Focus Assist

Maybe I'm getting old. It used to be that shooting video included staring down the barrel of a viewfinder. Though in black and white, picture was clear, and void of stray light. Today, most video is shot with the shooter looking at an LCD screen, often a foot or more away from their face and without protection from ambient light sources. As a result, capturing clear video that's in focus has become more difficult. Fortunately, many cameras now come with focus assist built in. For long-time videographers, focus assist is a nice, but unnecessary feature. For others, it's a necessity.

Focus assist generally comes in two forms. The first is essentially a digital zoom. You see this most frequently on DSLR cameras and interchangeable lens cinema cameras where the user is expected to occasionally use prime lenses (ie. lenses that can't zoom). The typical strategy for focusing video has always been to zoom in on your subject, focus, then zoom out to your intended framing. Obviously no optical zoom makes this impossible. To get around the lens limitation, users press the focus assist button (often represented by a magnifying glass icon), to magnify the image by a factor of five or more. From there, getting accurate focus is easy. Just don't forget to reset the magnification afterwords.

The second form of focus assist is algorithm based. It analyzes the image and highlights areas of very high contrast with a sharp proximity. For example, if part of your image contains adjacent black and white elements, if the size of the transition between the black and white portions are only a couple of pixels, the focus assist will highlight that transition as an indicator of sharp focus. If the transition happens over dozens of pixels, it will be ignored to indicate soft focus. (SmallHD explains this nicely on their YouTube channel). The highlighted edges look strange on a monitor or viewfinder, as they are often very obnoxious colors (like bright green), but they are impossible to miss and easily tell the shooter what's in focus. This type of focus assist is often built into newer camcorders, but is also in many external monitors and EVFs meant to be paired with camcorders with insufficient or no focus assist built in.


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The more I shoot, the more I tend to rely on focus assist. Going from a setup with sophisticated focus assist on the job to no focus assist for home movies is a difficult transition. Has using focus assist made me a better shooter or a worse one? Without a focus assist, I'll occasionally take a shot that is just off of ideal focus. When using a focus assist, that never happens. The answer to whether or not focus assist is a necessity or a crutch probably lies somewhere in between. Adapting to using a focus assist will make video shot with it better, while possibly making video shot without it slightly worse. Why worse? For me it comes down to not taking the time to set up shots that I used to.

What do you think? Do you use focus assist? If so, has your video improved as a result? Do you feel crippled without it? Leave a comment below and tell us how you focus.

Mike Wilhelm is Videomaker's Content Director.


  1. There are times that I rely on a Peaking function for focus, sometimes as a tool when utilizing depth of field and  of course times when I'm to lazy to get the loupe out of my pack. There are times when I just can't use it though as it will complicate framing the shot. So to me it fall under a "useful tool".

  2. I just bought myself a Canon Rebel T4i.  One of its appeal for me was its autofocus function.  The problem is that the camera is constantly evaluating and re-evaluating its focus;  the images go in and out of focus continuously.  Now if I want to go manual, I have a hard time focusing on the tiny monitor. I'm often guessing if I'm in focus or not.  So would a bigger external monitor be the solution for me?  As of now, I'm disapointed by the AF functtion.  It's  not usable in a continuous way during a shot.  I use it only when my object of focus is not moving during the shot.  I then AF before calling action.

  3. Mike,


    Yes digital zoom brings some relief but for static subjects.  How do you keep a subject in focus using AF when it is moving within the frame?  The lens keeps adjusting back and forth continuously making the shot unusable.  Can  a lens like the 18-200 mm STM work better , let's say when a shot is about a person walking towards the camera?

  4. Great question, Alex-

    If you're shooting subjects moving in an unpredictable fashion, DSLRs are just plain going to be more difficult. Their autofocus systems are generally made for still photography and don't adjust very smoothly. If your subject is doing something planned, like walking toward or away from the camera, and you need to maintain focus, there are a couple things you can do. The first is close down your aperture (if you can spare the light) to get a deep depth of field. Second, you can get a follow focus and put some masking tape around the lens where you can mark your start and finish locations. Focus on the subject where they start and mark the tape. Then focus where they end and mark the tape. As they move, simply adjust the focus ring to move your markers between the two focal points.

    Alternatively, you could get one of these:

  5. Thanks Mike for comment and link.  The following review may offer some hope re. using adequate AF on the T4i.   'l

    5.0 out of 5 stars This is THE video lens for the Canon Rebel T4i/650D. August 31, 2012
    Amazon Verified Purchase
    The short version: If you plan on shooting any video with the Rebel T4i, this is the lens you want to buy first.

    The long version: While the new video autofocus feature on the Rebel T4i will work with any Canon autofocus lens, the 18-135mm STM focuses quickly and silently, unlike the 18-55mm lens in the other kit. The 18-55 has a noisy, slow focus motor that constantly 'hunts' for focus while taking video, and the whirring of the non-STM motor is easily picked up by the built-in microphones. It doesn't sound so bad in the field, but once you playback your video clips, it's magnified a great deal, making the audio nearly useless for any serious work. But even with recording Audio separately, the constant focus blur makes the video completely useless anyway, so you may ask yourself why the T4i has any video capability at all.

    Enter the 18-135mm STM. Not only is the STM motor virtually silent, it's fast enough to keep focus while you shoot (well, depending on the speed of the target, of course. No lens can work miracles.) This is the lens all those gorgeous sample videos on the Canon website were taken with, and with good reason.

    Now, there are certainly better Canon lenses for still photography–this isn't even an "L" series lens–but for video work on the T4i, it's hard to beat. It is also both quicker to focus and quieter than the other STM lens currently on the market, the 40mm STM pancake. (I can barely hear the 40mm focus, but the 18-135mm really is next to silent.)

    My recommendation for anyone looking to purchase the Rebel T4i/650D is to insist on the 18-135mm kit and forget the 18-55mm kit. I wish I had. Not that the 18-55 is a horrible lens–it's a decent starter kit lens for still shots. But at the time of this writing, the 18-135mm STM lens is nearly impossible to find by itself–and it lists on the Canon site for $549 when they do have it in stock. So you can either pay the $250 premium for the 18-135 kit over the 18-55 kit, or pay $550 later when you realize the 18-55 won't "do for now" until you can afford the 18-135 STM lens, if you want to take advantage of the video autofocus of the T4i. I suggest the 18-135 kit, plus another $199 for the 40mm STM pancake. That will give you a good solid pair of lenses to get started with, while you save up for an L-series telephoto.

    ''. On Amazon.
  6. I unexpectedly found myself using my wife's Nikon 1 to capture video of a choral group, and I found myself panning around the chorus. The light levels were far less than ideal, and I was aware that the autofocus was cranking pretty hard — and then, after around five minutes of filming, the camera simply stopped and I got an error message that the camera had overheated!  It was true.  When I touched the back of it, it was hot to the touch.  Never had that happen before — but maybe a FYI for those who expect to capture long bursts of video with DSLRs not designed for the task.

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