(...And Why it Might Not Matter So Much!)
Many beginning photographers (and so seasoned ones as well) are in the habit of only using the center autofocus point of their DSLRs. This is understandable given how much simpler it is to simply focus and recompose as opposed to the multiple button presses it takes to position the active AF point over an off-center subject. The problem with this is that it degrades the focus accuracy.
Even for people who should know better, there's a strong temptation to only use the center focus point because the difference can often be subtle and... well, you know, the laziness thing. However, for quality photography it's the difference between a nice image and a stunning one. You can't achieve razor sharp images if you don't place your focus point precisely, so to that end, all the reasons why focus and recomposing should be something to get out of the habit of:
Clearing up One Misconception about Focus
Every photographer starts from somewhere. There's a misconception about focus lock that occurs from time to time with beginning photographers. Focus lock is actually two separate actions: the first is the acquisition of focus, and the second is the prevention of the autofocusing system from any further acquisition attempts. In other words, when focus is "locked", the position of the lens is fixed. What focus locking does not mean is the cameras will keep the subject in focus when the frame is recomposed. In other words, the term should be more correctly described as "Focus and locking."
An analogy: Locking focus is like extending a tape measure. Once you've extended the tape measure and locked it in place, you can move the end point any which way you like, but the distance that the tape measure is locked at remains the same. The key to why focus and recomposing lies with this fact: the distance that the lens is locked at is the cause of the focus error when recomposing.
The Geometry of Focus
Put simply, if you focus on a subject, and then recompose to move it off to the side, you are creating a "backfocus" situation of your own making. The effect can be subtle or pronounced, depending on how far you are recomposing, and how far the subject is from you. Here's a diagram explaining why focus and recomposing pushes the focus point behind your subject. (Click to see the diagram in full-size.) This depicts a situation where the photographer wants to focus on the subject and then recompose to place it to the left of the frame. (In other words, locking focus on the subject and then panning the camera to the right.)
- Focus is locked at the point on the left where the subject is (position 1). Note that the plane of focus is perpendicular to the imaginary line connecting the camera and the initial focus point.
- The photographer now pans the camera to the right, moving the subject to the left of the frame.
- However, because the focus is locked, the distance that the lens is focusing at is effectively the radius "r", which sweeps out an arc from position 1 to position 2.
- Once the picture has been recomposed, the final plane of focus has been shifted to according to the sweep of the arc between position 1 and position 2. The distance of the radius r is still the same, but the subject is now effectively closer to the subject relative to the final plane of focus.
In other words, if you focus and recompose, you are basically causing the camera to back focus. How much this occurs depends on how far the subject is to you and how far from the center of the frame you've placed your subject.
But Wait, It (Potentially) Gets Worse
The above illustration assumes that plane of focus is perfectly flat. In practice most lenses don't have flat planes of focus and exhibit what is called field curvature, which is how far the plane of focus deviates from the center of the frame. For most lenses, the field curvature is away from the camera. This means that the back-focus that focus-recomposing induces can be exaggerated by the optical characteristics of the lens being used:
In this example, the subject ends up not only being closer than the final locked focus point, the true focus plane curves away from the camera. However, once again, the above diagram, though correct in terms of theory, is not representative of true life (you would be essentially focus and recomposing from the center to the very edge on a wide angle lens, something that most people would never have need of). At distances more in keeping with real photography the effect is rather subtle.
Field curvature is one of the least well documented aspects of lens design. Most synthetic tests are done with flat targets, so field curvature is rarely quantified in lens reports. You can get a sense of it by looking at resolution tables and MTF charts. That's because the drop off in resolution from the center of the lens to the corners is predominantly down to field curvature. Put another way, if you focus with an off center focus point, you get good resolution at the expense of the center of the frame losing resolution. Field curvature varies according to lens design, focal length, and focus distance... meaning that it's a complex phenomenon to describe.
So How Bad is It Really?
In theory, focus-recompose is a bad thing. In practice, lots of people get away with it. If you are shooting a landscape scene... it doesn't have distant, it could be within a short-to-middle distance, focus/recompose will produce a dramatic focus shift that would give most people pause for thought to not do it again. For the same amount of re-positioning, the error in focus distance becomes greatly magnified at wider angles and longer distances.
