Written By onci on Thursday, March 18, 2010 | 11:03 AM
Understanding Exposure by Ed Cordes
I am writing this article for the photographers who visit this site who are relatively new to serious photography. Many of you are already completely knowledgeable about this subject. However, there isn’t a week that goes by that I don’t get detailed questions from friends and co-workers about how to use their camera correctly and get the proper exposure. Many of them have purchased an upper level or mid level DSLR and still shoot on “P” or “Green Square” and do not get the most out of their camera investment.
The discussion of exposure is one of the most complicated, but important aspects to successful image making. It is simply defined as the amount of light hitting the sensor of your camera. However, selecting the correct amount of light to present to your sensor is what separates the good photographers from the mediocre.
The human eye is capable of seeing a wide range of light values from dark shadows to bright highlights and still be able to discern detail. The digital sensor of your camera is vastly limited compared to your eye. The trick is to choose a compromise volume of light that balances the darkest areas of a scene with the brightest to produce a pleasing image, recording the largest amount of detail possible.
The following may seem tedious, but in practice it is easier to understand than it is to write down. There are volumes written on this subject that go into great detail on the physics and math involved in exposure. However, in practice only a few principles need to be understood.
In photography we discuss the volume of light hitting a sensor as the Exposure Value or EV. The EV is determined by the combination of shutter speed, aperture, and ISO. A specific volume of light is called a “stop”. The volume of light between stops is 100%. So, a subject exposed at EV 3 has 100% more light than that exposed at EV 2.
If you change the aperture by one full value, the volume of light hitting the sensor changes by 100%. Changing the SS by one full value also changes the light hitting the sensor by 100%. Therefore, each of these changes varies the EV by 1.
To make matters confusing, aperture values, or technically focal values, are not linear and not denoted in even integers. Your camera will list them as 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, and 22. These numbers are derived from multiples of the square root of 2. Your camera or lens may list values between these, but these numbers represent “full stops” of EV. The smaller the F number the larger the diameter of the lens diaphragm (more light); the larger the F number the smaller the lens diaphragm (less light). Don’t worry about the math proving this; just accept it and memorize it.
The shutter speeds listed by your camera in full stops are B (for unlimited exposure), various values more than 1 sec, 1 sec, ½, ¼, 1/8, 1/15, 1/30, 1/60, 1/125, 1/250/ 1/500, 1/1000, etc. Each of these values represents ½ of the time and therefore a 100% change, therefore a 1 full stop difference.
The EV is determined by setting the SS and F stop. Your camera meter will assist you in determining the correct EV. Once the EV is determined the SS and F can be varied maintaining the equivalent EV but using different values for SS or F. The camera maintains the same EV automatically if you are in any mode other than “M”.
So, if your camera meter tells you that 1/125 at f 16 is the correct EV for the scene you are photographing you can change the SS and F to maintain the same EV but achieve different photographic effects.
If you need to stop fast action you may want to change the SS to 1/1000 (less light). To maintain the same EV you will need to change the F to 5.6 (more light). As mentioned above, most cameras with modern meters and lens functions will automatically adjust these ratios for you. As you change one value the other will automatically change the correct amount unless you are shooting in manual mode.
Part 2 coming up next.
Posted by onci at 11:03 AM