Most digital cameras, while on auto-settings, use metering. Metering is when the camera takes information from the frame and averages how much light is cast on the subject, determining what kind of exposure is needed to correctly light the subject. There are typically a few types of metering: center weighted average, partial, spot, and multi zone.
"Correct exposure" is determined by the sensitivity of the medium (film) used. Sensitivity is referred to as film speed, and is measured on a scale by the ISO.
Faster film needs a shorter exposure and has a higher ISO rating.
Exposure is a combination of the length of time light is allowed to fall on the medium, and the level of illumination received. Exposure is controlled by the lens aperture (how wide the hole is/how much light is allowed in) and the shutter speed (how long the shutter is left open).
The principle of reciprocity says that if you expose a medium for a longer period of time, that a reciprocally smaller aperture is needed to acheive the same exposure. For example, if you use a lower, faster f-stop (for a shallower depth of field) you would need to change your aperture accordingly. The f-number (1.4, 5.6, 8, etc) tells you what the diameter of the entrance pupil is in terms of the effective focal length of the lens. It is the quantitative measure of lens speed.
There is a saying "f/8 and be there", meaning that being at the scene is more important than the technical details. f/8 will typically give a good depth of field, when all other settings are reasonable for the lighting. Usually, medium f-numbers (typically between f/5.6-f/8)
Another rule or saying is the "sunny 16 rule". This says that on a sunny day, using an aperture of f/16 and a shutter speed close to the reciprocal of the ISO speed will give you a clear, correctly exposed shot.
To adjust aperture, there is some math involved. Each f-stop faster (each lower f-number) is twice as fast as the f-stop before it. If you have f/5.6 with a shutter speed of 1/125 of a second, and you want to use a shutter speed of 1/500 of a second, you would need to adjust your f-stop to f/2.8. (125) x (2.2)= (500)
Using different exposures can create interesting effects on a photo!
(info via wikipedia, photos via flickr, artist noted)
This shot was captured thanks to a very quick exposure.
Thanks to banAI on Flickr.
The "movement" of the water was created by using a long exposure.
Thanks to shawnhoon on Flickr.
3 comments:
Very good ... some more research to do ...
First paragraph ... exposure determined by more than just the shutter (you say this later).
Some ideas for further research.
What are sample shutter settings?
What are some sample F-Stops? How is an F-Stop calculated?
What is a "Stop" (for shutter and for aperture).
What is the relationship between Shutter and Aperture? (Specifically based off of the settings you've talked about)
What camera settings allow you to set the aperture and shutter.
What is metering? How does the camera meter?
Good work again!
So ... if the camera meters a scene to 1/125th second shutter and F5.6. And you want a faster shutter for stopping action say 1/500th. Using the rule of reciprocity what should the aperture be set to?
Once you figure that out ... read and respond to the research of three other students. Correct them where they're wrong, suggest further research, provide links if you want ...
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