With people and portraiture, the difference in sharpness can be marked to subtle. If you do a focus-recompose from a person's chest to to their eyes at a distance of 10 feet, the image will be soft, but it won't necessarily be unusable... or un-rescuable in post processing. You might even mistake the blurring for handshake and think nothing of it. However, if you are shooting closer, as in a close up on somebody's face, a focus-recompose from the nose to eyes might not even register unless you have a trained eye and shoot portraits for a living.
In other words, the detriment to image quality is always there, but the actual impact is as much a subjective thing as it is an objective one. In some cases, focus-recomposing is unavoidable. This is especially true of the Canon 6D or the Nikon D610, where the focus points are bunched in the center of the frame.
For a visual reference, this is an example of focus-recomposing with a Nikon D800 and 50mm f/1.4G at a distance of approximately 6 feet. The aperture was set to f/1.4 to minimize the depth of field. As an exercise, this is a fairly common shooting distance for candids. There's a saying that the D800 magnifies faults in shooting technique, and that holds true for this example: the autofocus is very precise but the resolution can be unforgiving if you aren't careful. Here's the full scene, focused properly with a focus point on the far left:
|Nikon D800 with AF-S 50mm f/1.4G shot at f/1.4|
Here's what a 100% crop looks like:
... and this is what you get if you attempt a focus-recompose with the center focus point (click to see at actual size):
Though it's not catastrophic, the degradation in image quality is fairly noticeable. The focus point is clearly no longer on the logo, and it's not just the sharpness and edge definition that suffers. There's a drop off in overall contrast and lateral chromatic aberration is more prominent. Assuming that the focus-recompose movement was the equivalent of a 10 degree arc, the vital statistics for this scene go like this:
- Focus point deviation = 2.8cm (1.1")
- Total Acceptable Depth of field (front and back) = 11cm (4.3")
- Deviation in terms of AF Adjustment value = -4
So in other words, focusing with sloppy technique in the above situation is the equivalent of shooting with a lens that requires a -4 setting in the AF fine tune menu. The deviation is within the acceptable depth of field, but bear in mind that all DOF scales that report near and far limits of acceptable focus rely on subjective criteria; as you can see, even within the limit, once you deviate from optimal focus, the degradation in image quality is visible.
It's worth noting that focus-recomposing with a Nikon camera that's set to Matrix Metering changes the exposure slightly as well: even though the focus is locked, the metering isn't, and what happens is that the camera fives you an exposure that's in-between the initial frame and the recomposed frame. You can see that occurring in this next example. Stepping outside of the confines of the camera store, here is a situation shot with a Nikon D7000 and AF-S 35mm f/1.8 G DX, shot at f/1.8 and approximately 12 feet. First the full picture, focused properly with a right-side AF point; focus point is on the left eye of the statue (the one on the right side of the picture).
|Wu Chang by Tianna Kaczor, 2012|
And sample of the same scene using a focus-recompose of the center AF point:
Note the increase in overall brightness that is a direct result of using focus-recompose. In the first shot, the D7000 did a good job of prioritizing the exposure and colour of the statue, but in the second shot, the camera has also made an attempt to bring up the exposure of the foliage, and in doing so, over-exposing the subject.
You might also notice that despite the lengthy discussion above, the recomposed shot looks a tad bit sharper than the first shot. This is probably down to a difference in mechanical tolerance, but it bears in mind that the vital statistics are different than in the example shot in the camera store:
- Focus point deviation = 5.5cm (2.1")
- Total Acceptable Depth of field (front and back) = 76cm (2'6")
- Deviation in terms of AF Adjustment value = -2
In other words, there isn't much of a difference in this situation between doing it the proper way and doing it the quick-and-dirty way. There's plenty of depth of field to work with and the actual distance that the focus point deviates by is small relative to the subject distance. It's the equivalent of shooting with a lens that requires a -2 AF fine tune adjustment. Just as a reminder, a general rule of thumb is that adjustment factors of +/-2 are generally not worth worrying about.
Take Home Message(s)
- It's worth getting into the habit of focusing properly
- Bad focusing technique is like the functional equivalent of shooting with a mis-tuned lens.
- Stopping down increases the depth of field and masks the detrimental effects of focus-recomposing
- Watch out for what's happening with the exposure when you focus-recompose with the camera set to Matrix Meter mode.
- Sometimes it doesn't matter how you focus, but experience helps in knowing when it is okay to break the rules.
With thanks to Broadway Camera.