Photography
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PHOTOGRAPY
Introduction
Photography involves producing a two-dimensional image that is a reproduction of a three-dimensional scene. A camera is designed to expose film to light in a controlled way. Although the field of photography is much too complex to cover in a single experiment, this experiment will highlight some essential elements of digital photography.
Theory
The camera is designed to control the light that strikes the film to form the image. It consists of the following parts: (1) a lens system to focus the light and form a sharp image; (2) a variable aperture to control the size of the opening through which the light passes; (3) a shutter to control the amount of time during which light enters the camera; and (4) a CCD (charge coupled device) to detect and record the image.
The digital camera focuses automatically by changing the distance between the lens and the CCD, so we will focus our attention on the aperture and shutter speed. The two must be balanced in order to achieve an appropriate image exposure.
The size of the aperture is described by the fnumber of the aperture. The fnumber is defined as the ratio of the focal length of the lens to the diameter of the iris diaphragm opening. Since the focal length of the lens commonly remains fixed, increasing the size of the lens opening causes a decrease of the fnumber. Since the amount of light entering the camera depends on the area of the opening, the intensity of the image is inversely proportional to the f-number squared.
The amount of light received at the CCD is also proportional to the time the shutter is open. This is often called the shutter speed. A faster shutter speed corresponds to a shorter time. If the fnumber is increased, the light intensity on the film would be decreased, unless the exposure time is increased to compensate. Increasing the f-number by a factor of two requires increasing the exposure time by a factor of 4 to produce the same exposure.
Most digital cameras have an automatic setting, which determines appropriate aperture and speed settings based on the level of ambient light. Since we want to explore the effects of the settings, we will not use the automatic setting. Instead we will use the PAS setting, which allows you to manually set some of the properties. This manual setting is especially important if you are concerned with the depth of field of your photograph or if you would like to photograph a moving object without blurring.
When photographing a moving object, the image will be blurred if the object moves even a small amount during the exposure time. To avoid this, a very short exposure time should be used for photographing moving subjects. When the exposure time is short, a wide aperture will likely be needed to compensate, especially in low light.
Using a wide aperture has consequences, too. It produces a smaller depth of field than more narrow apertures (high f-numbers). "Depth of field" is the range of distance in the field being photographed in which the objects are in sharp focus. For example, under some circumstances objects 10 feet from the camera can be in sharp focus while those 3 feet from the camera are blurred. In general, the larger the fnumber, the wider the depth of field. A large depth of field is often a good thing, as it brings many objects at different distances into focus, but it is not always desirable. Often when photographers are taking portraits, they use a large aperture (small f-number) to produce a narrow depth of field, so that only the subject of the portrait is in clear focus.
Whenever a picture is taken, these issues should be considered: Is a large or small depth of field desirable? If so, the aperture should be chosen accordingly. Might objects in the field of view move or there is a possibility that the camera might move? If so, it is desirable to use the highest shutter speed practical, which is often limited by the maximum aperture available.
Procedure
Before taking any pictures, review the PH 202 lenses lab setup. The operation of the lens in a camera is very much like the part of that experiment in which a converging lens is used to make a sharp image on a screen which is very near the lens. In this case, is the image enlarged or diminished? Upright or inverted?
Turn on the digital camera, turning the dial to PAS mode. In this experiment, we will not use the flash, so press the flash button on top of the camera (the button with the lightening bolt icon) until the view screen reads “flash off. “
If no view appears on the screen on the back of the camera, press in the joystick in the middle of the dial. You should now be able to view the field which you are photographing. Also note the items across the bottom of the screen. On the left is a letter, which indicates which properties of the exposure can be set manually. Use the joystick to scroll through and select “S” for speed. This allows you to set the exposure speed. Now use the joystick to select the third item. This number is the time the aperture is open in seconds. The joystick will allow you to scroll through different values of exposure time, all the way up to 4” or 4 seconds. Notice that, under the S-setting, you do not have access to the second item. This is the f-number, which the camera will set automatically to give good exposure. If need be, you can change the value on the right to increase the exposure (+ numbers) or decrease the exposure (- numbers). The camera then will automatically change the f-number to accommodate this.
Throughout this experiment, be sure to record the aperture setting (fnumber) and shutter speed used for each picture.
Part I — Camera Movement
In the first part of the experiment, you will study the effect of camera movement on the ability to resolve fine detail in a photograph. Set up the camera on the tripod approximately 4.5feet from the Edmund Resolving Power Chart on the wall. This chart is designed to test camera lenses. It has a large number of patterns with varying line frequency. The camera should be at such a distance that when the chart is viewed through the viewfinder it fills the viewing area. Set the shutter speed at 1/30th of a second and adjust the exposure adjustment to 0.0, indicating a normal exposure. Record the aperture f-number. Click the shutter to make the exposure.
Remove the camera from the tripod and hold it in your hand at the same distance from the chart. Using the same aperture setting and shutter speed, take another picture of the chart. This second exposure will measure how steadily you were able to hold the camera.
Part II — Picture Quality
Another very important parameter in digital photography is the quality of the digital photograph made. Press the menu button, and scroll down to the “picture quality” menu. It should read “Better” currently. Select this menu by pushing in on the joystick. Scroll and select “Best” quality. Take another picture of the chart, with the camera on the tripod, with the same speed and aperture settings as in part I. Then return the quality setting back to “Better.”
Part III — Depth of Field
For this part of the experiment it will be necessary to position the camera to take pictures of the series of numbers that are mounted on the long wall. The camera should be mounted on the tripod. Center the camera on number 5, but be sure as many numbers as possible are in the field of view. Take photographs of the numbers with the widest aperture, the narrowest aperture, and one size in between. To set the aperture, use the “A” setting, instead of the “S” setting. Now you can select the aperture, and the camera will automatically set the speed to obtain a good exposure. Try to avoid shutter speeds slower than 1/30 second or faster than 1/250 second. The exposure adjustment should be the same for all three exposures.
Part IV — Subject Movement
For the final part of the experiment, take three pictures of moving people and/or objects. Use the tripod, and same thee aperture/speed settings used in part III.
Download your pictures to the computer using the camera port, and analyze them for your report.
Analysis
Part I
Examine the images of the Edmund Resolving Power Chart, zooming in as needed. Look for the finest lines (greatest line frequency) in which the three lines in the group are distinguished as separate lines. In which of the two images of the chart are the finest lines resolved? Which of these images was the one made by holding the camera in your hand? Why is the resolving power better or worse in this picture?
Part II
Examine the image made on the “Best” setting, and compare its resolving power to that of the tripod image from Part I. Zoom in on each image until you begin to see graininess or individual pixels. At what level of zoom does this pixilation become obvious in each image?
In what ways is “Best” quality better than “Better?” Under what circumstances might this quality difference be important?
Compare the size (in kB) of the “Better” and “Best” image files. Why wouldn’t you always use “Best” quality for your photos?
Part III
Examine the images of the series of numbers on the long wall. In which numbers can you see the individual shapes that make up the number? Why are some numbers in focus while others are not? Which aperture value(s) gave the best depth of field?
The light entering the camera is proportional to the exposure time and inversely proportional to the f-number squared, if the ambient lighting doesn’t change. Divide the exposure time by the f-number squared for each of the three images. Is the value approximately the same for all? Why or why not?
Part IV
Examine the images of moving subjects. Are the subjects blurred in any of the images? Which shutter speeds gave the least blurring? Why? Was it difficult to get a fully exposed image without blurring? Under what circumstances might it be impossible to get good exposure without blurring?
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PHOTOGRAPY