Rudy Winston
Rudy Winston

Rudy Winston has over 17 years experience with Canon USA's Pro Products team, and has been responsible during that time for training Canon's staff on new products, creating presentations for customers and dealers, numerous writing projects, and providing technical assistance to professional and amateur photographers.

Differences between Zone AF and AF Point Expansion

September 15, 2014

With the launch of the EOS 7D Mark II and its introduction of a second Zone AF Area option, called Wide Zone AF, it seems like this would be a great time to quickly review how users can enlarge the size of their active AF point and what the differences are between Zone AF and AF Point Expansion.

Start with the basics — AF Area selection

Canon calls the whole business of changing the size of an AF point “AF Area selection.” You’re changing the size of the active AF Area that will be used to focus upon your subjects. There are some instances where using just a single, small AF point will allow you to precisely take your time and put the sharpest focus right on that leading edge of the nearest eye in a tight portrait, or on that drop of water on the petal of a flower. And there are other times, like trying to follow a fast-moving bird in flight or a zigzagging athlete, where having a larger cluster of AF points might be a better choice.

AF Area options let the photographer leverage the powerful 65-point AF system in the EOS 7D Mark II and set it up however they choose. And, it’s easy to change AF Area settings on the fly — it doesn’t even require going into the menu to do.

A quick review of the EOS 7D Mark II’s AF Area options
  • Single-point AF
    This is the basic AF Area choice. The photographer works with one AF point only, and that point can be moved manually anywhere within the 65-point AF array
  • Spot AF
    Again, a single AF point is manually selected but now, the sampling area for AF is reduced in size, making even more precise focus on specific areas of a subject possible
  • AF Point Expansion — 4 points
    A cluster of 5 total AF points (four points surrounding one primary AF point), which can be manually moved anywhere within the 65-point array too
  • AF Point Expansion — 8 points
    An even larger cluster of AF points, with a box of eight points surrounding one primary AF point. Once again, this cluster of points can be moved manually anywhere within the 65-point array
  • Zone AF
    Looks almost the same as 8-point AF Expansion, but behaves differently. It’s a grouping of AF points, which can be moved to any of nine pre-defined locations. We’ll discuss why Zone AF is different later in this article
  • Wide Zone AF — first time ever in a Canon EOS camera
    A truly huge grouping of AF points, gathered into one working “zone.” Any of three positions can be selected: center, left, or right. It’s not possible to fine-tune its position or just “nudge” it over by a row or so
  • Automatic AF point selection
    Identified by the thin black borders or brackets that surround the entire AF area. All AF points are active; camera automatically selects the active AF point to use. Automatic AF point selection is different in One-Shot AF than in AI Servo AF; that’s a topic for a future article
When should you consider using a larger AF Area?

Using a larger AF sampling area can be a real benefit in a couple of common situations. One example is when focusing on subjects that don’t have a lot of detail or texture for the AF system to lock on to (especially if they’re moving!).

For subjects that may be moving so fast or erratically that to keep one single, small AF point on a known detailed part of the subject might be too limiting. Imagine shooting a professional slalom skier, rapidly advancing left, right, left again through the poles on a downhill run. Now, imagine doing that with a long, heavy telephoto lens with the challenge of trying to keep one small AF point on his or her face. Not easily done! In cases like this, using a larger AF Area still allows the shooter to manually “tell” the camera where to focus (center or off-center), but now reduces the need to precisely follow a fast and erratic moving subject with a small AF point.

Understanding AF Point Expansion

Canon’s AF Point Expansion gives two options in recent cameras like the EOS 7D Mark II, as well as EOS 5D Mark III and EOS-1D X:

  • 4-point Expansion
  • 8-point Expansion

Either way, you have one primary AF point in the middle of the “cluster” of expanded AF points, surrounded by the expanded points. It’s important to understand how this enlarged AF Area functions. Regardless of whether you’re in One-Shot AF mode or AI Servo AF, AF Expansion will always try to focus initially with the central, “primary” AF point — no exceptions.

