Owner`s manual

3.3. ALTERNATIVE TESTING OF VIGNETTING 19
FIGURE 3.2: Results of M42 adapter tests (2)
1. Set the zoom to the 35mm focal length.
2. Set the exposure mode to M (manual), the shutter speed to “bulb”, and the aperture to the maximum
(f4 at 35 mm focal length). As the maximum aperture varies with focal length on the 24–105 zoom,
remember to set the aperture AFTER setting the focal length.
3. Remove lens cap, lens hood, and filter.
4. Open the camera back and fire the shutter, holding the shutter open (a cable release helps here).
5. Point the camera at a bright wall or the sky and look through the back of the camera with one eye,
about 12–18 inches from the film gate. You will see the rectangular film gate, the black baffling in
the mirror box, and a bright circle (the rear element of the lens).
6. If you are holding the camera horizontally in the normal position and rotate the camera about a
vertical axis, your line of sight through the lens appears to move horizontally towards one edge of
the film gate. However, we actually want to check for vignetting at the corners of the frame, so
instead rotate the camera about an axis displaced somewhat from the vertical, such that your line of
sight through the lens appears to move diagonally from the centre of the film gate towards a corner of
the film gate. It does not matter which corner, but I will assume the lower left corner in the following.
7. As you rotate the camera, you will see in the bright circle an area of darkness emerge from the upper
right. As you rotate the camera more, the area of darkness will move towards the lower left, and you
will be able to see that the area of darkness is an out-of-focus image of the interior baffling in the
lens. Continue rotating the camera until the area of darkness reaches the lower left corner of the film
gate. What you have just seen is the ”normal” behaviour of the lens, and is the reference that we will
use to compare the effect of the lens hood and filters.
8. Attach the lens hood and repeat step (7). What you see will be identical to step (7), i.e. the lens hood
does not obstruct the light falling on the film plane. This is what you would expect if the lens hood
is correctly designed.