Exposure tells how much light is captured by the camera. More specifically exposure is a "per area" metric, thus different formats (i.e. different film or sensor size) capture different amount of light with the same exposure parameters. Exposure is a combination of three parameters:
These three elements in combination determine the amount of light that enters a camera to create an image. Of the parameters exposure time (i.e. "shutter speed") simply tells the lenght of time the film or image sensor is exposed to light, f-number tells how large the aperture of the lens is and scene luminance tells how bright the subject is, or how much light comes from the scene towards the lens.
Multiple exposure is where a single frame is exposed more than once, the total exposure is the sum of all the exposures for that frame.
There is no single correct exposure for a specific photograph - however for maximum technical image quality, to capture the least noisy image, one should capture as much light as possible - have as large exposure as possible without overexposure. A picture taken at f/22 at 1/4 seconds allows the same amount of light to hit the film or sensor as a picture taken at f/2.8 at 1/250 second. Of course, not many people can hand hold a camera at 1/4 of a second and get a sharp image and you might not have enough depth of field at f/2.8; therefore, every exposure is a compromise. The photographer must decide how much depth of field they need, while also thinking about how much of the action they want to freeze (or blur), what focal length lens they are using, can they use some sort of support, etc.
To summarize these aspects of exposure:
- Camera support: the longer the exposure (the slower the shutter speed), the greater the chance of blurring due to camera shake. Unless the camera is attached to a sturdy tripod, a higher shutter speed should be used.
- The same is true if one is trying to photograph fast action.
- The same is true when using long lenses: the longer the lens, the higher the shutter speed should be.
- Depth of field: to increase depth of field (to get parts of the scene at greater distances in focus), a smaller aperture (higher f-stop number) should be used.
- To decrease depth of field (focus only on subject and let everything else go out of focus), a large aperture (lower f-stop number) should be used.
- To reduce influence of noise one should maximise exposure.
Over- and underexposure[]
Over- and underexposure are unwanted effects in exposure over either the whole frame, or parts of it. Overexposure happens when too much light is captured by part of the sensor or film, destroying some or all details from that area. Underexposure is the opposite - capturing so little light that details in the relevant part can not be separated from the noise in the exposure. Over- and underexposure are errors in exposure. If the scene has a very difference in brightness between the shadows and highlight, the image sensor or film may not be able to capture the whole dynamic range of the scene. In such cases one will either have to sacrifice details either from highlight or shadows or from both ends, or if using digital camera, capture the scene by combining multiple exposures.
Exposure Bracketing & Compensation[]
- Exposure compensation is the practice of allowing more or less light into the camera than its light meter suggests is appropriate.
The light meter is most often tricked by strongly backlit scenes such as a bright sky behind the subject, or strong light reflecting off sand, water or snow. To compensate, many cameras allow for the exposure to be increased (or decreased) by increments of 1/3 of an f-stop, generally to a maximum of 2 full f-stops.
- Exposure bracketing is the technique of taking multiple photos at different f-stops to ensure at least one of them will be correctly exposed.
- In digital photography the different shots from bracketing can also be combined in post processing to capture a larger dynamic range of the subject.
Generally, three shots are taken: One at the camera or light meter's suggested exposure, and one at perhaps one f-stop darker and then one f-stop lighter.
High Dynamic Range (HDR) photos often use three photos with bracketed exposures to create a composite photo with enhanced detail in areas of strong shadow or light that could not all be captured in any one single exposure. Rather than modify the aperture, it's preferable to shoot the three photos with different shutter speeds so that the depth of field remains constant.
An alternative is to shoot multiple photos with the same exposure and combine them at the post processing stage. This technique is often used in astrophotography, but can also be for other forms of photography.
Exposure Modes[]
- Aperture priority, shutter priority, manual and program - what are the pros and cons of each and when would you use them?
- Average, spot, center weighted and flash metering systems
- zone system
- Sunny 16