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Focusing
Super 8 Cameras
The 'diopter' bug-bear!
What I hope to do here is explain why there is a certain difficulty
with focusing super 8 cameras, then tell you how to avoid some
of them. First and foremost is the issue of the viewfinder 'diopter'.
If you don't want to know why this is a problem, just how to fix
it, then skip this explanation and go to steps 1 - 4 below.
Super8 cameras are not like conventional 35mm SLR still cameras.
With SLR cameras, when you look through the viewfinder you are
(by and large) looking at an image on a ground-glass screen inside
the camera. Both this screen and the film plane are exactly the
same distance from the camera lens. This means that if the image
seen through the viewfinder on the ground glass is in focus, then
the image on the film plane will also be in focus when the shutter
is opened.
With super 8 cameras, things are not quite so rosy.
In the main, super 8 cameras utilise what is called 'aerial image'
focusing. Instead of an image landing on a piece of ground glass
inside the camera, the image simply 'floats' in the air where
the ground glass would have been. When you look through the viewfinder
- which is itelf a little 'close up' or 'diopter' lens - your
eye itself focuses on this 'floating' focused image and you see
what the film will see. The trouble is not all eyes are the same.
Some eyes when looking through the viewfinder diopter will focus
where the aerial image should be, while other eyes might be focused
in front or behind it. When this happens, these 'deviant' eyes
will still see a focused aerial image, BUT IT WON'T BE AN IMAGE
EXACTLY THE SAME DISTANCE FROM THE LENS AS THE FILM PLANE. This
means that while the scene in front of the camera may look in-focus
to the camera operator looking through the viewfinder, the camera
lens will nonetheless be projecting an out-of-focus image onto
the film. Bum.
To solve this problem, the viewfinder diopter can be ajusted to
suit different people's eyes. Having an ajustment, however, means
that it MUST be ajusted for every user! Diopter ajusters come
in a few different styles. It may be necessary to screw the viewfinder
eyepiece in or out to make the ajustment, or there may be a small
knurled dial that needs to be rotated. Sometimes there is a diopter
lock of some sort which is a useful thing in avoiding accidentally
changing the setting.
So here's how to set the viewfinder diopter:
1. Focus the camera lens on infinity (the extreme long distance
focus setting).
2. Zoom the lens all the way in (making distant objects 'bigger'
in the frame).
3. Point the camera at a distant object - as far away as possible,
but at least 30m or so.
4. Looking throught the viewfinder, ajust the diopter until the
image is in focus.
Of critical
value here (at 4) is the focusing aid found in most (but not all)
camera viewfinders. The best is the 'split image' focusing aid.
Another common one is the 'microprism' aid. These focusing aids
if present can be seen in the centre of the viewfinder image area
- usually inside a little circle. The split image device has an
upper and a lower semi-circle. The images inside these two semi-circles
will only line up one above the other when the camera is correctly
focused. When not in focus, the image inside the two semi-circles
are shifted out of alignment with each other. With these devices,
the diopter can be readily set by making the ajustment while looking
at a distant vertical object like a church spire. With the 'microprism'
type focusing device,correct focus is indicated by greatly exagerating
the blur of a wrongly focused image. Only a correctly focused
image (and a correctly set diopter) will give a clear image through
the microprisms.
Check your dipter setting regularly. Be carful that it doesn't
get changed accidently through handling. Check it again and again
while you shoot.
Practical Tips for Focusing:
Here's a rule of thumb: the more 'zoomed in' a lens, the more
critical focus is - the 'wider' the lens, the less critical the
focus. If you are using lens focal length of say 15mm or less,
then generally, you don't need to worry much about focus unless
your subject is very close to the camera. If you don't want to
worry much about focus, then shoot wide!
If the subject you are filming is generally a long way away -
say 20 meters or so - then focus is also not much of a problem.
Just put the lens on infinity and then you can zoom in and out
all you like and your subject will be clear.
The danger area for focus is when you are wanting to zoom in a
bit on your subject, and your subject is in the middle distance
(say 10 meters) or less from the camera. This is where you must
be the most careful. Here's a few things you can do:
1. If possible, before you take your shot, zoom all the way in
on your subject and focus very carefully, then pull out to the
framing you want. 'Zoom in and focus' is a standard cinematography
technique. Its the best way to get a crisp image. It also means
that if you do zoom in during filming, your subject will stay
in focus.
2. Always trust the distance markings on the lens more than you
trust the image in the viewfinder. Check your focus using a tape
measure if possible. Most cameras have a film plane indicator
printed somewhere on the body of the camera. This looks like a
circle with a vertical line through it. Measure the distance from
your subject to this spot on the camera, and set the camera lens
to this distance - it will be right! In less critical situations,
you can just guess this distance by eye and set your lens accordingly,
as long as you are not zooming in too close.
And if you
need a shot that is tricky to focus but is absoulutely critical
for the success of your film, then take more than one shot with
different focus settings (you can do this with exposure too, and
it is called 'bracketing'). Also, just for safety sake, take a
slightly wider shot as well if you can, just to be sure.
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