Photography

3

PHOTOGRAPY

Introduction

Photography involves producing a two-dimensional image that is a reproduction of a three-dimensional scene. 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.

In this experiment we will be using a digital SLR camera. The "SLR" stands for "single lens reflex". A CD with an e-book about this particular camera is available in lab. Open the e-book to the section entitled “Meet the Camera System.” Familiarize yourself with the following parts and controls on the camera; then keep the e-book handy for reference.

On/off Switch / Shutter Release
Exposure Mode Dial / Manual Focus Collar
Camera Body Focus Mode Switch / Auto Focus/Manual Switch (on lens)
Color Monitor / USB Port
Main Command Dial / Subcommand Dial
EV +/- Select / Delete Button
LCD Data Panel / Review Button
Viewfinder / Multiselector Switch

Three important components of the camera are:

(1) a lens system to focus the light and form a sharp image; (2) a variable aperture (opening) to control the amount of light passing through the lens; (3) a shutter to control the amount of time that the aperture is open and light can pass through.

Focus

Focusing of the image can be done manually or automatically. Unless a particular focus distance is called for, you can allow the camera to auto focus. To use the camera in auto focus mode, be sure the camera body focus mode switch is set to AF (auto focus) and the auto focus/manual switch on the lens is set to M/A (manual/auto).

To manually focus, be sure the camera body focus mode switch is set to M (manual), then turn the manual focus collar (the inner collar on the lens) until the subject is in focus. Notice that as you rotate the manual focus collar, the focus distance reads out in a display near the collar (in both meters and feet). To focus at a particular distance, rotate the collar until that distance is indicated in the display.

Exposure Modes

The exposure mode dial allows for many different settings regarding the exposure of the image. One choice is Auto, in which the camera automatically chooses the exposure settings to get normal exposure. This is often used if there is nothing unusual about the photo you are taking, as it allows you to take pictures in different situations without stopping to figure out the best settings. However, as we are doing an experiment, we want to be in control of the parameters, so we will use A (aperture mode) and S (shutter speed mode) settings. Also useful are the various pre-programmed settings such as portrait mode, as well as P mode, which allows you to program another setting, and M mode, in which you manually set all settings.

F-number and A-Mode

In A-mode you can manually set the aperture or size of the opening through which light passes into the camera. This changes 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. The f-number (sometimes called the f-stop) appears in the upper right corner of the Data Panel (if it is not in view, press the +/- button). Each doubling of the fnumber decreases the light entering the camera by a factor of four, since the area through which the light passes increases with the square of the diameter.

If the fnumber is increased, the light intensity is decreased; thus the exposure time must be increased to maintain the same level of exposure in the photo. In A-mode, the camera adjusts the exposure time (or shutter speed) automatically as you adjust the aperture.

Shutter Speed and S- Mode

In S-mode, you control the shutter speed, and therefore the length of time the aperture is open. Increasing the time would increase the level of exposure of the photograph, if the f-number weren’t increased to compensate. In S-mode, the camera adjusts the aperture automatically as you adjust the exposure time.

The shutter speed appears in the upper left corner of the Data Panel. The symbol ″ next to the number in the shutter speed means seconds, so 2″ means the shutter is open for two seconds. However, if the symbol does not appear in the shutter speed, that means it is a fraction of a second. For example 60 means the shutter is open for 1/60th of a second.

Under some light conditions, it may be impossible for the camera to get the exposure desired with the aperture or shutter speed setting you have chosen. In this case, the camera displays “LO” or “HI” in place of the shutter speed or aperture readout, indicating that the exposure will be too low (dark) or too high (bright). You should try to avoid taking pictures under these circumstances, if possible.

F-number/Shutter Speed Tradeoff

"Depth of field" is the range of distances in the field of view 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, and the greater range of distances are in focus.

To get a great depth of field, however, requires a small aperture, which in turn usually requires a slower shutter speed. When the shutter is open for a longer time, subjects have a greater likelihood of moving, and that movement can blur a picture. Serious photographers consider both the depth of field they need and the possibility of subject movement before deciding on their choice of aperture and shutter speed.

Procedure

Throughout this experiment, be sure to record the aperture setting (fnumber) and shutter speed used for each picture. If you forget to record as you take the picture, you can find the information in review mode. To enter review mode, press the review button. Now pressing the right arrow scrolls you through photo information.

If a picture is taken which will not be used in the analysis, record the settings, record the reasons it won’t be used, and delete it from camera memory.

If your pictures are turning out too dark to analyze well, you can increase the exposure by holding down the +/- button and rotating the main command dial. Select a small positive numbers to increase exposure slightly. Be sure that all photos taken in a single part of this experiment are taken with the same exposure setting.

Part I — Shutter Speed and Camera Movement

In this 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. The image should be in sharp focus. Using S-mode, set the shutter speed at 1/30th of a second. Click the shutter to make the exposure. Remove the camera from the tripod and hold it in your hand at approximately the same distance from the chart. Using the same aperture setting and shutter speed, carefully take another picture of the chart. This second exposure will measure how steady you were able to hold the camera.

Take four more pictures of the chart, holding the camera in your hand — two with faster shutter speeds and two with slower speeds.

Part II — Shutter Speed and Subject Movement

In this part of the experiment, you will study the effect of subject movement on the clarity of a photograph. Take five pictures of a moving subject. Use the tripod for all five pictures, and use the same speed settings used in part I.

Part III — Aperture and Depth of Field

In the third part of the experiment, you will determine the depth of field for different aperture settings. For this part of the experiment it will be necessary to position the camera to take a picture of the series of numbers placed at varying distances from the camera, ranging from 25 cm to 8 m. The camera should be mounted on the tripod. Focus the camera at 1 m. Take a series of four pictures using A-mode. Vary the aperture from one photo to the next, with at least a factor of four between the lowest and highest f-numbers. The camera will automatically set the shutter speed, but try to avoid speeds slower than 1/15th of a second or faster than 1/500th of a second.

Upload all useful pictures to the computer using the USB cable, 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 distinguishable.

Compare the two images taken with a speed of 1/30th of a second. In which of the two images of the chart are the finest lines resolved — the one on the tripod or the one made by holding the camera in your hand? Why does the resolution depend on whether or not you use a tripod?

Now compare the three images made while holding the camera in your hand. Which has the best resolution? Which has the worst? Explain why this is the case.

Part II

Examine the images of the moving subject. 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?

Part III

Examine the images of the series of numbers. Which numbers are in focus in each image? Why are some numbers in focus while others are not? Which aperture value(s) gave the best depth of field? Which gave the least depth of field? Does this agree with expectations?

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?

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PHOTOGRAPY