Depth-of-Field (DOF) Preview
The depth-of-field or in other words - the distance range which is covered
by the in-focus field - is dependent on the object magnification.
Object magnification = size of the object on film compared to the real
world. e.g. magnification 1:100 = the object is 100 times bigger in the
world than on film. 1:100 would be e.g. a full body portrait (=18mm
on film -> 18mm * 100 = 1.80m for a person in the real life).
At magnification 1:100 you have a smaller DOF than at 1:1000 and a bigger
DOF than at 1:10. Stopping down to e.g. f/8 has a different effect on the
DOF - it increases but in increases by a different degree. At 1:1000 this
may cause the DOF to cover everything in the scene (foreground to inifinity
distance). However, at 1:10 you have increased you DOF by just a few centimeters
(the background is still blurred).
And this is the sense of the DOF preview button - because the effect
on the DOF is different dependent on the object magnification it offers
you the possibility to PREVIEW the effect of your selected aperture BEFORE
you actually expose the film (=hit the shutter release button).
The following slide show demonstrates the visual effect in the viewfinder
when pressing the Depth-of-Field preview button (DOF button) at different
aperture settings using a 100mm f/2.8 based on an object distance of about
5m. The lamp is the point of focus here. Select one of the shown aperture
settings and observe the effect on the background (the castle) - requires
without pressing the DOF button
after pressing the DOF button
(with the shutter speed set accordingly)
In the middle and right image you should be able to see that the castle gets
"sharper" at smaller aperture settings due to the increased depth-of-field. The
left pictures shows the viewfinder image which remains unaffected by any change
of the aperture setting because without pressing the DOF preview button the
viewfinder ALWAYS shows you the image at the widest aperture setting - the
aperture gets only stopped down via the DOF preview button or during the exposure
of the film.
If you press the DOF button at smaller apertures this has obviously also
the effect that the image gets darker because less light passes through
the aperture. The depth-of-field is not about your main focus point - it
is about the degree of fore- and background blurr.
You may wonder why you don't see any effect in your viewfinder other
than the viewfinder image gets darker. This can have a couple of reasons:
a slow speed, standard zoom - if you have a slow speed zoom the difference
in DOF is not that obvious unless your main object is very close (= the
object magnification on film is relatively high). Just play with f/5.6
and f/11 above only - there's little difference - this would be similar
with e.g. a 28-80/4-5.6 lens.
you have a low quality (=dull) viewfinder (typically a roof-mirror type
viewfinder common in base class cameras).