Background: How do Autofocus Systems Work?
Autofocus systems work by detecting contrast in the image, and measuring how that contrast changes as the lens focuses in and out. When the contrast is high “enough” the autofocus system tells the body it’s ok to take the picture.
Contrast means the difference between the brightest and darkest area of the image. This is why autofocus systems sometimes have problems trying to focus on smooth surfaces; even when in focus the brightest and darkest area of a smooth subject are not that different. Autofocus systems also have a minimum amount of light they can see, which is why autofocus systems can struggle in dark environments and why the DF Body projects a red pattern of light on to the subject.
One Size Does Not Fit All
But how contrasty is contrasty “enough”? For most cameras the manufacturer sets this threshold and you have no control. This one threshold is used no matter what kind of photography is being done. Take two extremes: tripod-based landscape photography and run-and-gun photojournalism, even though the two applications have completely different priorities from the photographers point of view. In slow, precise, diligent landscape photography there is no problem giving the camera as long as it needs to scan through various focus distances to determine the focus with absolute precision. In run-and-gun photojournalism the photographer is very concerned with the camera taking the picture quickly so that they can capture a particular moment, expression, or event; for this kind of photography if the autofocus engine is “pretty confident” it has the right focus it is more important to the photographer that the image is captured immediately than to wait another fraction of a second for the autofocus engine to re-scan the focus-range again to gain higher confidence of focus.
The upshot is that no focus system can perfectly fit every need, especially in medium format where larger sensors, higher resolution, shallow DOF, and sharper lenses demand higher standards of focus accuracy and larger/heavier lenses make it harder to achieve this accuracy.
The Solution? Give the Photographer Control
Phase One and Mamiya worked together to devise a creative solution to this issue and implemented it in the DF Body: the autofocus system can be set (via custom function #19) to either “speed” or “accuracy”. Understanding the advantages of each mode is critical to easily getting consistent focus.
Speed: The autofocus system will allow the camera to fire when it’s “fairly sure” of the focus. It is recommended you use a higher f-stop (e.g. f/8-f/16) when using this mode so that slight variances of focus will be covered by the depth-of-field.
Accuracy: The autofocus system will continue to scan focus back and forth until it is “very sure” of the focus. This can result in some “hunting” especially if the subject is low-contrast. However it provides the highest degree of assurance that the image will be perfectly focused on the intended subject, even when shooting wide-open (e.g. f/2.8)
Which Mode Should You Choose?
The default mode is speed. Whether you should change from the default “speed” to “accuracy” depends on your shooting style. If you’re shooting tabletop, product, landscape, architecture, interiors, etc then the “accuracy” setting will improve your focus accuracy and only cost you a fraction of a second more when the camera occasionally hunts for the perfect focus position. If you’re shooting moving subjects handheld the [Speed] setting will reduce the frequency with which the focus hunts and decrease the time between pushing the shutter release (or starting the focus using rear-button focus method).
Advanced hint: in [Speed] mode you can half-tap the shutter to focus the camera, and then release and half-tap the shutter again to fine-tune the focus. The final effect is similar to using [Accuracy] mode. When making major changes to focus distance (e.g. from infinity to a close-up or vice-versa) this is often needed when in [Speed] mode as the initial focus attempt often comes up a bit shy of moving the focus all the way to the subject.
How Do I Change Modes?
- Rotate the main body dial to CF (for “Custom Functions”)
- Rotate the shutter dial until you are on CF#00 (“SEL”)
- Rotate the thumb-dial until you’ve selected to use Custom Function Set A, B, or C.*
- Rotate the shutter dial until you are on CF#19 (“AF_2”)
- Rotate the thumb-dial until you’ve selected to use Option 0 [Speed] or to Option 1 [Accuracy]
- Rotate the main body dial back to your desired shooting mode
Users of the Phase One P40+/P65+ can also use the interface on the digital back “DF Setup” to access and change the setting. The Phase One IQ will also allow for control of the DF custom functions on the back’s LCD in a future firmware update.
*When using the [blank] Custom Function Set (rather than A, B, or C) the camera uses all defaults and you cannot change any custom functions. This ensures that any user can pick up any DF and put it on defaults by simply changing CF#00 to [blank].
Note: Different Camera Defaults
Currently, the default setting for the AF mode is different depending on which camera body you use. The DF body defaults to setting “0” for Speed, while the DF+ body defaults to “1” for Accuracy.