Building on the basics of Shutter, Aperture and ISO to control Exposure.
Welcome to the Brown Season!
Spring has sprung, but the grass is not quite green yet and the trees haven't started leafing. Things are just kind of blah if you are an outdoor photographer so let take a look at some of the items that can be confusing in regards to getting a good exposure.
What is Exposure? When is a cake done?
A cake is done when it reaches a particular internal temperature. What affects that internal temperature? Oven temperature, time in oven and thickness of the cake. For a photograph that is Aperture, Shutter time, and ISO.
The images below show a proper exposure followed by an under exposed image and an over exposed image.
The complete Guide to Understanding Exposure:
Most of the controls in your camera are about controlling Shutter speed, Aperture and ISO.
Shutter Speed is the time that the shutter (think curtains) remains open. The longer the curtains remain open the more light that will enter the camera and affect the sensor and visa-verse. This control lives in the camera body. This value is most of the time expressed in a fraction of a second, like 1/200th and it can also be longer than one second.
Aperture is the size of the opening in the lens. The larger the opening of the aperture (lower value number), the more light that will enter the camera and affect the sensor and vice versa. This control lives in the lens and is expressed in the ratio of the focal length vs. the aperture diameter, like f/3.5.
ISO is the measure of sensitivity of the sensor. The more sensitive the sensor (larger ISO number) the less light the sensor needs to capture the exposure.
There are pros and cons about the choices that you make, but the end result is that you want a good exposure of your subject. Exposure is the right amount of light to capture the desired image. Too much light and your image is ‘over exposed’ and too white. Not enough light and your image is ‘under exposed’ and too dark.
Lets look at the Tv – Av – ISO seesaw.
If you make the aperture smaller and don’t make any other changes, the see-saw will be out of balance and will get an under exposed shot.
To correct this situation you can either make the shutter speed longer… Or you can increase the ISO sensitivity to balance the exposure.
The point is to keep the see-saw in balance.
Meanwhile, if instead you increase the shutter duration (smaller fraction) and don’t change any other setting, you will let too much light in to the sensor and get an over exposed image – too light.
To correct this you need to decrease the size of the aperture (larger number)…
Or decrease the ISO sensitivity.
In Auto and Semi-auto modes your camera will attempt to balance the exposure according to its metering mode. This may be OK or it may not be OK. When it’s not OK then you have to take control and set the camera in manual mode and be the boss of the shot.
