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Adobe Photoshop CS2 |
Level
3 |
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Lesson 5 |
In this lesson you will learn the
relationship between file size,
pixel dimension, image resolution
and print dimension. |
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Introduction |
In digital photography,
file size is determined by the
number of pixels in an image. This
measurement is expressed in
kilobytes (K), megabytes (MB) or
gigabytes (GB).
Pixel dimension is a measurement
that refers to the number of pixels
along the height and width of the
image. If you change the pixel
dimensions of an image, you will
change the file size.
The following lesson will show you
the results of a decrease in file
size. |
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Step #1: |
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Open a file that has a high
resolution – approximately 300 ppi.
The file opens at 33.33% zoom level. |
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Step #2: |
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From the “Toolbox”:
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Select the
[ Zoom ]
tool. |
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Step #3: |
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Position the Zoom tool over the
flowers and click three times. |
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Step #4: |
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The zoom level now changes to 100%.
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Examine the image.
The image has fine detail, sharp
edges and subtle color. |
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Step #5: |
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Go to the “Menu bar”.
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Choose Image > Image Size. |
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Step #6: |
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The “Image Size” dialog box
appears.
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From the “Pixel Dimensions”
section, we see that:
The image’s pixel dimensions are
1800 pixels x 1200 pixels.
The file size is 6.18 MB.
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From the “Document Size”
section, we see that:
The image’s print dimensions are 6”
x 4”.
The resolution is 300 ppi. |
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Step #7: |
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Click the check box marked
[Constrain Proportions]
to preserve the image’s
width-to-height ratio.
Click the check box of [Resample
Image]. This will allow us to
modify the resolution independently. |
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Step #8: |
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In this image, we want to change the
pixel dimensions to 300 pixels in
width.
So, under the “Pixel Dimensions”
section:
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In the text box labeled “Width”,
change the 1800 to 300. |
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Step #9: |
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As the pixels dimension in
“Width” changes to 300, the
value in the text box labeled
“Height” automatically changes
to 200.
The file size also automatically
changes to 175.8 K.
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Look at the “Document Size”
section:
The print dimensions of the image
automatically change to 1” x 0.667”.
The image resolution remains
unchanged. |
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Step #10: |
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If we want to change the pixel
dimensions but maintain the print
dimensions, we should:
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Skip Step #8, and
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Start from the original values. |
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Step #11: |
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Go to the “Document Size”
section:
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In the “Resolution” text box,
change 300 to 50. |
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Step #12: |
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As image resolution changes to 50
ppi:
The print dimensions remain the same
at 6” x 4”.
Look at the “Pixel Dimensions”
section:
The pixel dimensions automatically
change to 300 x 200.
The file size also automatically
changes to 175.8 K.
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Click [ OK ]
to apply the changes. |
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Regardless of whether we choose Step
#8 or Step #11, the image’s file
size and pixel dimensions decrease.
As pixel dimensions decrease, the
image’s on-screen display size is
immediately reduced. |
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Step #13: |
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From the “Toolbox”:
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Select the
[ Zoom ]
tool. |
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Step #14: |
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Position the Zoom tool over the
flowers and click five times. |
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Step #15: |
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Using the [ Hand ] tool, drag
the mouse to pan over the image so
we can see the same area as we did
in Step #4. |
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Step #16: |
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Examine the image.
As the file size decreases, the
image becomes coarse-looking with
jagged edges.
In fact, when you decrease the
number of pixels in an image, you
are deleting information from the
image. As a result, the image’s
quality decreases. |
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Before |
After |
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Conclusion |
In this lesson, you learned about
the relationships that exist among
file size, pixel dimension, image
resolution and print dimension.
We also showed you that when file
size decreases, the image’s quality
also decreases. |
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