Physics 109, Lab 12: Digital Photography

Physics 109, Lab 12: Digital Photography


In this lab we will investigate a typical
modern point and shoot digital camera to see how it works.
The camera you will be using is Canon G7, a point and shoot camera which can be controlled
from the computer. In particular you will learn about focus,
exposure time, aperture and ISO. In automatic mode a digital camera will choose
these settings for you, but if you want to take creative photographs, you have to know
how to choose them yourself In digital cameras , the image sensor is a
charge-coupled device, or “CCD” . When the sensor is exposed to light, the charge
distribution changes. This new charge distribution is then read
off and processed into an image. Whereas in film camera, the image is formed
on a film which is a length of plastic that has been coated with certain chemicals. When
a picture is taken, the film is exposed to light in a controlled manner, using lenses,
an aperture and shutter. To begin the lab
Switch on the camera and connect to the computer. Then open up the zoombrowser ex software that
you will use to control the camera. Click on acquire and camera settings and then connect
to camera. then select the remote shooting tab and click “start remote shooting”.
select the folder where to save the images. The camera is now connected and you can set
the camera settings. Set the ISO to 400 AE mode to manual, exposure time which is referred
to as Tv in zoombrowser to 1/50 and aperture that is Av to 4.5. Then press release to grab
a picture. You can then zoom into the picture to see the pixels
us the slider on the upper left of the window to zoom in and out.
Zoom in until you can see the pixels, and out again until you can’t see the pixels
any more You can also check the image size by checking
the properties panel. When you change the focus on the camera, you
are changing the “image distance”- the distance from the lens to the image sensor– while
leaving the focal length the same. This is done by moving the lens relative to the sensor.
If the image distance is changed, but the focal length is not, this has the same effect
as changing the object distance . You can see this in the lens equation. If f is constant
then the left hand side of equation should remain the same that means if either i or
o is varied the other one has to change to get a proper image in focus. Increasing the distance between the lens and
the real image actually increases the total size of the real image.
The same basic thing happens in a camera. As the distance between the lens and the real
image increases, the light beams spread out more, forming a larger real image. Here you can see the camera changing the image
distance by moving the lens away from the sensor. in this section you will test the exposure
time, using the light-emitting diode(LED), and an audio generator. Begin by setting the
frequency of the audio generator to 10HZ. Set the exposure time to 1/10 of a second
and ISO to 100. Take pictures of the diode, with different values of exposure time and
by changing the frequency of flashes accordingly. The number of bright dots varies with different
settings of frequency and different values of exposure time. In these photographs, the
exposure time set to 1/10 of a second, the red led is on frequency 10hz, green one on
20Hz and yellow on 30 hz, that is why only one red dot is visible whereas there are two
and three green dots. The aperture of a camera is the hole through
which light has to pass in order for a picture to be made. The bigger the aperture, the more
light gets in, and the brighter is the image captured.
Here, you can observe the changing aperture by looking at the center of the lens. Next we come to ISO. ISO controls the sensitivity
of the image sensor to light. The higher the ISO number, the more sensitive the camera
is. In this photo the graininess is caused due to a high iso number. Whereas this is
a photo of the same object with lower iso number. Next we study the relation between aperture
and depth of field. First place a notebook 20 cm in front of the camera and lock the
auto focus, then set the aperture to 8 and exposure to 1/10 of a second. then change
the aperture to 2.8 and exposure to 1/80 and take another photograph. You will observe
that the photograph with small has a larger depth of field as compared to the one with
bigger aperture.

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