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SLR
Viewfinder Optics
by
Peter Rowlands and Peter Scoones
Reproduced
from in focus 52 (August
1994)
The
old saying that 'The Good Lord giveth and taketh away'
was probably inspired by designers of SLR viewfinder optics,
for, in trying to provide the best viewing image underwater,
certain compromises, both optically and physically, have to
be accepted.
The
problem is caused by our eyes being further away from the
viewfinder than would be the case on land. The thickness of
the housing plus the distance from your mask to your eye increases
the viewing distance by at least 1 ' with a compact housing
and a low volume mask. Use a larger housing and a bigger mask
and the viewing distance could increase to up to T'. Underwater,
the small amount of water between your mask and the camera
will give a useful slight enlargement. So for absolute certainty
you would be advised to try the viewfinder underwater to establish
its true performance.
Some
top of the range SLR camera such as the Nikon F, F2, F3, and
F4, Canon FI and Pentax LX, have optional viewfinders, known
as Action or Sports finders. These are much larger prisms
which replace the standard viewfinder and allow the full frame
to be viewed from up to T' away. This offers the best possible
viewing underwater, but at a price (of both the camera and
the viewfinder) and increased bulk (of both carnera system
and housing).
Until
the introduction of the Nikon F801, most standard SLR viewfinders
in housings resulted in the photographer not being able to
see the whole frame. The advantage of the F801 is that it
has a 'high eypoinC viewfinder prism, which allows the
whole frame to be viewed from up to 19min away on land. This,
when combined with a housing whose viewfinder port is touching
the camera viewfinder, results in virtually full frame viewing
of the fully magnified image. A quick roll of the eye or slight
movement of the head will let you scan the whole image area
including the viewfinder readout. The high-eyepoint design
gives the largest practical viewing image and represents a
design which provides optimum viewfinder performance.
'The
good Lord ' comes in where housing design results in
increased viewing distance and the need for additional optical
devices, which allow the full frame to be viewed. In providing
full frame viewing, the apparent image in the viewfinder has
to be placed further away from the eye. This results in a
significant reduction in image size (not often pointed out
by manufacturers of such optical devices) and a slight loss
of light transmission. This loss of light increases as the
image size increases, hence there is a trade off between size
and brightness. The image size reduction will make manual
focusing more difficult, but if you use autofcwus then of
cousre focussing is no problem. The reduction in image size
can also cause shortsighted people problems when trying to
read the viewfinder information panel.
So
there you have it. Action/Sports finders provide a no-compromise
optical solution; high eye point viewfinders represent the
most efficient design which, in well designed housings, offer
the fullest image size but without the corners being visible;
and viewfinder enhancers provide full frame viewing with the
corners visible but with a reduction of image size.
The
best way to choose the system you require is to look through
each type and see which suits you best. |
