter is pressed all the way. The amount by which the iris is opened is determined by the f-stop.(f2,8 / f3,6 / f8,0 are typical f-stop numbers)

The moment the shutter button is pressed all the way the following will happen. With a SLR camera first the mirror will move out the way to open the image path from the front of the camera lens to the sensor. A compact camera like the G7 does not have a mirror and the G7 will immediately go to the next step.

Next the shutter will briefly open to let through the light (image) and it will close again. If the shutter speed, the iris opening and the ISO settings were all OK then the picture will be correctly exposed on the image sensor. If any of these variables were incorrectly adjusted then the image sensor will be incorrectly exposed.

I like to discuss ISO briefly with you. You can think of ISO as the normal water tap in your kitchen tub. If you open the tap water will come out. If you in your weekend house in the mountains and your son jumps in the shower while the water is running in the kitchen then the water pressure will fall. To maintain a constant flow of water you need to open the tab in the kitchen more. You can think of the ISO as the tap in the kitchen, if the light intensity decrease at night then you need to open the tap (up the ISO) to maintain a constant signal strength.

The above picture shows two different iris openings. The large diameter opening is marked f2,8 and the small diameter f22. Typically one would expect the numbering to be f2,8 for the small opening and f22 for the large opening. Its unfortunately not the case and its therefor important for you to memorize this “picture” in your mind. Small apertures have large f-stop numbers and large apertures have small f-stop numbers.

At this point we need to ask the question, how does the camera know what the outside exposure is. The answer is very simple. Build into the camera is a exposure sensor and that tells the camera exactly what the exposure value is of the picture you want to take. Typically in the camera specification you will read TTL. That says the exposure was read Through The Lens (TTL).

What system or measurement is used to determine different exposure levels. In photography there is a well developed system to determine how much exposure is reaching the image sensor and is generally referred to as stops. For example one could say I increased the exposure with one stop or I decreased the ISO with one stop. What is a full stop? In short a full stop change, will double or half the exposure reaching the image sensor.

Another way of looking at it is to have a physical look at the iris. If I have a large diameter iris opening of f2,8 and I “close” the opening to next smaller diameter of f4,0 then I will half the exposure reaching the image sensor with one full f-stop If I go back to f2,8 then I will double the exposure reaching the image sensor.

The stops indicated on your camera is not necessary in full f-stops, often a camera is marked in fractions of a stop, like half stops or quarter stops. For example the next aperture after f2,8 on most cameras is f3,2, then next f3,6 and then only f4.0. This in-between stops is to help the photographer to make fine adjustments.

A practical way to test what we have discussed is to set your camera in aperture (A) priority mode. Set the ISO to 100. Point the camera at a subject and press the shutter halfway. You will see that the camera has selected a shutter speed that will balance the exposure based on the fixed ISO and the iris opening (f-stop). Next is to change the aperture from say f2,8 to f3,6. Again point the camera to the subject and press the shutter halfway. The shutter speed will now be lower to again balance the three variables and to give a good exposure.

With this knowledge in mind you can now give manual exposure a try. Typically you will have a exposure scale on the camera screen showing you what the current exposure reading is. If the exposure reading is negative then you need to get more light onto the sensor. To do that you can open the shutter for a longer time or you can open the iris wider open or you can up the sensor sensitivity.

Well folks now that you know all about exposure, you can take a afternoon off, to go and to be creative. Some pictures you will over expose because YOU want to, others you will take into the sun and you could decide for a specific set of settings again all based on what YOU want. There is no right or wrong, I believe the better you master exposure the better you will be able to translate your creativity into beautiful pictures.

Exposing the image sensor correctly is unfortunately not all we like to do when discussing exposure. To become good and creative photographers we need to understand how each of the three, aperture (iris opening), Shutter Speed and ISO react on changes and what happens when each of these variables are adjusted.

Important for you to benefit most from this last part of the discussion is to practice the concepts over and over.

We will use the picture to the right to explain a few concepts. The view in front of the camera is the image view.

Points A, B and C represents three different poles. C is the furthest away from the camera.

Pole C represents the total depth of view. In some cases we like everything from the lens to pole C to be in focus. Then we refer to a deep, depth of view.

If we want only pole B to be well focused and the rest to blur then we refer to a shallow depth of view.

How does one achieve different depth’s or views? The aperture setting will determine the depth of view, or how much in front of the camera lens is in focus. With f22 selected typically everything will be in focus. With f2,8 only a small part, like for example one pole will be in focus. The pole on which the camera was focused on will be the one that is in focus with a large aperture (f2,8).

The disadvantage of selecting a deep depth of field is that it requires a small aperture (f22). A small aperture will only let through a small amount of light and therefore one will have to decrease the shutter speed. Decreasing the shutter speed will keep the shutter open longer and then more light will reach the image sensor. The disadvantage of lowering the shutter speed is possible camera shake. For example a slow shutter speed of 1/20 seconds will result in camera shake if you do not use a tripod. One could then decide to rather up the ISO and not to lower the shutter speed. That will make the sensor more sensitive and less light is required to expose the image sensor adequately. The disadvantage of upping the ISO is that it creates more picture noise will.

Folks and the above paragraph summarize exposure in a nutshell. To help you understand the main points I decided to list them, each with a small description.

Aperture	Build into the lens is the Iris Mechanism. Its similar to the pupil of the human eye and it controls how much light or “exposure” will pass through the lens. A small aperture is indicated with a large f-stop number and a large aperture is indicated with a small f-stop number. Aperture determines the depth of field in a picture.
Shutter Speed	Build into the camera body and in some cases the lens is the shutter mechanism. The shutter mechanism is a highly specialized part and typically allow shutter speeds from 30 seconds to 1/4000 of a second. The shutter determines how long will the sensor be exposed.
ISO	ISO determines the sensitivity of the image sensor. If one turn the sensitivity right up to say ISO 1600 then the image sensor becomes highly sensitive to light. That also causes the sensor to sometimes incorrectly react on incoming light. The result is image noise in the picture.
Depth of Field	Depth of field is a term used to describe how much of the picture in front of the camera is in focus. Aperture is directly influencing depth of field. A large aperture (f2,8) will have a shallow depth of field and a small aperture (f22) will have a deep depth of field
Image Noise	As said image noise is directly related to the ISO setting on a camera. With most SLR cameras its save to use up to ISO 800 and with most compacts ISO 400 is OK. Remember that in most cases a good software package like Neat Noise Removal Software will solve noise related problems.
Camera Shake	This is most probably the most critical aspect of all. A good picture with camera shake is a bad picture. Learn how to hold the camera correctly and determine down to what shutter speed you typically OK. If you have shaky hands then you need to stay at higher shutter speeds.

Folks the above is a very practical discussion on exposure and how it function. I can only recommend that you visit Wikipedia if you require more help or information. To have a good understanding plus a feel of the above is critical.

Siegfried