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If I change the aperture for a particular photo, why don't I see a change in the brightness of the image in the viewfinder?

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You will not see a change of brightness through the viewfinder as the aperture is only stopped down when the photo is taken. This allows the auto focus sensors to focus accurately as many cannot focus below certain apertures. It also provides more light for you to compose your shot with.

Many DSLRs have a "Depth-of-Field preview" button that stops the aperture down to what it will be when the photo is taken. You will notice a difference in the light and depth of field when this button is pressed but only if the chosen aperture is both smaller than the maximum of your lens and what your focus screen lets in. This can be anywhere between F/2 (measured) and F/3.8 (reported).

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    \$\begingroup\$ You won't notice a difference in brightness or DOF when pressing the DOF preview button using a stock focusing screen with lenses faster than about f/2.2 as the focusing screen doesn't transmit light from oblique angles. \$\endgroup\$
    – Matt Grum
    Oct 26, 2012 at 7:45
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    \$\begingroup\$ Thanks @MattGrum I was not aware of that, maybe you could edit it to be more accurate. \$\endgroup\$
    – damned truths
    Oct 26, 2012 at 8:18
  • \$\begingroup\$ See also this answer to a related question: photo.stackexchange.com/a/27104/1943 \$\endgroup\$
    – mattdm
    Oct 26, 2012 at 11:02
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If your camera has an electronic viewfinder (DSLR or mirrorless) the brightness of the viewfinder is set independently of the brightness as exposed. Often these cameras have a setting buried in a menu somewhere to adjust to the screen's brightness to the actual exposure that the camera anticipates using.

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    \$\begingroup\$ Some mirrorless camera's have electronic viewfinders, but DSLRs certainly don't. They have optical viewfinders, which is part of the reason that they are still the preferred choice of professionals. \$\endgroup\$
    – damned truths
    Nov 2, 2012 at 4:41

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