You should see a preview of what will be in acceptable focus at the
selected taking aperture. The smaller the aperture the greater the
distance both ahead of and behind the main point of focus that will also
be in good focus. Of course, the smaller the aperture the darker the
view and the greater the difficulty of seeing much of anything let alone
if it's in focus or not.
However, if the lens is set to its wide open aperture there will be no
difference in what you see by pressing the DOF preview since the
aperture will not change. Also, depth of field increases with short
focal lengths and distance from the camera. If you're using a short
focal length lens or are focused sufficiently far away from the camera
(or both) you may not be able to see any significant difference.
To get a better handle on what you should be seeing you can use your
prime lenses which are always equipped with a DOF scale. For example,
assuming you have the standard 50/1.8 lens: Note the red mark used to
indicate focusing distance. Also note that, on both sides of that red
mark are aperture marks of 4, 8 and 16. These marks indicate the DOF
range at the marked aperture when the lens is focused at the distance
indicated by the red mark. For example; using the red mark, set the
lens focus at 2 meters. Now look at the marks for f/16 to the left and
right of the red mark. They show that the DOF ranges from a nearest
focus of 1.5 meters to a farthest focus of 3 meters. Assuming you make
a print not larger than about 8x magnification (the standard on which
these scales are based), objects from 1.5 to 3 meters should appear in
good focus. Also note that the DOF is substantially reduced as you open
the aperture wider or shorten the focusing distance. If you have
shorter or longer focal length lenses you'll see that their scales are
different.
Finally, note the infinity symbol on the focusing scale. This is used
along with the DOF scale to do what is called hyperfocal focusing.
Assume the lens is set at f/8 and the focus is set at infinity with
infinity set over the red mark. There is, of course, nothing in focus
beyond infinity and the nearest distance that's in focus can be seen to
be 10 meters about the f/8 mark to the left of the red mark. Now move
the infinity symbol to the f/8 mark at the right of the red mark. If
you observe the red mark you'll see that the lens is actually focused at
10 meters. But you can see from the DOF scale that everything should
actually be in acceptable focus from about 5 meters to infinity. This
is useful for landscape work where you want infinity to be in focus but
you also want to capture as much of the foreground as possible.
When using DOF scales some folks consider the standard lens markings to
be not critical enough for big enlargements. Something that looks sharp
in a small print may not look so sharp in a larger print. If that's the
case then use the next larger aperture to set your scale. In the
hyperfocal example above I would probably actually be shooting at f/11
(for best compromise between DOF and lens sharpness) but base the DOF
using f/8 on the scale rather than f/11.
Hope that helps.
Chuck Norcutt
Nicolas Mitchell wrote:
> Help!
> I simply do not understand what I am seeing when I press the DOF
> preview button on my OM kit. Would anybody be so kind as to try an
> help me get it?
> All the very best,
> Nic
--
_________________________________________________________________
Options: http://lists.thomasclausen.net/mailman/listinfo/olympus
Archives: http://lists.thomasclausen.net/mailman/private/olympus/
Themed Olympus Photo Exhibition: http://www.tope.nl/
|