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 adjusted to suit different people's eyes. Having an adjustment, however, means that it MUST be adjusted for every user! Diopter adjusters come in a few different styles. It may be necessary to screw the viewfinder eyepiece in or out to make the adjustment, 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.
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, adjust 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 adjustment while looking at a distant vertical object like a lone tree, a power pole or a tower. 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 diopter setting regularly. Be carful that it doesn't get changed accidently through handling. Check it again and again while you shoot.
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.
3. 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.
Check whether your camera has a 'macro' lens settingThis can be another trap. Some cameras will allow you to film very close by switching the lens into a 'macro' mode. Often this is done by pulling the zoom lever outwards such that it becomes possible to rotate the zoom on the lens further than normal. Or else it can be a little switch that allows the focus ring of the camera to move beyond its normal closest position. If your camera has one of these settings you need to know, because most of the time, you don't want to be shooting in macro! It is a classic to see entire rolls completely out of focus because the shooter didn't know about the camera's macro setting.