TAME YOUR AUTO FOCUS

By Lorraine Grula
Internet Video Gal

Unless you spend quite a bit of money on a video camera, you will no doubt have one with auto focus.  Most people want auto focus because it sounds easy.  Sometimes auto focus does indeed make your life as a videographer easier.   But the truth is, auto focus can also be a royal pain in the butt.  If you don’t know how it works, auto focus can ruin your video.   Have you ever experienced the joy of an auto focus camera that can’t decide, so it wildly switches focus every half second?   

An auto focus camera guesses at what you want in focus based on parameters such as how far away subjects are from the lens and where subjects are placed inside the frame.  Camera engineers study customer habits and they know people usually want whatever is in the middle of the screen to be what’s in focus.  So they program the camera to automatically focus on the center image, six to ten feet away. 

Of course, sometimes we humans like to be creative and that befuddles camera designers and their magical lenses.

Overriding the automatic focus on a small camera is usually more trouble than it’s worth.  (Press button 4, turn dial x, engage function P, stand on your tippy-toes, hold your breath and squint…) 

Auto focus lenses work best when you have them on the widest possible setting.  In other words, zoomed all the way out.  The picture will ALWAYS be in focus with the lens zoomed all the way out because a wide-angle lens inherently has an extremely wide depth-of-field and there’s nothing for the lens to think about.    

(Depth-of-field refers to how much of a picture, measured linearly, is in focus.  Say for example, everything from five feet to ten feet in front of the camera is in focus. Anything closer than five feet and further than ten feet is out of focus.  That five foot in-focus range is your depth-of-field.  )
 
The depth-of-field in this baby picture is very shallow.  Notice how the blanket behind and in front of her are both out of focus.  This selective focus makes the baby predominant in the shot. 

How much depth-of-field a picture has depends largely on two things:
• How “long” the lens is that you’re using 
• How much light you have available 

A “long” lens refers to a telephoto lens, or one that can see a long distance off and still get a close-up.   This is what you have when you are zoomed in. 

The term “wide-angle” lens is the exact opposite and refers to a lens that gets a wide shot even when the camera is close to the subject.  This would be equivalent to a zoomed-out setting. 

A zoom lens, which is what virtually all video cameras have, zooms in and out, changing from a from wide angle to telephoto.   As it changes from wide to telephoto, its characteristics change too. 

On the telephoto setting (zoomed all the way in) your zoom lens will have a shallow depth of field.  It’ll be hard to keep anything in focus.   Add the fact that the auto focus gets confused and you have the forever-fun experience of watching your auto zoom go bonkers. 
 
A wide-angle lens, or one zoomed all the way out, has a deep depth-of-field.  In fact, an extremely wide-angle lens has infinite depth-of-field; so it’s impossible to take an out-of-focus picture with an extreme wide-angle lens.  

What does this mean in practical terms?  Well, first, you can ignore salespeople when they brag about how much zoom a camera has.  You really won’t zoom in as much as you will zoom out. 

If you want a close-up of something, walk up to it with the camera, do NOT just zoom in on it while you stand a far distance away.   It’s a lot easier to get a good close-up if you stand extremely close to something and zoom all the way out.   Your focus will be better and the shot will be steadier.  (We’ll talk more about camera shake in the chapter on handholding steady without a tripod.)  

Instead of bragging about zoom capacity, camera salespeople should educate customers about how good the macro focus feature is. 

Most video cameras today, even the very inexpensive, are capable of focusing on a subject with the lens extremely close.  This is a feature called macro focus.   Macro focus works best when you place the camera within an inch or two of something and zoom all the way out. 

Try this on your fingertip or maybe a postage stamp.  Pick a bright spot in the room, zoom all the way out and stick your free hand directly in front of the lens.  Experiment within a range of one to three inches.  You should be able to get an extreme close-up of your fingerprint that fills the screen.   That’s truly amazing. 

As a lifelong photography enthusiast, the quality macro provided by modern video cameras make me drool with amazement.   In fact, this is the single biggest criteria I use when evaluating inexpensive camcorders.  You can easily test macro in the store with the fingertip example. 

Macro does not work when the lens is zoomed in.  It’s a wide-angle feature only.  On most inexpensive cameras, it will automatically kick in when the camera is extremely close to something.    

Using your wide-angle setting and relying on the macro feature for close-ups will improve the quality of your video tremendously. 

LIGHTING AFFECTS FOCUS
Another factor that might make it difficult for you to focus well is if you’re in a very low-light situation.   Not having much light makes any lens have a more shallow depth-of-field.  That’s a basic law of physics and photography. 

With a shallow depth of field, auto focus lenses tend to perform poorly.  To alleviate the situation, the only thing you can really do is add more light.  Any lens will have a greater depth-of-field if there is abundant light.