If it’s aimed at something, like the gray feathers of the seagull in the picture seen earlier in this article, and there’s not enough detail (or insufficient light) for that primary AF point to achieve focus, the surrounding AF points are immediately called upon. If any of them are able to achieve sharp focus, the lens is immediately driven to focus it. And if you’re in One-Shot AF, the central point and the outer point(s) that achieved focus will be highlighted in the viewfinder.

So the key thing to understand is that you should begin by trying to put the central point in that expanded cluster of AF points upon an important part of your subject with detail — and that could be anything from a portrait subject’s face to the foreground of a snow-covered field. Think of that central, primary point as being the place where AF will happen most of the time. AF Point Expansion is a great safety net, so to speak, for those instances where the camera simply can’t latch on to whatever you first aimed it at.

Just to be clear on three additional points:

  • AF Point Expansion (either 4-point or the larger 8-point variety) can be used in any shooting mode other than the EOS 7D Mark II’s Full Auto “green zone”
  • Once you select either the 4-point or 8-point AF Expansion from the AF Area options, you can move the cluster of AF points manually anywhere in the 65-point AF array. If you place the central, “primary” AF point at the absolute outer edge of the array, there are no additional AF points to surround that primary point — so you’ll only have surrounding AF points on the side of the primary point facing inward, toward the center of the AF array
  • An expanded group of AF points cannot move on its own to follow a subject as it darts around the frame. The photographer is always free to move it manually, and the EOS 7D Mark II has important shortcuts to make that a pretty quick and painless operation. But once you position an expanded cluster of points in one place (center or off-center), it stays there until it’s manually moved by the photographer.
Zone AF:

Zone AF may appear to be more of the same, but it has one fundamental difference: Zone AF always tries to focus on the nearest part of a subject that’s within the zone of points. Again, no exceptions. Understand that this is fundamentally different than AF Point Expansion, which starts focus with the central, primary AF point — without regard for the “closest” subject falling within the enlarged AF Area.

In some instances, this can be very useful — if you’re shooting a moving subject like a vehicle and want to keep the nearest part of it in sharpest focus, if you set Zone AF and can position the zone within your finder so it includes the front of the vehicle, it’ll work to keep it sharp.

Another thing to consider about Zone AF: when working in AI Servo, it’s historically been the only way to have the camera focus-track a broad area and maintain sharpest focus on the nearest subject. So it can be useful, for instance, if shooting a group of runners rounding a turn on a track, where you might want the lead runner in sharpest focus.

The new Wide Zone AF on the EOS 7D Mark II occupies a substantial part of the picture area, taking the concept of Zone AF to even bigger levels. It still functions the same way — whether in One-Shot AF or AI Servo AF, it tries to keep sharpest focus on the nearest object it detects falling within the zone.

Zone AF can be useful as well for quick, run-and-gun style candid shooting at parties or events, where you don’t want focus to fall accidentally on the background. However, you obviously have to be careful applying Zone AF. There are plenty of situations where you won’t want the absolute nearest part of a subject in sharpest focus (think of a tight portrait with a telephoto lens, where you more than likely would prefer optimum sharpness on the eyes, not on the point of a subject’s nose!).

This is one of the distinctions between AF Point Expansion and Zone AF. Both are certainly useful, but knowing exactly how they work is the goal of this article. It certainly can help you out when facing a subject or scene and deciding if one AF Area choice might be better than another one.

Traditional Zone AF has a pre-defined number of AF points in the zone and you can move it to any of 9 specific locations. Again, the Wide Zone AF can be positioned in any of three specific locations within the 65-point AF array: center, left or right. Unfortunately, neither Zone AF option allows the user to manually change the size of a zone, nor can you “nudge” it slightly to any location other than those pre-defined for it.

Summary:

Making an AF point larger can sometimes be a truly effective way to maintain sharp focus on an erratically moving subject, or to preserve focus on the nearest subject if it's moving and not in the same area of the frame, from shot to shot. Especially with the new Wide Zone AF option, it's a reminder to understand what these AF Area settings do and how they differ. Knowing that can make it an easy decision to reach for one, when circumstances call for it.

The CDLC contributors are compensated spokespersons and actual users of the Canon products that they promote.

http://learn.usa.canon.com

© 2016 Canon U.S.A., Inc. All Rights Reserved. Reproduction in whole or in part without permission is prohibited.