Photo Exposure: In photography, exposure is the total amount of light allowed to fall on the photographic medium (photographic film or image sensor) during the process of taking a photograph. Exposure is measured in lux seconds, and can be computed from exposure value (EV) and scene luminance.
Correct exposure
The "correct" exposure for a photograph is determined by the sensitivity of the medium used. For photographic film, sensitivity is referred to as film speed and is measured on a scale published by the International Organization for Standardization (ISO). Faster film requires less exposure and has a higher ISO rating. Exposure is a combination of the length of time and the level of illumination received by the photosensitive material. Exposure time is controlled in a camera by shutter speed and the illumination level by the lens aperture. Slower shutter speeds (exposing the medium for a longer period of time) and greater lens apertures (admitting more light) produce greater exposures.
An approximately correct exposure will be obtained on a sunny day using ISO 100 film, an aperture of f/16 and a shutter speed of 1/100th of a second. This is called the sunny 16 rule: at an aperture of f/16 on a sunny day, a suitable shutter speed will be one over the film speed (or closest equivalent).
Ultimately there is no such thing as "correct exposure", as a scene can be exposed in many ways, depending on the desired effect a photographer wishes to convey.
Exposure is controlled by 5 factors:
1. Aperture
2. Shutter Speed
3. Film/ISO Speed
4. Focal length of the lens
5. Metering - how your camera measures light
Photograph: This is the end result of your photo exposure. With slides, it's the picture you see in the slide viewer or projected on a screen. This is as near to how you saw the image through the camera as you'll get and is really only affected by the film brand you use (Kodak, Fuji, Agfa, etc.) as each will respond in slightly different ways to light.
if you use negative/print film, it's one step removed from your photo exposure since the lab that produces the final print uses machines that are set by human operators, at least to some degree and there's a certain amount of subjective assessment in getting the color balance right in the final photo. How many times have your photos come back from the lab with a color cast? Or looking flat, lacking vibrancy? A lot of that's down to a bad lab rather than a bad photo exposure.
If you use a digital camera, then your photograph will appear on your monitor or TV Screen. How it appears will depend on the brightness, contrast and color settings of your screen. In addition, your camera may do some in-camera processing on your image such as sharpening, adjusting the brightness and contrast and maybe trying to balance the colors before the image is finally stored on your memory card.
A Digital SLR provides an option for storing images in what's called RAW mode. This means the camera does absolutely no processing of the image before writing it to the memory card. It's then up to you to import the image into an image processing package like Adobe PhotoShop, PhotoShop Elements or Paint Shop Pro so you can manipulate the image exactly as you want.
Whatever digital image you do take, whether in RAW mode or as a preprocessed JPG, may need some tweaking before you're completely happy with it. This is where digital supercedes film. Using the image manipulation packages mentioned above, your PC becomes an electronic darkroom and gives you much more control over the final image. You'd need a darkroom, enlarger and chemicals to do that with film-based photos. You could of course invest in a film scanner to convert your film images into digital scans. But it's one more expense.
If you want to print out your digital image, you need a color inkjet printer. To get the best results, you need to calibrate it with your monitor so the colors you see on it (and are happy with) are the one that get printed out. Alternatively, you can hand your digital images over to a color lab and have them print them out for you. Prices these days are very reasonable.
That final image, the one you're happy with is the photograph. It may be very close to the photo exposure you originally made or it may differ substantially because you tweaked brightness, contrast or color or the lab did a lousy job of printing from your negatives.
The photograph is the end result.
Aperture & Shutter Speed
Aperture and shutter speed work together. Aperture is measured in f-stops and shutter speed in seconds or fractions of a second. Because both are calibrated so that each setting changes by a factor of two (newer digital SLRs break that rule) they are, to some extent, interchangeable in controlling the amount of light that makes up the photo exposure.
The amount of light entering the camera with a photo exposure setting of f/5.6 at 1/250 sec is the same as f/8 at 1/125 sec; i.e. half the shutter speed and double the aperture. Make changes in one setting and alter the other correspondingly and, all things being equal, the photo exposure will be the same.
Shutter speed and aperture have quite different pictorial effects. Aperture affects the depth of field - how much of what you see in the viewfinder will be in focus; shutter speed determines whether movement is blurred (slow shutter speed) or frozen (fast shutter speed).
Which one you give priority to depends on how you see your subject and what you want to capture in your photo. SLR cameras frequently provide program modes that prioritise aperture or shutter speed; e.g. in aperture priority mode, you set the aperture to a value for the photo you want to take and the camera adjusts the shutter speed accordingly.
ISO Speed
Film/ISO speed determines how fast the film or CCD sensor reacts to light. The slower the speed, the slower the reaction to light so longer shutter speeds are required which, in turn, will affect the apertures you can use.
The ISO speed is fixed for film-based cameras. Those silver squares you see on the film casing tell the camera what the film's ISO speed is.
On digital cameras, the ISO speed determines how fast the sensor responds to light. This setting can be changed to any ISO setting the camera provides, so you have much more freedom in the pictures you can take. You could take one photo exposure at ISO 50 in good daylight conditions, the next one might be at ISO 1600 to bring out detail at night and so on.
Both film and CCD sesors suffer from grain. Both have fine granularity at low ISO settings and both have noticable grain at high ISO settings. So there is a trade-off between the amount of grain you want in your final image and the ISO speed that's called for in any given situation.
Focal Length
Lenses come in different focal lengths ranging from 16mm fisheye lenses through 28mm and 35mm wide angle lenses to the standard 50mm, and 135mm and 500mm telephoto lenses. And then you have the zoom lenses that traverse various focal lengths such as 28-80mm, 75-210mm and so on. Adorama have a huge selection of lenses and accessories, so you should find something there to suit your camera.
The actual definition of focal length is a bit technical. It's defined as the distance in mm from the optical center of the lens to the focal point, which is located on the sensor or film if the image is "in focus". The camera lens projects part of the scene onto the film or sensor. The field of view (FOV) is determined by the angle of view from the lens out to the scene and can be measured horizontally or vertically. Larger sensors or films have wider FOVs and can capture more of the scene. In other words, the longer the focal length, the closer objects appear through it and the smaller the area of the scene you can see (because you're zooming in on a smaller part of it).
Focal length affects the depth of field in your pictures which in turn affects the shutter speeds you can use, all of which are governed by the ISO speed you're using.
Metering
Your photo exposure is the key to successful landscape shots. You'll find a guide to the main metering options offered by SLR and creative compact cameras here.
One thing to beware of is burned out skies or dull featureless foregrounds. These occur when there's a large difference in brightness between the sky and the foreground. In such cases, you're better off using a graduated-filter to bring the brightess difference within the range that the film or digital sensor can record. Your first step is to determine the strength of filter you'll need. The simplest method is to take a meter reading with the ground filling the whole of the frame without the filter in place. Now repeat this step but this time with the sky filling the frame. The difference between these two readings will indicate the strength of graduated filter that you need. A 1-stop difference in the readings will need a 0.3 ND graduated filter, a 2-stop difference a 0.6 ND grad, while a 3-stop difference will require a 0.9 ND grad.
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