Focus, Depth of Field part1

The ability to manually focus your camera is a critical skill at any level of video production.

Note:
Manual focus is so important that most professional cameras don't even have an auto-focus feature.

Some Focus Jargon

Soft: Out of focus
Sharp: In focus
Depth of Field: The range of distances from the lens at which an acceptably sharp focus can be obtained
Pull focus: Adjust the focus to a different point during a shot

How to Use the Manual Focus

Focus RingFirst of all, locate the focus control. Professional cameras usually have a manual focus ring near the front of the lens housing. Consumer-level cameras usually have a small dial (Note: you may need to select "manual focus" from the menu).

Make sure the camera is set to manual focus.
Zoom in as tight as you can on the subject you wish to focus on.
Adjust the focus ring until the picture is sharp. Turn the ring clockwise for closer focus, anti-clockwise for more distant focus.
Zoom out to the required framing — the picture should stay nice and sharp.
If the picture loses focus when zoomed out, check the back-focus and make sure the macro focus is not engaged.

If you need to adjust your focus on the fly (for example, you're in the middle of shooting the Prime Minister's speech when you realise her face is soft), it helps to know which way to turn the focus ring. If you go the wrong way and defocus more, even if you correct yourself quickly you've drawn attention to your camera work. Try comparing the background and foreground focus. If the background is sharper than the subject, then you need to pull focus to a closer point (and vice versa).
Note: You will usually find the sharpest focus occurs at about the middle iris position.

Difficult Focus Conditions
You'll notice that focusing is more difficult in certain conditions. Basically, the more light coming through the lens, the easier it is to focus (this is related to depth of field). Obviously it will be more difficult to focus in very low light. If you're really struggling with low-light focus, and you can't add more lighting, try these things:

Make sure your shutter is turned off.
If your camera has a filter wheel, make sure you're using the correct low-light filter. Remove any add-on filters.




If your camera has a digital gain function, try adding a little gain
(note: this compromises picture quality).
Stay zoomed as wide as possible. If your lens has a 2X extender, make sure it's on 1X.

Back Focus

If you find that your focus is sharp when you are zoomed in but soft when zoomed out, your back focus needs adjusting. This normally only happens to cameras with detachable lenses — consumer-level camera users shouldn't have to worry about it.

Technical Note: Back focus refers to the "focal flange length". This is the distance between the rear lens element and the CCD.

You will need:
A camera with a back focus ring. It will be located toward the rear of the lens housing.
A back focus chart like the one pictured is helpful, but any object with sharp contrast will do.

How to Adjust the Camera Back Focus

1. Set your camera on a tripod or stable mount, with your subject (back focus chart or other contrasting object) at least 20 metres/70 feet away (or as far as possible).

2. Your iris should be wide open, so it's better to perform this operation in low light. Alternatively, add some shutter speed or a ND filter.

3. If your lens has a 2X extender, switch it to 1X.

4. Zoom in on your subject.

5. Adjust the focus normally until the picture is sharp. If you're using a back focus chart, the centre of the chart will appear blurry - your focus is sharpest when the blurred circle is smallest. (You can simulate this effect by looking at the chart above and defocusing your eyes.)

6. Zoom out.

7. Loosen the back-focus ring's locking screw, and adjust the ring until the picture is sharp.

8. Repeat steps 3-6 until the focus is consistently sharp.

9. Tighten the back-focus locking screw.

The Focus Pull

The focus pull (AKA rack focus) is a creative camera technique in which you change focus during a shot. Usually this means adjusting the focus from one subject to another.

The shot right begins focused on the woman in the foreground, then adjusts focus until the woman is sharp.

The focus pull is useful for directing the viewer's attention. For example, if there are two people in shot but only one is in focus, that person is the subject of attention. If the focus changes to the other person, they become the subject. This is often used in drama dialogues — the focus shifts backwards and forwards between the people speaking. A slightly more subtle trick is to focus on a person speaking then pull focus to another person's silent reaction.

Focus Throw / Defocus

Throwing focus usually means dropping focus completely. This can either refer to certain parts of the picture (e.g. the background) or to the entire picture.

Throwing focus on part of a picture can't normally be done during a shot — it requires making adjustments such as changing filter or adding shutter.
Throwing focus on the whole picture can be done at any time simply by turning the focus ring until focus is completely lost. This can be used as an opening/closing shot or as a transition between shots. It can also be used for various effects, such as a point of view shot from someone who is drunk or groggy.

In this age of digital editing, focus-throwing for effect is more commonly achieved in post-production. This adheres to the general guideline that it's safer to record pictures "dry" (without effects) and add effects later.

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Gold Coast Video Camera Club